Chromosome fragility and the abnormal replication of the FMR1 locus in fragile X syndrome

Abstract

Fragile X Syndrome (FXS) is a learning disability seen in individuals who have >200 CGG•CCG repeats in the 5' untranslated region of the X-linked FMR1 gene. Such alleles are associated with a fragile site, FRAXA, a gap or constriction in the chromosome that is coincident with the repeat and is induced by folate stress or thymidylate synthase inhibitors like fluorodeoxyuridine (FdU). The molecular basis of the chromosome fragility is unknown. Previous work has suggested that the stable intrastrand structures formed by the repeat may be responsible, perhaps via their ability to block DNA synthesis. We have examined the replication dynamics of normal and FXS cells with and without FdU. We show here that an intrinsic problem with DNA replication exists in the FMR1 gene of individuals with FXS even in the absence of FdU. Our data suggest a model for chromosome fragility in FXS in which the repeat impairs replication from an origin of replication (ORI) immediately adjacent to the repeat. The fact that the replication problem occurs even in the absence of FdU suggests that this phenomenon may have in vivo consequences, including perhaps accounting for the loss of the X chromosome containing the fragile site that causes Turner syndrome (45, X0) in female carriers of such alleles. Our data on FRAXA may also be germane for the other FdU-inducible fragile sites in humans, that we show here share many common features with FRAXA.

Somatic intronic microsatellite loci differentiate glioblastoma from lower-grade gliomas

“…Since some microsatellites are associated with DNA ‘fragile sites’, locations within chromatin susceptible to constrictions or break-points that are linked to cancers and neuro-developmental disorders, we analyzed our MST loci to determine which are located in these regions, as a possible mechanism for tumor potentiation [18, 19]. BRWD2 , found in our GBM signature, is located at a break-point on chromosome 10 and allelic deletions within 10p, 10q 25-26, and 19q 13.3-13.4 are the most common alterations in glial tumors[20, 21].…”

Repeat instability during DNA repair: Insights from model systems

“…Deletions and translocations at the site of CGG expansions have been documented, which provides indirect evidence that a double-strand break (DSB) occurred on the chromosome. Expanded repeats at the FMR1 gene are associated with very late replication and problems with replication initiation and/or elongation (Gerhardt et al ., 2014a; Hansen et al ., 1993, 1997; Subramanian et al ., 1996; Yudkin et al ., 2014). …”

A Female Patient with FMR1 Premutation and Mosaic X Chromosome Aneuploidy and Two Sons with Intellectual Disability

“…Cells were cultivated in RPMI 1640 GlutaMAX medium (Gibco) with 15% fetal bovine serum (Gibco) and antibiotics. Metaphases were prepared in Carnoy's fixative as previously described [Yudkin et al, 2014]. G-banding was performed according to the standard method.…”

“…GPR50 is a more telomeric sequence than FMR1 and helps to analyze the results because the fragile site FRAXA is located close to the telomere and difficult to detect. FISH with 2 probes was performed with suppression of repeated DNA by human C o t20 DNA as previously described [Yudkin et al, 2014].…”