Retinoblastoma

For the first time, preimplantation genetic diagnosis for retinoblastoma has ensured the birth of a healthy infant who does not carry the genetic germline for the devastating disease, reported Dr. Kangpu Xu and his colleagues at Cornell University, New York.

"Preimplantation genetic diagnosis for retinoblastoma offers families the opportunity to potentially avoid the loss of life, eyes, and vision in subsequent generations to this genetic cancer syndrome," the investigators said. "These considerations, as well as the avoidance of a decision to terminate an ongoing pregnancy, are seen as the benefits of this technology over other prenatal (postimplantation) diagnostic options" (Am. J. Ophthalmol, 137[1]: 18-23, 2004).

The singleton pregnancy resulted from an in vitro fertilization (IVF) cycle that yielded 12 embryos: 7 embryos were found to be unaffected and 5 were implanted.

The 33-year-old father is affected with retinoblastoma. Individuals with the disease have a 50% chance of passing the germline retinoblastoma gene (RB1) mutation to their offspring. The couple already has one daughter affected with the cancer. Although her retinoblastomas were successfully treated in infancy, at age 2 years she was diagnosed with a pineal tumor, a common occurrence since these children face a sharp increase in the risk of developing multiple cancers.

About 1% of patients with bilateral retinoblastoma develop a second primary neoplasm each year; at least 50% of these patients die from their tumors, they said.

The couple underwent two IVF cycles. In the first cycle, 12 oocytes were fertilized and underwent a single-cell genetic analysis by nested polymerase chain reaction. Four embryos were identified as unaffected, and three were transferred. A clinical pregnancy did not occur.

In the second cycle, 20 oocytes were retrieved, and 12 were fertilized and analyzed. Seven embryos were identified as unaffected, and five were implanted.

A live birth resulted from the singleton pregnancy. A peripheral blood sample from the female infant indicated the absence of the paternal RB1 mutation. Ophthalmic examination at birth and through age 6 months revealed normal fundi.

Preimplantation genetic diagnosis has been in use for more than a decade and can currently identify more than 50 genetic disorders, including sickle cell anemia, Huntington disease, and cystic fibrosis. In the case of RB1, however, the technique is not infallible. It may fail to identify the mutation in a gene that contains allele drop out--a phenomenon in which one of the two alleles may not amplify during PCR, or in which the gene's amplification efficiency is much lower than that of the other allele.

Allele drop out is believed to occur in 5%-15% of the tests, Dr. Xu said in an interview. In half those cases, the drop out will be of the normal allele, indicating an affected embryo. However, when the mutated allele drops out, a false-negative result will occur.

"The rate of misdiagnosis is less than 5%, partly because allele drop out occurs more frequently in 'poor' embryos, which will not be transferred or will not implant in the uterus," Dr. Xu told this newspaper. "To reduce the rate of allele drop out or misdiagnosis, a closely linked DNA marker should be used." In that case, the rate of misdiagnosis can be reduced to less than 1%.

Dr. William Mieler, professor of ophthalmology at Baylor College of Medicine, Houston, said in an interview that "if the test is positive you can stop the process without ever having to make a decision about abortion."

BY MICHELE G. SULLIVAN

Mid-Atlantic Bureau

COPYRIGHT 2004 International Medical News Group
COPYRIGHT 2004 Gale Group