Hyperlipoproteinemia type I

Alzheimer's disease (AD), a devastating brain disorder, is the leading cause of dementia in the elderly population, affecting 5-10% of the group over age 65 and increasing rapidly to 24-47% for those over age 85 (Evans et al. 1989; Bachman et al. 1992). It is estimated that there are more than 4 million cases of AD in the United States, and AD is considered a major public health problem. Alzheimer's disease is characterized by various pathological features, including (1) the presence of amyloid (starchlike) containing senile plaques in the brain's extracellular parenchyma, especially in the hippocampus and cerebral cortex, which affects normal cognitive function; (2) the occurrence of intracellular neurofibrillary tangles that contain proteins twisted into paired helical filaments, which affects neurons; and (3) the deposition of amyloid plaques along the walls of blood vessels in the central nervous system, which causes vascular damage.

There are two types of AD: familial AD and sporadic AD. In familial AD the disorder segregates in families, following an autosomal dominant inheritance pattern. Because of variation in age at onset, familial AD is further categorized into early onset (=65 years of age). In sporadic AD no obvious family history is indicated. Sporadic AD is also further categorized into early-(=65 years age) onset forms.

Familial AD is genetically heterogenous with the involvement of at least four chromosomal locations identified by linkage studies (Table 1). (Table 1 omitted) Chromosome 21 (Goate et al. 1991), chromosome 14 (Schellenberg et al. 1988), and a yet to be identified chromosome for the Volga-German type of AD (Schellenberg, Bird et al. 1992) are implicated in early-onset familial AD. Although the gene on chromosome 14 has yet to be identified, the gene on chromosome 21 codes for a membrane glycoprotein called amyloid precursor protein (APP). A peptide of 39-42 amino acids, called Beta-amyloid peptide (APP), is derived from APP and is a major component of the core of senile plaques; therefore it is a prime suspect for the cause of AD. Initially, a fourth gene in the 19q13 chromosome region was implicated in late-onset familial AD (Pericak-Vance et al. 1991; Schellenberg, Boehnke et al. 1992). Subsequent association studies identified the APOE gene on chromosome 19 as the susceptible gene not only for late-onset familial AD (Strittmatter, Saunders et al. 1993) but also for early-and late-onset sporadic AD (Saunders, Strittmatter et al. 1993; Mayeux et al. 1993).

Here, I review current data regarding the association of APOE polymorphism with AD and various hypotheses put forward to explain this association. Before discussing these, however, I provide a brief background of the biochemical, physiological, and genetic aspects of apoE. For details on these matters readers are referred to the excellent reviews by Weisgraber (1986), Mahley (1988), and Davignon et al. (1988).

Biochemical and Physiological Characterization of ApoE

Human apoE is a monomeric glycoprotein of 34 kD, and it consists of 299 amino acid residues. The protein is synthesized as pre-apoE with 317 amino acids containing an 18-residue signal peptide that is cleaved cotranslationally. Although the major site of apoE synthesis is the liver, apoE is also synthesized in several other tissues, including the brain, the lungs, the spleen, the kidneys, and the macrophages.

ApoE, originally identified as a constituent of very low density lipoprotein (VLDL) (Shore and Shore 1973), was later found to be a normal component of high-density lipoprotein (HDL) and chylomicrons. ApoE plays a pivotal role in lipid metabolism by mediating the uptake of apoE-containing lipoprotein particles by receptor-mediated endocytosis. ApoE binds with high affinity to two cell receptors, LDL receptor and the LDL-receptor related protein (LRP).

ApoE participates in all three known pathways involved in lipoprotein metabolism: transport of dietary lipid from the intestine to the liver (exogenous pathway), transport of lipid from the liver to extra-hepatic cells (endogenous pathway), and transport of cholesterol from extra-hepatic cells to the liver (reverse cholesterol transport pathway). In the exogenous pathway chylomicrons are synthesized in the intestinal mucosal cells in response to dietary fat and cholesterol and thereafter are secreted into lymph as triglyceride-rich lipoprotein particles. In circulation, chylomicron-associated triglycerides are hydrolyzed by lipoprotein lipase and are converted into chylomicron remnants. These remnants are rich in cholesterol and are removed rapidly from plasma because of the presence of apoE on their surface, which recognizes and binds with LRP on the liver cells.

In the endogenous pathway VLDL is produced by the liver. As with chylomicrons, most triglycerides on VLDL are hydrolyzed in extra-hepatic tissues by lipoprotein lipase, resulting in the formation of, first, intermediate-density lipoprotein (IDL) and finally low-density lipoprotein (LDL). During this metabolic process, most of the IDL particles are cleared from plasma by means of LDL receptors on the liver cells, which recognize apoE on the surface of these particles.

The elimination of excess cholesterol from extra-hepatic tissues, either synthesized in or delivered to peripheral cells, is mediated by the liver through excretion in the bile. HDL serves as a vehicle to transport the excessive cholesterol from extra-hepatic tissues to the liver by a reverse cholesterol transport pathway. ApoE is involved in one of the two alternative pathways by which HDL cholesterol is transported back to the liver. Those HDL particles that harbor apoE on their surface are taken up in the liver by receptors that recognize and bind with apoE.

It is clear that apoE plays an important role in the redistribution of lipids between various tissues in the body from their sites of synthesis to their sites of utilization and excretion.

APOE Genetic Polymorphism

Since 1977, when it was first discovered that a genetic variant in the APOE gene was associated with a rare form of lipid defect, called type III hyperlipoproteinemia (Utermann et al. 1977), the APOE gene has gained the status of one of the most thoroughly studied human genes, primarily because of the profound effect of APOE polymorphism on plasma cholesterol variation. The APOE gene, which is closely linked with the APOC1 and APOC2 genes on chromosome 19q13.2, is genetically polymorphic with the occurrence of three common alleles: APOE*2, APOE*3, and APOE*4. These three alleles control the expression of three homozygous phenotypes (APOE 2,2, APOE 3,3, and APOE 4,4) and three heterozygous phenotypes (APOE 2,3, APOE 3,4, and APOE 2,4).

The three alleles differ from each other by point mutations, resulting in amino acid substitutions at either codon 112 or codon 158 (Figure 1). (Figure 1 omitted) The APOE*3 allele is considered the parental allele and has cysteine at position 112 and arginine at position 158. The APOE*2 allele differs from the APOE*3 allele by the replacement of arginine by cysteine at position 158, and the APOE*4 allele differs from the APOE*3 allele by the replacement of cysteine by arginine at position 112 (see Figure 1). These structural differences in the three APOE alleles are believed to affect the function of apoE in lipid metabolism. The APOE*2 allele product binds defectively to its receptors, resulting in reduced in vivo catabolism of APOE*2-containing lipoproteins, whereas the APOE*4 allele shows an increased in vivo catabolism compared with the APOE*3 allele product. Compared with the most common APOE*3 allele, the APOE*4 allele is associated with a significantly higher plasma total cholesterol, higher LDL cholesterol, higher apoB, and lower apoE, whereas the effect of the APOE*2 allele is in the opposite direction (Davignon et al. 1988).

The APOE locus is polymorphic in all human populations but with variable allele frequencies (Table 2). (Table 2 omitted) Among the populations studied to date, the APOE locus is least polymorphic in Mayan Indians, with a gene diversity of only 16%. New Guineans are highly variable at this locus, with a gene diversity of 60%. The gene diversity value in US whites and in Europeans is 35-40%; in blacks it varies from 45% to 50%. In Polynesians this value is about 52%, whereas in Asians the gene diversity at the APOE locus is only 27%. Several points about the distribution of APOE allele frequencies in various population groups are notable: the aboriginal populations of Australia and America are characterized by the absence of the APOE*2 allele, whereas African and Oceanic populations are characterized by a relatively high frequency of the APOE*4 allele.

APOE Polymorphism and Alzheimer's Disease

The first clue that the APOE gene might be associated with susceptibility to AD came when Strittmatter, Saunders et al. (1993) found a strikingly elevated frequency of the APOE*4 allele in 30 randomly selected AD cases from 30 families with late-onset AD compared with 91 age-matched controls (0.50 vs. 0.16). The high frequency of the APOE*4 allele remained unchanged when Strittmatter analyzed all 83 AD patients in the 30 families. A significant high frequency of the APOE*4 allele in the patient group was at the expense of the other two alleles; the frequencies of the APOE*2 and APOE*3 alleles were 0.04 and 0.44, respectively, in the AD group compared with 0.10 and 0.73, respectively, in the control group. Soon after, the same research group also demonstrated a highly significant association between the APOE*4 allele and late-onset sporadic AD (sporadic AD accounts for about 70% of Alzheimer's disease), including both clinically diagnosed and autopsy-verified AD cases (Saunders, Strittmatter et al. 1993). Subsequently, several research groups independently confirmed the overrepresentation of the APOE*4 allele in late-onset familial and sporadic AD (see Table 3). (Table 3 omitted)

In various studies conducted so far the frequency of the APOE*4 allele varies from 0.42 to 0.52 in the late-onset familial AD group and from 0.24 to 0.47 in the late-onset sporadic AD group. The consistently higher frequency of the APOE*4 allele in the familial AD group may be due to the inclusion of related individuals. The variation in the APOE*4 allele frequency among different studies in the sporadic AD group may relate to different diagnostic criteria used in different studies, because current clinical criteria used for diagnosis of AD are less reliable than autopsy results (Haines 1991; Tsai et al. 1994). Inspection of Table 3 shows a similar high frequency of the APOE*4 allele (==0.40) in the six late-onset sporadic AD studies that used autopsy-confirmed AD samples but different values in clinically diagnosed AD cases. The mean frequency of the APOE*4 allele, calculated from 17 late-onset sporadic AD studies listed in Table 3, is 0.36.

A significant association of the APOE*4 allele with early-onset sporadic AD has also been found (see Table 3) (Okuizumi et al. 1994), indicating that the APOE*4 allele is a major risk factor for AD regardless of age at onset. However, this association remains to be confirmed in early-onset familial AD because the only reported study included just 16 subjects with familial early-onset AD and did not find a significant association with the APOE*4 allele (Saunders, Strittmatter et al. 1993). This may be due to the small number of AD cases used or the overriding effects of specific mutations at chromosome 14 or chromosome 21 in the families used.

An interesting and dramatic aspect of the association between the APOE*4 allele and AD comes from findings that the effect of the APOE*4 allele is dose related and inversely correlated with age at onset. These findings provide further evidence that the APOE*4 allele may be involved in the pathogenesis of AD. Individuals with two copies of the APOE*4 allele (genotype APOE*4/*4) are at significantly higher risk of developing AD at an earlier age than those who carry either one copy (genotypes APOE*3/*4 or APOE*2/*4) or no copy (genotypes APOE*2/*2, APOE*2/*3, and APOE*3/*3) of the APOE*4 allele. The odds of developing AD with at least one copy of the APOE*4 allele is about five times that for developing the disease without the APOE*4 allele. This ratio increases approximately threefold if the person is homozygous for the APOE*4 allele.

In one study of late-onset familial AD, Corder et al. (1993) found that risk for AD increased from 20% with no copy of the APOE*4 allele to 47% and 91% with one and two copies of the APOE*4 allele, respectively. Mean age at onset decreased from 84 to 75 to 68 years as the number of APOE*4 copies increased from 0 to 1 to 2, respectively. This inverse relationship between the APOE*4 allele dosage effect and age at onset has been confirmed in another large late-onset familial AD study (Payami et al. 1994) and in a late-onset sporadic AD study (Poirier et al. 1993).

In addition to the universal findings that the APOE*4 allele confers an increased risk of developing AD, other data suggest that the presence of the APOE*2 allele may protect against AD, thus providing further support for the direct involvement of the APOE gene in the etiology of AD. The average frequency of the APOE*2 allele in the general population is 0.08. Eighteen of the 26 studies listed in Table 3 provide a frequency of the APOE*2 allele in AD cases in the range from 0.005 to 0.08. Of these, nine studies found a frequency of 0.02 or less, and only one study found a frequency of 0.08. An apparently protective effect of the APOE*2 allele is in keeping with the association of the APOE*2 allele with longevity (Schachter et al. 1994).

Mechanisms of Association of Apolipoprotein E with Alzheimer's Disease

Allelic Association, Despite the striking association of the APOE*4 allele with AD, the APOE*4 allele is neither necessary nor required for the development of familial or sporadic AD. Not all individuals with the APOE*4 allele develop AD, even at older age, and not all individuals with AD are carriers of the APOE*4 allele. This implies that the APOE*4 allele is a strong susceptibility marker that, in conjunction with other genetic or environmental factors, significantly increases an individual's risk of developing AD. Alternatively, the APOE*4 mutation is in strong linkage disequilibrium with a cis-acting causal mutation either in the APOE gene itself or in some linked gene on chromosome 19. The idea that the genetic variation at the APOE locus is involved in conferring high susceptibility to AD is strongly supported because the effect of the APOE*4 allele is dose related, two copies of the APOE*4 allele are associated with earlier age at onset and rapid progression of AD, and apoE is involved in the pathogenic pathways of AD. With this in mind it is important to note that the allele frequency of APOE*4 (see Table 2) and the proportion of individuals who carry this allele (Figure 2) vary widely among population groups. (Figure 2 omitted) Although only 14% of Asians and 25% of whites carry the APOE*4 allele, almost 50% of Africans, Polynesians, Melanesians, and Australian Aborigines do. The proportion of carriers of the APOE*4 allele is also relatively higher (-40%) in some Amerindian and Eskimo populations. Although the incidence of AD is not well documented in these aboriginal populations, these populations might be at higher risk of developing AD should they experience the demographic shift toward longer life expectancy similar to the one that has already occurred in Western European and American white populations.

Pathogenic Association. The notion that apoE may play a causal role in the pathogenesis of AD comes from various lines of evidence. ApoE has been immunolocalized in the defining pathological lesions of AD, including the extracellular senile plaques and intracellular neurofibrillary tangles (Namba et al. 1991; Wisniewski and Frangione 1992; Wisniewski et al. 1993; Han et al. 1994). beta-amyloid peptide (AbetaP) is a major component of the central core of amyloid deposits; a protein known as tau is associated with the formation of neurofibrillary tangles in neurons. ApoE binds with both AbetaP and tau and therefore may directly participate in determining the neuropathology of AD. Whether the binding of apoE with AbetaP or tau or both promotes the clinical features of AD is controversial (Marx 1993). Because the role of AbetaP in plaque formation has been studied more extensively than the role of tau in the formation of neurofibrillary tangles, the most favored hypothesis at present is that AbetaP is the primary lesion in AD, and other events, such as neurofibrillary tangles, neuronal cell loss, vascular damage, and dementia, are consequences of amyloid deposition (Hardy and Higgins 1992; Travis 1993; Suzuki et al. 1994).

This hypothesis is further strengthened by the identification of several pathogenic mutations in the APP gene that cosegregate with certain families with early-onset AD. Mutations in the APP gene have also been linked with an autosomal dominant form of hereditary cerebral hemorrhage with amyloidosis of the Dutch type (HCHWA-D), which causes the accumulation of potentially amyloidogenic fragments and increased production of AbetaP (Selkoe 1990; Felsenstein et al. 1994). Furthermore, patients with Down syndrome, in which there are three copies of the APP gene, develop deposits of AbetaP in senile plaques as they age. These findings strongly support the hypothesis that AbetaP deposition may precede other neuropathological features of AD. However, a recent report on APOE-allele-specific binding with tau has provided the strongest support yet that neurofibrillary tangles are a significant culprit behind the initial pathology of AD (Strittmatter et al. 1994). The possible mechanisms of the association of apoE with AbetaP and tau leading to AD are discussed in what follows.

Apolipoprotein E and Amyloid Plaques. Before discussing the pathogenic mechanism of apoE binding with AbetaP, it is important to understand the alternative pathways by which the APP gene is proteolytically cleaved to release the normal secreted APP and the potentially pathogenic AbetaP. The APP gene undergoes alternative splicing to yield several protein forms that are designated by the total number of amino acids in each. The largest known APP form consists of 770 amino acids; the other forms include 751, 714, 695, 677, 563, or 365 amino acids [see Kosik (1992) and Suzuki et al. (1994)]. With the exception of the 563 and 365 forms, all other APP forms contain the AbetaP sequence. APP is a membrane-bound protein consisting of an NH sub 2 2-terminal domain of 699 amino acids (in APP-770) in the extracellular or intraluminal region (depending on the type of membrane), a short transmembrane region of 24 amino acids, and a 47 amino acid long COOH-terminal tail in the cytoplasma (Figure 3). (Figure 3 omitted)

AbetaP is a small fragment of 39-42 amino acid residues (although it may extend to 43 residues in some cases) derived from APP by alternative processing (Figure 3). AbetaP begins 99 amino acid residues from the COOH-terminal of APP, covering 28 amino acids in the extracellular-intraluminal region and extending to the first 11-15 hydrophobic residues in the transmembrane region (Esch et al. 1990; Sisodia et al. 1990; Shoji et al. 1992). The secretion of ==100-kD soluble APP, consisting of 686 residues of the NH sub 2 -terminal domain, is mediated by an APP-secretase enzyme that cleaves the APP at position 15 within the AbetaP sequence in the extramembrane region. This generates a 9-kD membrane-bound COOH-terminal fragment, 83 amino acids long, that begins at residue 17 within the AbetaP sequence. The single amino acid at position 16 is thought to be removed by an exopeptidase after the initial cleavage of APP. This so-called constitutive processing of APP, which involves cleavage in the interior sequence of AbetaP, prevents the intact formation of AbetaP and thus precludes its deposition in senile plaques.

The release of intact AbetaP is achieved by alternative processing in which APP is cleaved at two different positions, one at or near the NH sub 2 terminal of AbetaP and the other in the transmembrane region. Early reports suggested that these potentially amyloidogenic peptides are generated in the endosomal-lysosomal pathway, but Shoji et al. (1992), Busciglio et al. (1993), and Felsenstein et al. (1994) have shown that AbetaP can be formed in the secretory pathway by enzymatic cleavage.

The binding of apoE with AbetaP has been confirmed by in vitro studies (Strittmatter, Saunders et al. 1993). Although both the common (apoE3) and the variant (apoE4) forms bind to synthetic AbetaP, the binding of apoE4 to AbetaP is much more rapid and stronger than the binding of apoE3 to AbetaP (Strittmatter, Weisgraber et al. 1993). Furthermore, APOE*4 homozygosity is associated with an increased amount of AbetaP in senile plaques and vascular deposits more than APOE*3 homozygosity (Schmechell et al. 1993; Rebeck et al. 1993).

Additional evidence that apoE may be present in amyloid plaques comes from findings that the LDL-receptor related protein (LRP), which recognizes and binds apoE-containing lipoproteins, has been colocalized with apoE in senile plaques (Rebeck et al. 1993). The question arises, however, as to how the apoE4 isoform is associated with more beta-amyloid plaque deposition in brain cells. The potential pathogenicity of the apoE4 isoform has been implicated by several mechanisms. In the brain apoE is synthesized in astrocytes. Its synthesis is increased following neural injury in both the peripheral and the central nervous systems and is implicated in neuronal repair and regeneration (Mahley 1988). Because the accumulation of AbetaP is considered neurotoxic and can lead to neural injury (Hardy and Higgins 1992; Behl et al. 1994; Goodman and Matson 1994; Hensley et al. 1994), this may result in an increased supply of apoE to the site of injury, and as a result of the binding of apoE with AbetaP, more senile plaques form. Because apoE4 binds more rapidly and strongly with AbetaP, this might explain the difference in the extent of plaque formation between individuals carrying the APOE*3 and the APOE*4 alleles (see Figure 3).

Recently, however, Whitson et al. (1994) showed that the interaction of rabbit apoE, which shows 80% similarity to human apoE3, with AbetaP does not increase AbetaP neurotoxicity. They showed that at physiological concentration apoE may in fact play a neuroprotective role in the brain, and its binding with AbetaP may interfere with this role by reducing the available pool of apoE. According to this hypothesis, the apoE4 isoform, which binds to AbetaP more rapidly and more avidly than the apoE3 isoform, may not be as effective as apoE3 in abolishing the neurotoxicity of AbetaP and thus is associated with increased amyloid plaques.

Rebeck et al. (1993) proposed another mechanism by which the APOE*4 allele may promote plaque formation. As mentioned, apoE binds to two receptors: LDL receptor and LRP. In brain cells the expression of LRP appears to be greater than the LDL receptor, and this difference is more pronounced in patients with AD, especially in reactive astrocytes and senile plaques. Based on the observation that plaques may not accumulate over the course of AD, Rebeck et al. (1993) proposed that apoE-AbetaP complexes are cleared from the neutrophil by LRP and that the APOE*4 allele is associated with impaired clearance of these complexes because of either altered apoE-AbetaP interaction or lower levels of apoE. Because AbetaP binds to apoE in the lipoprotein-binding domain and not in the receptor-binding domain of the protein (Strittmatter, Weisgraber et al. 1993), it is conceivable that the metabolism of both lipoproteins and apoE-AbetaP complexes is mediated by the same route of receptor-mediated endocytosis. If this hypothesis is correct, then AD patients with the APOE*4 allele, which is associated with elevated plasma cholesterol levels in the general population, will also have abnormal cholesterol levels. Indeed, it has been shown previously that patients with AD have abnormally high levels of plasma cholesterol and apoB (Giubilei et al. 1990); the basis of this observation could not be explained at that time. Now that we know that the frequency of the APOE*4 allele is high in patients with AD and that the APOE*4 allele is associated with high cholesterol and apoB levels in the general population, it is highly likely that the AD group studied by Guibilei et al. (1990) had a high frequency of the APOE*4 allele. It will be important to confirm the relationship of the APOE*4 allele with lipoprotein metabolism in patients with AD in future studies because these patients may also be at higher risk of developing atherosclerosis.

Apolipoprotein E and Neurofibrillary Tangles. Neurofibrillary tangles, a pathological hallmark of AD, are made up intracellularly of paired helical filaments (PHFs), which are composed of a highly phosphorylated protein called tau. Tau is a microtubule-associated protein (MAP) whose normal function is to promote and stabilize microtubule assembly, that is, to organize the intracellular structure in the cytoplasm and to transport molecules between the cell components and nerve terminals. However, phosphorylation of tau by some kinases reduces its stabilizing effect (Steiner et al. 1990). Normal or AD-soluble tau contains at least one phosphorylation site in the COOH-terminal portion of the protein, whereas PHF-associated tau (PHF-tau) is abnormally phosphorylated at more than seven sites in the whole molecule (Hasegawa et al. 1992). Therefore the hyperphosphorylated tau does not bind and stabilize microtubules, leaving tau to clump into tangles and causing the microtubules to destabilize and eventually collapse in the brain of AD patients.

Tau is encoded by a single gene that undergoes alternative splicing to yield at least six protein isoforms of 50-68 kD containing between 341 and 441 amino acid residues (Goedert et al. 1989a,b). There are inserts of either 29 or 58 residues in the NH sub 2 terminal and 3 or 4 internal repeats of 31 or 32 residues in the COOH-terminal half of the protein. The COOH-terminal repeats are responsible for binding to microtubules.

The implication that apoE may participate in the pathology of neurofibrillary tangles comes from observations that apoE is immunolocalized in neurons and neurofibrillary tangles and that it binds to tau (Strittmatter et al. 1994). However, the binding of apoE to tau is isoform specific: Tau binds to apoE3 but not to apoE4. This unusual finding led Strittmatter et al. (1994) to formulate a provocative hypothesis that emphasizes the absence of the APOE*3 or APOE*2 allele rather than the presence of the APOE*4 allele in patients with AD. According to this hypothesis (Figure 4), the binding of apoE3 isoforms and possibly of apoE2 isoforms with tau will protect them from becoming hyperphosphorylated, thus slowing the formation of neurofibrillary tangles. (Figure 4 omitted) On the other hand, the inability of apoE4 to bind to tau will leave tau to be phosphorylated abnormally, and eventually neurofibrillary tangles will form.

The isoform-specific binding of apoE to tau is believed to be mediated by cysteine residues. As shown in Figure 1, apoE3 and apoE2 have one and two cysteine residues, respectively, whereas apoE4 has none. Tau has one or two cysteine residues (depending on the type of isoform), which are located in the microtubule binding region. This region is also believed to form the core of PHF (Crowther et al. 1992), and cysteine present in this region appears to form antiparallel dimers of tau in PHFs. The presence of cysteine residues in apoE2 or apoE3 isoforms thus enables them to bind to tau and prevents its self-assembly.

This hypothesis is intriguing and, if proved correct, may provide a rationale for therapy wherein the missing protective factor (apoE2 or apoE3) is supplemented rather than the culprit product (apoE4) being replaced.

The question that still needs to be resolved, however, is how apoE gets inside neurons, because in the brain apoE is primarily synthesized by astrocytes and macrophages (microglia). Although apoE has been immunolocalized in neurons (Han et al. 1994), currently, no data suggest that apoE might be synthesized in neurons. Alternatively, apoE may be internalized in neurons through LDL receptors or LRP-mediated endocytosis and may interact with other cellular proteins, including tau (Nathan et al. 1994).

Conclusions

The evidence presented here clearly shows the important role of apoE in the pathogenesis of AD. However, it also emphasizes that, like all other complex genetic disorders, the underlying cause of AD is not rooted in one gene. Although the APOE*4 allele has emerged as the most powerful genetic tool yet to be described in AD research, it is neither necessary nor required for the expression of AD. Nevertheless, the role of the APOE*4 allele as a major susceptibility marker for AD is undeniable. The existence of individuals with the APOE*4 allele who do not develop AD even at advanced age and of individuals who lack the APOE*4 allele and still develop the disease emphasizes the involvement of other genetic and/or environmental factors. Other candidate genes that may play a significant role in the pathogenesis of AD include those whose products are implicated in the pathological lesions of AD. For example, in addition to apoE, alpha l-antichymotrypsin, cathepsin, and alpha-trypsin are associated with senile plaques. Similarly, in addition to tau, there are other microtubule-associated proteins, such as MAP sub 2 . Also, linkage studies suggest two additional genes, one on chromosome 14 and the Volga-German AD gene, which can predispose individuals to early-onset autosomal dominant AD. Characterization of these genes in patients with AD who lack the APOE*4 allele will be a step toward revealing additional genetic susceptibility markers.

With the knowledge that the APOE*4 allele promotes amyloid plaques (and possibly neurofibrillary tangles), we can envision therapeutic treatments that will either block the interaction of apoE4 and AbetaP or increase interaction between apoE and tau. Currently, tacrine is the only approved drug to treat AD. However, its interaction with APOE genotypes is unknown. Knowing that patients with a particular APOE genotype respond more or less favorably to this drug than patients with other APOE genotypes would be another step forward in treating AD in selected patients.

The exciting new discoveries, however, also have implications on ethical, psychological, social, and legal fronts. For example, can results of APOE genotyping be used to predict the risk of developing AD or the age of onset in the general population? Can results of APOE genotyping be used for diagnostic purposes in family members of patients with AD who are carriers of the APOE*4 allele? Should symptomatic persons be tested for APOE genotypes? The answer to the first question is no, because the APOE*4 allele is a susceptibility marker not absolutely required for the disease. Although APOE genotyping may predict an asymptomatic family member's risk of developing AD, in the absence of a specific treatment of the disease the knowledge of being a carrier of the susceptible allele will have great psychological implications. Is it ethical to tell a person that he or she might develop a devastating neurodegenerative disease for which there is no known cure? Furthermore, healthy individuals who are carriers of a defective gene may be discriminated against by health and life insurance companies. The answer to the last question is most probably yes, because given the association of the APOE*4 allele with the severity of pathological lesions, a substantial amount of the health care cost involved in routine screenings of patients for diagnostic purposes will be saved.

Despite the legitimate concerns and questions, we have learned a great deal about the genetics of AD and hope to have more dramatic discoveries in the near future that will increase our knowledge of this debilitating disease and ease its burden as a major public health problem in the United States.

Literature Cited

Asakawa, J., N. Takahashi, B.B. Rosenblum et al. 1985. Two-dimensional gel studies of genetic variation in the plasma proteins of Amerindians and Japanese. Hum. Genet. 50:222-230.

Bachman, D.L., P.A. Wolf, R. Linn et al. 1992. Prevalence of dementia and probable senile dementia of the Alzheimer type in the Framingham Study. Neurology 42:115-119.

Behl, C., J.B. Davis, R. Lesley et al. 1994. Hydrogen peroxide mediates amyloid beta protein toxicity. Cell 77:817-827.

Boerwinkle, E., S. Visvikis, D. Welsh et al. 1987. The use of measured genotype information in the analysis of quantitative phenotypes in man. II. The role of the apolipoprotein E polymorphism in determining levels, variability, and covariability of cholesterol, betalipoprotein, and triglycerides in a sample of unrelated individuals. Am. J. Med. Genet. 27:567-582.

Brousseau, T., S. Legrain, C. Berr et al. 1994. Confirmation of the epsilon4 allele of the apolipoprotein E gene as a risk factor for late-onset Alzheimer's disease. Neurology 44:342-344.

Busciglio, J., D.H. Gabuzda, P. Matsudaira et al. 1993. Generation of beta-amyloid in the secretory pathway in neuronal and nonneuronal cells. Proc. Natl. Acad. Sci. USA 90:2092-2096.

Chartier-Harlin, M.-C., M. Parfitt, S. Legrain et al. 1994. Apolipoprotein E, epsilon4 allele as a major risk factor for sporadic early- and late-onset forms of Alzheimer's disease: Analysis of the 19q13.2 chromosomal region. Hum. Molec. Genet. 3:569-574.

Corder, E.H., A.M. Saunders, W.J. Strittmatter et al. 1993. Gene dose of apolipoprotein E type 4 allele and the risk of Alzheimer's disease in late onset families. Science 261:921-923

Corder, E.H., A.M. Saunders, N.J. Risch et al. 1994. Protective effect of apolipoprotein E type 2 allele for late onset Alzheimer disease. Natur. Genet. 7:180-183.

Crowther, R.A., O.F. Olesen, R. Jakes et al. 1992. The microtubule binding repeats of tau protein assemble in filaments like those found in Alzheimer's disease. Fed. Eur. Biochem. Soc. 309:199-202.

Davignon, J., R.E. Gregg, and C.F. Sing. 1988. Apolipoprotein E polymorphism and atherosclerosis. Arteriosclerosis 8:1-21.

Deka, R., S.T. McGarvey, R.E. Ferrell et al. 1994. Genetic characterization of American and Western Samoans. Hum. Biol. 66:805-822.

DeKnijff, P., L.G. Johansen, M. Rosseneu et al. 1992. Lipoprotein profile of a Greenland Inuit population: Influence of anthropometric variables, Apo E and A4 polymorphisms, and lifestyle. Arterioscler. Thromb. 12:1371-1379.

Ehnholm, C.. M. Lukka, T. Kuusi et al. 1986. Apolipoprotein E polymorphism in the Finnish population: Gene frequencies and relation to lipoprotein concentrations. J. Lipid Res. 27:227-235.

Esch, F.S., P.S. Keim, E.C. Beattie et al. 1990. Cleavage of amyloid beta peptide during constitutive processing of its precursor. Science 248:1122-1124.

Evans, D.A., H.H. Funkenstein. M.S. Albert et al. 1989. Prevalence of Alzheimer's disease in a community population of older persons: Higher than previously reported. J. Am. Med. Assoc. 262:2551-2556.

Felsenstein, K.M., L.W. Hunihan, and S.B. Roberts. 1994. Altered cleavage and secretion of a recombinant beta-APP bearing the Swedish familial Alzheimer's disease mutation. Natur. Genet. 6:251-256.

Frisoni, G.B., A. Bianchetti, S. Govoni et al. 1994. Association of apolipoprotein with vascular dementia. J. Am. Med. Assoc. 271:1317.

Giubilei, F., R. D'Antona, R. Antonini et al. 1990. Serum lipoprotein pattern variations in dementia and ischemic stroke Acta Neurol. Scand. 81:84-86

Goate, A.M., M.C. Chartier-Harlin, M. Mullan et al. 1991. Segregation of a missense mutation in the amyloid precursor protein gene with familial Alzheimer's disease. Nature 349:704-706.

Goedert, M., M.G. Spillantini, R. Jakes et al. 1989a. Multiple isoforms of human microtubule-associated protein tau: Sequences and localization in neurofibrillary tangles. Neuron 3:519-526.

Goedert, M., M.G. Spillantini, M.C. Potier et al. 1989b. Cloning and sequencing of the cDNA encoding an isoform of microtubule-associated protein tau containing four tandem repeats: Differential expression of tau protein mRNAs in human brain. EMBO J. 8:393-399.

Goodman, Y., and M.P. Mattson. 1994. Secreted form beta-amyloid precursor protein protects hippocampal neurons against amyloid beta peptide-induced oxidative injury. Exp. Neurol. 128:1-12.

Haines, J.L. 1991. The genetics of Alzheimer disease: A teasing problem. Am. J. Hum. Genet. 48:1021-1025.

Hallman, D.M., E. Boerwinkle, N. Saha et al. 1991. The apolipoprotein E polymorphism: A comparison of allele frequencies and effects in nine populations. Am. J. Hum. Genet. 49:338-349.

Han, S.-H., C. Hulette, A.M. Saunders et al. 1994. Apolipoprotein E is present in hippocampal neurons without neurofibrillary tangles in Alzheimer's disease and in age-matched controls. Exp. Neurol. 128:13-26.

Hardy, J.A., and G.A. Higgins. 1992. Alzheimer's disease: The amyloid cascade hypothesis. Science 256:184-185.

Hasegawa, M., M. Morishima-Kawashima, K. Takio et al. 1992. Protein sequence and mass spectrometric analysis of tau in the Alzheimer's disease brain. J. Biol. Chem. 267:17047-17054.

Hensley, K., J.M. Carney, M.P. Mattson et al. 1994. A model for beta-amyloid aggregation and neurotoxicity based on free radical generation by the peptide: Relevance to Alzheimer's disease. Proc Natl. Acad. Sci. USA 91:3270-3274

Kamboh, M.I., K.K. Bhatia, and R.E. Ferrell. 1990. Genetic studies of human apolipoproteins. XII. Population genetics of apolipoproteins in Papua New Guinea. Am. J. Hum. Biol. 2:17-23.

Kamboh, M.I., S.T. DeKosky, and R.E. Ferrell. 1994. Overrepresentation of the APOE*4 allele in autopsy confirmed early- and late-onset sporadic Alzheimer's disease. Am. J. Hum. Genet. 55(suppl.):A154.

Kamboh, M.I., B. Sepehrnia, and R.E. Ferrell. 1989. Genetic studies of human apolipoproteins. VI. Common polymorphism of apolipoprotein E in blacks. Dis. Markers 7:49-55

Kamboh, M.I., S.W. Serjeantson, and R.E. Ferrell. 1991. Genetic studies of human apolipoproteins. XVIII. Apolipoprotein polymorphisms in Australian Aborigines. Hum. Biol. 63:179-186.

Kamboh, M.I., K.M. Weiss, and R.E. Ferrell. 1991. Genetic studies of human apolipoproteins. XVI. APO E polymorphism and cholesterol levels in the Mayans of the Yucatan Peninsula, Mexico. Clin. Genet. 39:26-32.

Klasen, E.S., M. Smit, P. DeKnijff et al. 1987. Apolipoprotein E phenotype and gene distribution in the Netherlands. Hum. Hered. 37:340-344.

Kosik, K.S. 1992. Alzheimer's disease: A cell biological perspective. Science 256:780-783.

Lucotte, G., S. Visvikis, B. Leininger-Muler et al. 1994. Association of apolipoprotein E allele epsilon4 with late-onset sporadic Alzheimer's disease. Am. J. Med. Genet. 39:26-32.

Mahley, R.W. 1988. Apolipoprotein E: Cholesterol transport protein with expanding role in cell biology. Science 240:622-630.

Marx, J. 1993. New Alzheimer's theory stirs controversy. Science 262:1210-1211.

Mayeux, R., Y. Stern, R. Ottman et al. 1993. The apolipoprotein epsilon4 allele in patients with Alzheimer's disease. Ann. Neurol. 34:752-754.

Namba, Y., M. Tomonaga, H. Kawasaki et al. 1991. Apolipoprotein E immunoreactivity in cerebral amyloid deposits and neurofibrillary tangles in Alzheimer's disease and Kuru plaque amyloid in Creutzfeldt-Jakob disease. Brain Res. 541:163-166.

Nathan, B.P., S. Bellosta, D.A. Sanan et al. 1994. Differential effects of apolipoproteins E3 and E4 on neuronal growth in vitro. Science 264:850-852.

Noguchi, S., K. Murakami, and N. Yamada. 1993. Apolipoprotein E genotype and Alzheimer's disease. Lancet 342:737.

Okuizumi, K., O. Onodera, H. Tanaka et al. 1994. ApoE-epsilon4 and early-onset Alzheimer's. Natur. Genet 7:10-11

Ordovas, J.M., L. Litwack-Klein, P.W.F. Wilson et al. 1987. Apolipoprotein E isoform phenotyping, methodology, and population frequency with identification of apoE1 and apoE5 isoforms. J. Lipid Res. 28:371-380.

Payami, H., J.A. Kaye, L.L. Heston et al. 1993. Apolipoprotein E genotype and Alzheimer's disease. Lancet 342:738.

Payami, H., K.R. Montee, J.A. Kaye et al. 1994. Alzheimer's disease, apolipoprotein E4, and gender. J. Am. Med. Assoc. 271:1316-1317.

Peacock, M.L., and J.K. Fink. 1994. ApoE epsilon4 allelic association with Alzheimer's disease: Independent confirmation using denaturing gradient gel electrophoresis. Neurology 44:339-341.

Pericak-Vance, M.A., J.L. Bebout, P.C. Gaskell Jr. et al. 1991. Linkage studies in familial Alzheimer disease: Evidence for chromosome 19 linkage. Am. J. Hum. Genet. 48:1034-1050.

Poirier, J., J. Davignon, D. Bouthillier et al. 1993. Apolipoprotein E polymorphism and Alzheimer's disease. Lancet 342:697-699.

Rebeck, G.W., J.S. Reiter, D.K. Strickland et al. 1993. Apolipoprotein E in sporadic Alzheimer's disease: Allelic variation in receptor interactions. Neuron 11:575-580.

St. George-Hyslop, P.H., R.E. Tanzi, R.J. Polinsky et al. 1987. The genetic defect causing familial Alzheimer's disease maps on chromosome 21. Science 235:885-890.

Saunders, A.M., K. Schmader, J.C.S. Brietner et al. 1993. Apolipoprotein E epsilon4 allele distributions in late-onset Alzheimer's disease and in other amyloid-forming diseases. Lancet 345:710-711.

Saunders, A.M., W.J. Strittmatter, D. Schmechel et al. 1993. Association of apolipoprotein E allele epsilon4 with late-onset familial and sporadic Alzheimer's disease. Neurology 43:1467-1472.

Schachter, F., L. Faure-Delanef, F. Guenot et al. 1994. Genetic associations with human longevity at the ApoE and ACE loci. Natur. Genet. 6:29-32.

Schellenberg, G.D., T.D. Bird, E.M. Wijsman et al. 1988. Absence of linkage of chromosome 21q21 markers to familial Alzheimer's disease. Science 241:1507-1510.

Schellenberg, G.D., T.D. Bird, E.M. Wijsman et al. 1992. Genetic linkage evidence for a familial Alzheimer disease locus on chromosome 14. Science 258:668-670.

Schellenberg, G.D., M. Boehnke, E.M. Wijsman et al. 1992. Genetic association and linkage analysis of the apolipoprotein CII locus and familial Alzheimer's disease. Ann. Neurol. 31:223-227.

Schmechell, D.E., A.M. Saunders, W.J. Strittmatter et al. 1993. Increased amyloid beta-peptide deposition in cerebral cortex of a consequence of apolipoprotein E genotype in late-onset Alzheimer disease. Proc. Natl. Acad. Sci. USA 90:9649-9653.

Selkoe, D.J. 1990. Deciphering Alzheimer's disease: The amyloid precursor protein yields new clues. Science 248:1058-1060.

Sepehrnia, B., M.I. Kamboh, L. Adams-Campbell et al. 1988. Genetic studies of human apolipoproteins. VII. Population distributions of polymorphisms of apolipoproteins A-I, A-II, A-IV, C-II, E, and H in Nigeria. Am. J. Hum. Genet. 43:847-853.

Seubert, P., T. Oltersdorf, M.G. Lee et al. 1993. Secretion of beta-amyloid precursor protein cleaved at the amino terminus of the beta-amyloid peptide. Nature 361:260-263.

Shoji, M., T.E. Golde, J. Ghiso et al. 1992. Production of the Alzheimer amyloid beta protein by normal proteolytic processing. Science 258:126-129.

Shore, V.G., and B. Shore. 1973. Heterogeneity of human plasma very low density lipoproteins: Separation of species differing in protein components. Biochemistry 12:502-507.

Sisodia, S.S., E.H. Koo, K. Beyreuther et al. 1990. Evidence that beta-amyloid protein in Alzheimer's disease is not derived by normal processing. Science 248:492-495.

Steiner, B., E.-M. Mandelkow, J. Biemat et al. 1990. Phosphorylation of microtubule-associated protein tau: Identification of the site for Ca sub 2+ -calmodulin dependent kinase and relationship with tau phosphorylation in Alzheimer tangles. EMBO J. 9:3539-3544.

Strittmatter, W.J., A.M. Saunders, D. Schmechel et al. 1993. Apolipoprotein E: High-avidity binding to beta-amyloid and increased frequency of type 4 allele in late-onset familial Alzheimer disease. Proc Natl. Acad. Sci. USA 90:1977-1981

Strittmatter, W.J., K.H. Weisgraber, M. Goedert et al. 1994. Hypothesis: Microtubule instability and paired helical filament formation in the Alzheimer disease brain are related to apolipoprotein E genotype. Exp. Neurol. 125:163-171.

Strittmatter, W.J., K.H. Weisgraber, D.Y. Huang et al. 1993. Binding of human apolipoprotein E to synthetic amyloid P peptide: Isoform-specific effects and implications for late-onset Alzheimer disease. Proc. Natl. Acad. Sci. USA 90:8093-8102.

Suzuki, N., T.T. Cheung, X.-D. Cai et al. 1994. An increased percentage of long amyloid beta protein secreted by familial amyloid beta protein precursor (betaAPP sub 717 ) mutants. Science 264:1336-1340.

Talbot, C., C. Lendon, N. Craddock et al. 1994. Protection against Alzheimer's disease with apoE epsilon2. Lancet 343:1432-1433.

Travis, J. 1993. New piece in Alzheimer's puzzle. Science 261:828-829.

Tsai, M.S., E.G. Tangalos, R.C. Peterson et al. 1994. Apolipoprotein E: Risk factor for Alzheimer's disease. Am. J. Hum. Genet. 54:643-649.

Utermann, G., M. Hees, and A. Steinmetz. 1977. Polymorphism of apolipoprotein E and occurrence of dysbetalipoproteinemia in man. Nature 269:604-607.

Utermann, G., A. Steinmetz, and W. Weber. 1992. Genetic control of human apolipoprotein E polymorphism: Comparison of one-and two-dimensional techniques of isoprotein analysis. Hum. Genet. 60:344-351.

Van Duijn, C.M., P. DeKnijff, M. Cruts et al. 1994. Apolipoprotein E4 allele in a population-based study of early-onset Alzheimer's disease. Natur. Genet. 7:74-78.

Weisgraber, K.H. 1986. The role of apoE in cholesterol metabolism. In Biochemistry and Biology of Plasma Lipoproteins, A.M. Scann and A.A. Spector, eds. New York: Marcel Dekker, 301-330.

Whitson, J.S., M.P. Mims, W.J. Strittmatter et al. 1994. Attenuation of the neurotoxic effect of Abeta amyloid peptide by apolipoprotein E. Biochem. Biophys. Res. Commun. 199:163-170.

Wisniewski, T., and B. Frangione. 1992. Apolipoprotein E: A pathological chaperone protein in patients with cerebral and systemic amyloid. Neurosci. Lett. 135:235-238.

Wisniewski, T., A. Golabek, E. Matsubara et al. 1993. Apolipoprotein: Binding to soluble Alzheimer's beta-amyloid. Biochem. Biophys. Res. Commun. 192:258-365.

Yu, C.-E., H. Payami, J.M. Olson et al. 1994. The apolipoprotein E/CI/CII gene cluster and late-onset Alzheimer disease. Am. J. Hum. Genet. 54:631-642.

Zubenko, G.S., S. Stiffler, S. Stabler et al. 1994. Association of the apolipoprotein E epsilon4 allele with clinical subtypes of autopsy-confirmed Alzheimer's disease. Am. J. Med. Genet. 54:199-205.

M. ILYAS KAMBOH(1)

1 Department of Human Genetics, Graduate School of Public Health, University of Pittsburgh, PA 15621.

Acknowledgments Support for this study was provided by the National Institute on Aging through grant AG07562, the National Heart, Lung, and Blood Institute through grant HL49074, and by a grant from the National Dairy Promotion and Research Board administered in cooperation with the National Dairy Council. I wish to thank Robert E. Ferrell and Steven T. DeKosky for reading the manuscript and making helpful comments and Kimberley Smithwick for clerical assistance.

Received 16 August 1994.

Copyright Wayne State University Press Apr 1995
Provided by ProQuest Information and Learning Company. All rights Reserved