Evidence for chromosome fragility at the frataxin locus in Friedreich ataxia

Abstract

Friedreich ataxia (FRDA) is a member of the Repeat Expansion Diseases, a group of genetic conditions resulting from an increase/expansion in the size of a specific tandem array. FRDA results from expansion of a GAA/TTC-tract in the first intron of the frataxin gene (FXN). The disease-associated tandem repeats all form secondary structures that are thought to contribute to the propensity of the repeat to expand. The subset of these diseases that result from a CGG/CCG-repeat expansion, such as Fragile X syndrome, also express a folate-sensitive fragile site coincident with the repeat on the affected chromosome. This chromosome fragility involves the generation of chromosome/chromatid gaps or breaks, or the high frequency loss of one or both copies of the affected gene when cells are grown underfolate stress or as we showed previously, in the presence of an inhibitor of the ATM checkpoint kinase. Whether Repeat Expansion Disease loci containing different repeats form similar fragile sites was not known. We show here that the region of chromosome 9 that contains the FXN locus is intrinsically prone to breakage in vivo even in control cells. However, like FXS alleles, FRDA alleles show significantly elevated levels of chromosome abnormalities in the presence of an ATM inhibitor, consistent with the formation of a fragile site.

The role of fork stalling and DNA structures in causing chromosome fragility

“…Expanded GAA repeats cause replication fork stalling in S. cerevisiae in a length‐dependent manner when present in >20‐40 copies on the lagging strand . GAA repeats are fragile sites on yeast chromosomes and in human cells . Further, GAA and GAAA predicted triplex‐forming repeats are enriched near translocation and deletion breakpoints in cancer genomes .…”

“…92 GAA repeats are fragile sites on yeast chromosomes 42 and in human cells. 93 Further, GAA and GAAA predicted triplex-forming repeats are enriched near translocation and deletion breakpoints in cancer genomes. 45 A naturally occurring H-DNA forming sequence in the c-myc promoter maps to the Bcl-2 major breakpoint region and implicates secondary structures in common c-myc translocations that occur in lymphomas.…”

On the wrong DNA track: Molecular mechanisms of repeat-mediated genome instability

“…A less known fact is that many other repeats are also fragile. In model experimental systems GAA, CAG, and ATTCT repeats exhibit length-dependent fragility (263)(264)(265)(266)(267)(268), whereas triplex-forming DNA sequences promote DSB formation in human cells (175). Additionally, breakpoints of genomic rearrangements in cancer are enriched for structure-forming tandem DNA repeats, such as (AT) n , (GAA) n , and (GAAA) n (269).…”

Close encounters: Moving along bumps, breaks, and bubbles on expanded trinucleotide tracts

“…MSH2-MSH3 plays roles as a co-factor in BER [94,98,99], transcription-coupled repair (TCR) [136–138] and double strand break repair (DSBR) [137,139,140]. Each of these pathways has been implicated in CAG instability in mice [58,77–79, 81–83, 94], and classically operates with distinct Family A, B, X or Y polymerases, depending on the pathway (Table 2). Does MSH2-MSH3 stimulate them all?…”

“…However, in one cell line isolated from a DM1 transgenic mouse, the rate of CTG tracts is unrelated to the rate of replication [76]. On long CGG tracts, replicative polymerases stall [77], and frequently break in cells [78,79]. As modeled in yeast and in mammalian cells, expansions during proliferation arise from rescue of stalled forks by recombination [80], break-induced replication (BIR) [81], and strand switching [82,83].…”