Linkage analysis in the fragile X syndrome using multiple distal Xq polymorphic DNA markers

Tightly linked polymorphic markers for fragile X syndrome and prenatal cytogenetic diagnostic experience

Linkage analysis using the polymorphic loci DXS369, DXS296, DXS297 and DXS306 was carried out on a cohort of 17 families segregating for fragile X syndrome. The observed recombination fractions at: DXS369 (Zmax = 3.02; theta = 0.06), DXS297 (Zmax = 2.92; theta = 0.0), DXS296 (Zmax = 3.82; theta = 0.0), DXA306 (Zmax = 4.55; theta = 0.05) confirm that these loci are tightly linked to FRAXA. Our experience in the cytogenetic analysis of 58 at risk pregnancies by chorionic villus or fetal blood sample examination documents a false negative rate in obligate carrier male pregnancies for CVS of 11% and for FBS of 3%.

Prenatal diagnosis and carrier detection in fragile X

Prenatal diagnosis was performed in 81 cases at risk for the fragile X syndrome. There were 12 fra(X)-positive cases, two of which showed low expression in cultured amniotic fluid cells. FUdR and high thymidine were used for induction of fra(X) (q27.3) expression in all cases. In 21 cases linkage studies were performed, 7 with probes for the loci DXS52, DXS98 and DXS105, 13 with probes for DXS369 and DXS296, DXS304 or DXS374 and one with the probe Do33 for DXS465. In 11 of these cases linkage analysis gave risk figures higher than 95% or lower than 5%, all in concordance with the cytogenetic findings. Discordance was found in three cases studied earlier, the two cases with low expression mentioned above and one cytogenetically normal case, which were now restudied with the new probes. RFLP-studies and linkage analysis was also performed for 24 cytogenetically fra(X)-negative females having a 50%, 25% or 12.5% risk of being carriers according to pedigree data. In 15 cases the risk dropped to 1% or less. Six of these women were pregnant and had asked for prenatal diagnosis but after genetic counseling prenatal diagnosis was avoided.