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IVS1-2A^T, IVS4-1G ^C.

*Novel mutations are underlined.

*Novel mutations are underlined.

3.4. Prenatal Diagnosis of Canavan disease

Aspartoacylase activity is low to undetectable in chorionic villus and amniocytes. Therefore, a prenatal diagnosis of Caravan disease cannot be made reliably by enzyme activity. As an alternative, the concentration of NAA in amniotic fluid can be determined using a stable-isotype dilution technique (Kelley, 1993). This method was used to detect Canavan disease in a pregnancy originally thought to be at low risk.

The mother was one-quarter Jewish and a carrier for the E285A mutation. Her husband, who was non-Jewish and of Turkish, Persian and Indian ancestry, tested negative for the three common Canavan disease mutations. Nevertheless, the couple elected to have the amniotic fluid analyzed at 17 weeks of pregnancy. The amniotic fluid NAA was reported as elevated and was again elevated when repeated at 21 weeks of gestation (Dr. Richard Kelley, Kennedy-Krieger Institute, Baltimore, Maryland). The fetus was aborted. The father's genomic DNA was sequenced and a mutation at IVS4+1G^T (Rady et al., 2002) was found. Both this mutation and the mother's E285A mutation were confirmed in the fetal tissue.

Mutation analysis of fetal cells is the "gold standard" for prenatal diagnosis of Canavan disease but may not always be possible. In another family with an affected child, we were able to positively identify a fetus as unaffected using polymorphisms within the ASPA gene. The mother carried the A305E mutation but it was not possible to identify the father's mutated allele since neither the father nor the affected child produced mRNA from this allele. The 693 C^T polymorphism could not be used because the father appeared to be homozygous whereas the affected sibling was heterozygous for this polymorphism. Sequencing of the father's genomic DNA revealed that he also carried a new variation at IVS2-284A^T on one of his ASPA alleles. His wife also carried this variation on one of her alleles and the affected sibling was homozygous for the new polymorphism. However, the fetus was heterozygous for the novel variation. With cDNA, the affected child had a one allele pattern, whereas the fetal cDNA contained two alleles confirming the diagnosis of an unaffected fetus.

Consequently, when one mutation remains unidentified in either parent, the search for polymorphisms may be a useful alternative to determine the genotype of an at risk fetus.

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