DNA-based prenatal carrier detection for group A xeroderma pigmentosum in a chorionic villus sample

Prenatal diagnosis of xeroderma pigmentosum and trichothiodystrophy in 76 pregnancies at risk

OBJECTIVE

Evaluation of results in a consecutive series of 76 prenatal diagnoses for xeroderma pigmentosum (XP) and trichothiodystrophy (TTD) made since 1977.

METHODS

UV-induced DNA repair synthesis was assessed by the autoradiographic measurement of the incorporation of (3)H-thymidine.

RESULTS

XP was diagnosed in 19 of the 76 investigated pregnancies at risk; cultured chorionic villus (CV) cells were used in 33 pregnancies with ten affected fetuses and cultured amniocytes in 43 pregnancies with nine affected fetuses. In four cases, CVS results were corroborated by subsequent investigation of amniocytes because maternal cell contamination in the CV cell culture was either present or could not be excluded. Uncertain results in two other cases with intermediate DNA repair capacity and severe maternal cell contamination required further investigation. Median time needed for cell culture and analysis was 25 days. To reduce intra-assay variations, a modification of the DNA repair synthesis assay has recently been developed. In this assay, patients and controls are investigated simultaneously in mixed cultures of cells labelled with polystyrene beads.

CONCLUSION

Reliable prenatal diagnosis for XP and TTD can be made by the demonstration of clearly reduced UV-induced DNA repair synthesis due to defective global genome nucleotide excision repair.

Dermatologic clues to inherited disease

Genetic diseases often have cutaneous manifestations, which can be the first or most prominent signs of the underlying problem. Recognition of these dermatologic clues allows prompt diagnosis and intervention. In this article, the authors have attempted to outline several important examples of genodermatoses.

The comet assay as a repair test for prenatal diagnosis of Xeroderma pigmentosum and trichothiodystrophy

Xeroderma pigmentosum (XP) and trichothiodystrophy (TTD) are autosomal recessive diseases associated with extreme cutaneous photosensitivity, a defect in nucleotide excision repair (NER), and genetic complexity. Severe prognosis and lack of treatment led families at risk to request genetic counseling. Unscheduled DNA synthesis (UDS) is the classic method for diagnosis and requires 4 to 5 wk before conclusion. The use of the alkaline comet assay (single cell gel electrophoresis assay) is proposed as a simple repair test for earlier prenatal diagnosis. Amniotic or chorionic villus cells in two pregnancies at risk for XP and one for TTD were examined in comparison with skin fibroblasts of family members or with repair-proficient or -deficient control cells. The comet assay and the UDS test were performed in parallel. In repair-proficient cells, DNA strand breaks due to the incision of UV-induced DNA damage result in increased migration of high molecular weight DNA in the comet assay. Fetal cells demonstrate repair capacity similar to that of fibroblasts. In incision repair-deficient XP and TTD cells, after post-UV incubation, migration does not occur and comet moments are reduced. Two fetuses belonging to two XP families responded normally and were diagnosed as unaffected. Fetal cells in a TTD family had reduced comet moments and a low UDS. This fetus was diagnosed and confirmed later as affected. Heterozygotes had normal responses with both assays. The comet assay offers discrimination similar to that of the UDS assay in identifying NER-deficient phenotypes. Practical advantages in view of prenatal diagnosis include the reduced number of cells required, a 24-h delay in obtaining results, and no need for radioactivity.