Fragile X affected males have an expansion of a CGG repeat and a hypermethylated CpG island 5' to the FMR-1 gene. Mosaic males with both a premutation and full mutation have been noted among the affected individuals. Such mosaic males are most easily identified by the presence of a methylated restriction fragment characteristic of the full mutation and an additional unmethylated fragment in the premutation range in Southern analyses with EcoR I and the methylation-sensitive enzyme Eag I and a probe such as StB12.3. We analyzed a group of affected fragile X males by Southern blotting and found 41% (61/148) to be mosaic. The 148 individuals were divided between 36 pairs of brothers and 76 unrelated males. Little difference in the number of mosaics was seen between the brothers and the unrelated males nor was the expected distribution of mosaicism in brother pairs different from observed. Thus, these data do not suggest a familial basis for mosaicism. Our observation that 41% affected fragile X males were mosaic is significantly higher than previous reports. The difference is likely due to technical modifications which permitted the identification of faint premutation bands in some patients. The high percentage of affected males with mosaicism seen here suggests that the occurrence of such individuals may be a much more frequent event than presently recognized.

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Miscarriage is the spontaneous loss of an embryo or fetus before the 20th week of pregnancy. Most miscarriages occur before the end of the first trimester (<13 weeks). Although many risk factors relate to this occurrence, genetic factors play the most important role. Chromosomal abnormalities, including both numerical and structural anomalies, underlie the majority of miscarriages. In this study, we employed a comprehensive approach using cytogenetic karyotyping, polymerase chain reaction (PCR)-based genotyping, and microarray-based comparative genomic hybridization (arrayCGH) in combination to analyze chromosomal profiles of 115 first-trimester miscarriages of Chinese women. Seventy cases (61%) were found to have chromosomal anomalies, of which 90% were numerical and 10% were structural. Cytogenetic karyotyping identified 78.6% (55/70), PCR assays 2.9% (2 triploids), and arrayCGH 18.6% (13/70) of the anomalies. In this study, a microdeletion of 108 kb and four microduplications sizing from 300 to 1460 kb were observed. An advantage of using this combination approach is that microsatellite genotyping and arrayCGH can be accomplished in spite of culture failure and maternal cell contamination. In addition, arrayCGH can detect submicroscopic chromosomal anomalies and gene dosage alterations.

The mutation responsible for fragile X syndrome and myotonic dystrophy involves the amplification of a simple trinucleotide repeat sequence, which increases in successive generations of affected pedigrees accounting for increasing penetrance of both disorders. This common molecular basis suggests that the two diseases may share other genetic features, but whereas myotonic dystrophy exhibits a significant founder chromosome effect, fragile X syndrome apparently has a very high mutation frequency. By haplotype analysis of microsatellite markers which flank the fragile X unstable element, we have uncovered evidence of founder chromosomes of the fragile X 'mutation'. Disorders caused by heritable unstable elements may therefore exhibit common genetic properties including anticipation and founder chromosomes.

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