Trinucleotide repeats at the FRAXF locus: frequency and distribution in the general population

Fragility Extraordinaire: Unsolved Mysteries of Chromosome Fragile Sites

Chromosome fragile sites are a fascinating cytogenetic phenomenon now widely implicated in a slew of human diseases ranging from neurological disorders to cancer. Yet, the paths leading to these revelations were far from direct, and the number of fragile sites that have been molecularly cloned with known disease-associated genes remains modest. Moreover, as more fragile sites were being discovered, research interests in some of the earliest discovered fragile sites ebbed away, leaving a number of unsolved mysteries in chromosome biology. In this review we attempt to recount some of the early discoveries of fragile sites and highlight those phenomena that have eluded intense scrutiny but remain extremely relevant in our understanding of the mechanisms of chromosome fragility. We then survey the literature for disease association for a comprehensive list of fragile sites. We also review recent studies addressing the underlying cause of chromosome fragility while highlighting some ongoing debates. We report an observed enrichment for R-loop forming sequences in fragile site-associated genes than genomic average. Finally, we will leave the reader with some lingering questions to provoke discussion and inspire further scientific inquiries.

A novel gene, FAM11A, associated with the FRAXF CpG island is transcriptionally silent in FRAXF full mutation

The cytogenetic expression of the FRAXF fragile site is due to an expanded, hypermethylated and unstable CGG repeat in Xq28. Normal individuals have 6-38 triplet repeats while individuals expressing the fragile site have expansions of greater than 300 triplets. Through analysis of the region adjacent to the fragile site, we have identified a approximately 2.6 kb cDNA originating from the FRAXF fragile site associated CpG island, and containing the unstable FRAXF CGG repeat in its 5' UTR region. This gene, FAM11A, comprises at least seven exons, shows alternative splicing, and extends over 35 kb of genomic DNA distal to the FRAXF fragile site. Analysis of the FAM11A cDNA sequence has identified a 1050 bp open reading frame encoding a 350 amino acid protein. We have also identified FAM11B a highly conserved (88% at the protein level) transcribed chromosome 2 retropseudogene. We show that the novel FRAXF fragile site associated gene FAM11A is transcriptionally silenced in a normal individual with a cytogenetically and molecularly detectable FRAXF CGG full mutation (fragile site). Finally, we were able to reactivate FAM11A transcription by treatment of a FRAXF lymphoblastoid cell line with the demethylating agent 5-azadeoxycytidine, thus demonstrating the critical role of FRAXF methylation in FAM11A silencing.