Frequency of tri- or multiradial configurations in fragile X identification

Fragile X screening program in New York State

Most fragile X [fra(X)] males in New York State have not been identified. Hence, a large number of female relatives are unaware of their risks for having an affected child. A program was established in New York State in 1987 to screen for the fra(X) syndrome in mentally retarded males with living relatives. The goal of the program is to identify affected males and inform their families about the diagnosis. In this way relatives would be able to assess their risks for having a fra(X) male. In order to identify the males a screening form was developed to assess 10 features which included physical characteristics, behavior, and family history. Males who exhibited at least 5 of these manifestations were selected for cytogenetic analysis. Any male who had macroorchidism or a family history of mental retardation was also included. A total of 995 males have been screened of which 352 (35%) were selected for cytogenetic analyses. Seventeen (10.5%) of the 161 completed studies were positive for fra(X). A large number of possible female carriers were identified in the families of the propositi. This program identifies fra(X) males in a population of the mentally retarded for whom there had been no previous diagnosis. By using a two-step procedure, it is possible to screen a large population of the mentally retarded for fra(X) without testing each male cytogenetically.

Distribution of diploidy, polyploidy, and endoreduplication in fra(X) positive and negative lymphocytes, amniocytes, and chorionic villi

Expression of fragile X [fra(X)] (q27.3) and endoreduplicated metaphases have been reported in methotrexate-treated (MTX) fra(X) cultures (Kerem B, Biotein R, Schaap T [1988]: Chromosoma 97: 6-10). Further, new data (Kimchi-Sarfaty C, Goitein R, Kerem B, Werner M, Medan B, Schaap T [1991]: Am J Med Genet, this issue) indicate that MTX may specifically induce polyploidy and endoreduplication in cells with the fra(X) mutation. To confirm and extend these results, we have studied short-term lymphocyte cultures incubated in M199, a folate deficient system, and RPMI-1640 in the presence and absence of 5-fluorodeoxyuridine (FUdR) exposure during the last day of a 4 day culture. No endoreduplicated cells were seen under these conditions and there was no change in the level of polyploidy. We also studied the distribution of polyploid and endoreduplicated cells in amniotic fluid and chorionic villus sample cultures from one fra(X) positive and 4 at-risk specimens. No increase in the incidence of polyploidy or endoreduplication was observed in cultures exposed to MTX for both 24 and 48 hours from a fra(X) positive amniotic fluid case. Cytogenetic results were fra(X) negative for the remaining 4 cases tested. There was significant discordance between our findings and those expected based on MTX-induced increased frequencies of polyploidy and endoreduplication. Thus, our studies do not confirm the reported correlation between the presence of FRAXA and increased frequencies of polyploidy and endoreduplication in MTX-exposed amniocyte cultures and there was no evidence for increased levels of polyploidy and endoreduplication in short-term fra(X) lymphocyte cultures exposed to non-MTX fra(X) induction.

Frequency of tri- and multiradial configurations in fragile X chromosomes

The appearance of tri- or multiradial configurations in fragile chromosomes is affected by the number of cell cycles in the folate antagonist system. In this study the lymphocytes were incubated for 96 h in a medium 199 without calf serum, and tri- or multiradial configurations were observed in 6 of 12 cases of fra(X) chromosome. The frequency ranged from 0.6% to 7.4% of fra(X) chromosomes.

Linked polymorphic DNA markers in the prediction of X-linked muscular dystrophy

Ten polymorphic DNA markers, including gene specific markers of loci DXS164 and DXS206, were tested for allele frequencies, degree of heterozygosity and linkage in 34 Finnish families with X-linked muscular dystrophy. With the exception of the BamHI RFLP of DXS164 subclone pERT87-15, allele frequencies and the degree of heterozygosity failed to show any significant deviation from the data published elsewhere. We document a high degree of linkage disequilibrium between several RFLPs belonging to locus DXS164. Our linkage data include one recombination between DMD and DXS164 enabling a tentative location of the mutation site distal to DXS164. The maximum lod score for linkage between the disease locus and DX164 was 7.828 at a recombination fraction of 0.02. According to our data DXS28 and DXS43 may be located further away from the disease locus than previously thought. We use only gene specific markers for genetic counselling. Excluding deletions, 97.1% of women were heterozygous for at least one such marker. A diagnostic procedure in which useful information can be obtained in over 90% of all diagnostic situations, using only four filters, is proposed.