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DiGeorge syndrome

22q11.2 deletion syndrome (also called DiGeorge syndrome and velocardiofacial syndrome) is a disorder caused by the deletion of a small piece of chromosome 22. The deletion occurs near the middle of the chromosome at a location designated q11.2. more...

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The features of this syndrome vary widely, even among members of the same family, and affect many parts of the body. Characteristic signs and symptoms include heart defects that are often present from birth, an opening in the roof of the mouth (a cleft palate or other defect in the palate), learning disabilities, recurrent infections caused by problems with the immune system, and mild differences in facial features. Affected individuals may also have kidney abnormalities, low levels of calcium in the blood (which can result in seizures), significant feeding difficulties, autoimmune disorders such as rheumatoid arthritis, and an increased risk of developing mental illnesses such as schizophrenia and bipolar disorder.

Because the signs and symptoms of 22q11.2 deletion syndrome are so varied, different groupings of features were once described as separate conditions. Doctors named these conditions DiGeorge syndrome, velocardiofacial syndrome (also called Shprintzen syndrome), and conotruncal anomaly face syndrome. In addition, some children with the 22q11.2 deletion were diagnosed with Opitz G/BBB syndrome and Cayler cardiofacial syndrome. Once the genetic basis for these disorders was identified, doctors determined that they were all part of a single syndrome with many possible signs and symptoms. To avoid confusion, this condition is usually called 22q11.2 deletion syndrome, a description based on its underlying genetic cause.

Symptoms

Individuals with a 22q11 deletion have a range of findings, including:

  • Congenital heart disease (74% of individuals), particularly conotruncal malformations (tetralogy of Fallot, interrupted aortic arch, ventricular septal defect, and truncus arteriosus)
  • palatal abnormalities (69%), particularly velopharyngeal incompetence (VPI), submucosal cleft palate, and cleft palate; characteristic facial features (present in the majority of Caucasian individuals)
  • learning difficulties (70-90%)
  • an immune deficiency regardless of their clinical presentation (77%)
  • hypocalcemia (50%)
  • significant feeding problems (30%)
  • renal anomalies (37%)
  • hearing loss (both conductive and sensorineural)
  • laryngotracheoesophageal anomalies
  • growth hormone deficiency
  • autoimmune disorders
  • seizures (without hypocalcemia)
  • skeletal abnormalities

Thymus, parathyroid glands and heart derive from the same primitive embryonic structure and that is why these three organs are dysfunctioned together in this disease. Affected patients (usually children) are prone to yeast infections.

Cause

The disease is related with genetic deletions (loss of a small part of the genetic material) found on the long arm of the 22nd chromosome. Some patients with similar clinical features may have deletions on the short arm of chromosome 10.

Read more at Wikipedia.org


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Two genes tied to common birth defect - genetic cause of DiGeorge's syndrome - Brief Article
From Science News, 3/10/01 by J. Netting

In 1963, physician Angelo M. DiGeorge discovered a set of birth defects that often affects the heart and is behind a host of other problems. These can include mild learning disabilities, severe retardation, and facial abnormalities such as down-slanting eyes, a bulbous nose, and cleft palate. Clinicians estimate that DiGeorge's syndrome affects 1 in 4,000 newborns and is the second most common cause of congenital heart defects.

Now, researchers report that defects in either of two specific genes may be behind the syndrome. In an article in the March 1 NATURE and two in the March NATURE GENETICS, three research teams working with mice describe disabling either of two genes, Tbx1 and Crkol, to reproduce many of the syndrome's effects.

Although researchers had pegged the syndrome's genetic basis to deletions in various sections of human chromosome 22 in 1978, the specific genes responsible have been elusive.

"It's an exciting business. There have been some 20 or more genes that have been [suspects], and none of them panned out," says DiGeorge at Temple University Children's Medical Center in Philadelphia.

"[Scientists] had basically hit a wall in traditional mapping," says Seigo Izumo, a cardiovascular researcher at Beth Israel Deaconess Medical Center in Boston. Instead of searching for individual genes in people, the authors of all three papers modified mice so the animals would have defective copies of either Tbx1 or Crkol.

In one of the NATURE GENETICS studies, Loydie A. Jerome and Virginia E. Papaioannou of Harvard Medical School found that embryonic mice without Tbx1 have malformations of the aortic arches, which become the major arteries leaving the heart.

In the report appearing in the March 1 NATURE, a group led by Elizabeth A. Lindsay at the Baylor College of Medicine in Houston found similar effects in their own version of mice without Tbx1.

Both groups found that Tbx1 must be disabled on both copies of the chromosomes that bear it to produce the heart defect. In people, however, it takes only one faulty chromosome to produce DiGeorge's, the researchers say.

The human version of Tbx1, specified as TBX1, has always been considered a good candidate for the main cause of the syndrome, says Marcia L. Budarf, leader of the lab at the University of Pennsylvania School of Medicine that first described the gene in humans. "It's clear that TBX1 is important to this syndrome, but it might not be the whole story in humans," she says. Budarf has studied over 100 DiGeorge's patients who don't have TBX1 deletions.

Akira Imamoto of the University of Chicago and his colleagues focused on Crkol--the mouse equivalent of the human gene CRKL, which is sometimes absent from patients with DiGeorge's syndrome. The embryos that were missing the gene had heart defects and facial abnormalities like those of patients with the syndrome, they report in NATURE GENETICS.

The effects of the syndrome and their severity vary even between people with identical deletions of chromosome sections. In many cases, the syndrome is so mild that it goes unrecognized. "We have adults who never knew they had it until they had an affected child," says Donna McDonald-McGinn a genetic counselor at the University of Pennsylvania Children's Hospital.

Finding the genes responsible could lead to genetic tests for the syndrome and help refine understanding of embryonic development, says Izumo.

COPYRIGHT 2001 Science Service, Inc.
COPYRIGHT 2001 Gale Group

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