Cytological evidence of defective template in the fragile X chromosome

Fragile sites, chromosomal lesions, tandem repeats, and disease

Expanded tandem repeat DNAs are associated with various unusual chromosomal lesions, despiralizations, multi-branched inter-chromosomal associations, and fragile sites. Fragile sites cytogenetically manifest as localized gaps or discontinuities in chromosome structure and are an important genetic, biological, and health-related phenomena. Common fragile sites (∼230), present in most individuals, are induced by aphidicolin and can be associated with cancer; of the 27 molecularly-mapped common sites, none are associated with a particular DNA sequence motif. Rare fragile sites ( ≳ 40 known), ≤ 5% of the population (may be as few as a single individual), can be associated with neurodevelopmental disease. All 10 molecularly-mapped folate-sensitive fragile sites, the largest category of rare fragile sites, are caused by gene-specific CGG/CCG tandem repeat expansions that are aberrantly CpG methylated and include FRAXA, FRAXE, FRAXF, FRA2A, FRA7A, FRA10A, FRA11A, FRA11B, FRA12A, and FRA16A. The minisatellite-associated rare fragile sites, FRA10B, FRA16B, can be induced by AT-rich DNA-ligands or nucleotide analogs. Despiralized lesions and multi-branched inter-chromosomal associations at the heterochromatic satellite repeats of chromosomes 1, 9, 16 are inducible by de-methylating agents like 5-azadeoxycytidine and can spontaneously arise in patients with ICF syndrome (Immunodeficiency Centromeric instability and Facial anomalies) with mutations in genes regulating DNA methylation. ICF individuals have hypomethylated satellites I-III, alpha-satellites, and subtelomeric repeats. Ribosomal repeats and subtelomeric D4Z4 megasatellites/macrosatellites, are associated with chromosome location, fragility, and disease. Telomere repeats can also assume fragile sites. Dietary deficiencies of folate or vitamin B12, or drug insults are associated with megaloblastic and/or pernicious anemia, that display chromosomes with fragile sites. The recent discovery of many new tandem repeat expansion loci, with varied repeat motifs, where motif lengths can range from mono-nucleotides to megabase units, could be the molecular cause of new fragile sites, or other chromosomal lesions. This review focuses on repeat-associated fragility, covering their induction, cytogenetics, epigenetics, cell type specificity, genetic instability (repeat instability, micronuclei, deletions/rearrangements, and sister chromatid exchange), unusual heritability, disease association, and penetrance. Understanding tandem repeat-associated chromosomal fragile sites provides insight to chromosome structure, genome packaging, genetic instability, and disease.

Fra(X) prenatal diagnosis: are endoreduplicated and polyploid cells useful diagnostic criteria?

Cytogenetic and molecular protocols for prenatal ascertainment of the fragile X syndrome and the associated fragile site at Xq27.3 are relatively reliable. Any new diagnostic method which becomes available still elicits much interest. Kimchi-Sarfaty et al. [1991] reported an increase in frequency of endoreduplication and polyploidy in fra(X) lymphoblasts and amniocytes when cultured with methotrexate (MTX) or fluorodeoxyuridine. Recently we analyzed the endoreduplication/polyploidy system using amniotic fluid, chorionic villus, and fibroblasts from fra(X) positive abortus cell cultures and from control samples. We observed no increased expression of endoreduplicated or polyploid cells in fra(X) positive amniocytes after exposure to MTX. The data presented here clearly dispute the value of endoreduplication/polyploid scoring as a diagnostic aid in prenatal fra(X) analysis.

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.

Endoreduplication and polyploidy in fragile X cells induced by methotrexate and fluorodeoxyuridine: implications for diagnosis

Lymphoblastoid cell lines from fragile X patients and amniotic cells from fragile X embryos, when cultured with methotrexate (MTX) or fluorodeoxyuridine (FUdR), showed a significant increase in endoreduplication and polyploidy. This phenomenon was not observed in fragile X lymphocytes or in lymphoblastoid cell lines and amniotic cells of normal control individuals. The relationship between the inducible fragile site at Xq27.3 and the inducible endoreduplication is discussed. The induction of endoreduplication and polyploidy in fragile X lymphoblasts and amniocytes is evaluated as a possible diagnostic test.

The role of recombination in the evolvement of the fragile X mutation

The frequency of recombination in the regions adjacent to the fragile X locus was studied in two groups of carriers: daughters of transmitting males and transmitters of maternally inherited fragile X chromosomes. Approximately one-half of the offspring of the former and one quarter of the offspring of the latter are recombinant. Recombinants and parentals are equally distributed among affected and normal offspring in the two groups. These results indicate that crossing-over at or around the fragile X locus occurs in every meiosis in daughters of transmitting males, although the recombinant chromatids do not necessarily carry the fragile X mutation. Hence, crossing-over is unequivocally associated with, but is not the direct cause of, the transition from the primary genetic lesion to the final mutation.