Fragile sites are heritable specific chromosome loci that exhibit an increased frequency of gaps, poor staining, constrictions or breaks when chromosomes are exposed to partial DNA replication inhibition. They constitute areas of
chromatin that fail to compact during mitosis. They are classified as rare or common depending on their frequency within the population and are further subdivided on the basis of their specific induction chemistry into different groups differentiated as
folate sensitive or non-
folate sensitive rare fragile sites, and as
aphidicolin,
bromodeoxyuridine (
BrdU) or
5-azacytidine inducible common fragile sites. Most of the known inducers of fragility share in common their potentiality to inhibit the elongation of DNA replication, particularly at fragile site loci. Seven
folate sensitive (FRA10A, FRA11B, FRA12A, FRA16A, FRAXA, FRAXE and FRAXF) and two non-
folate sensitive (FRA10B and FRA16B) fragile sites have been molecularly characterized. All have been found to represent expanded DNA repeat sequences resulting from a dynamic mutation involving the normally occurring polymorphic CCG/CGG trinucleotide repeats at the
folate sensitive and AT-rich minisatellite repeats at the non-
folate sensitive fragile sites. These expanded repeats were demonstrated, first, to have the potential, under certain conditions, to form stable secondary non-
B DNA structures (intra-strand hairpins, slipped strand
DNA or tetrahelical structures) and to present highly flexible repeat sequences, both conditions which are expected to affect the replication dynamics, and second, to decrease the efficiency of
nucleosome assembly, resulting in decondensation defects seen as fragile sites. Thirteen
aphidicolin inducible common fragile sites (FRA2G, FRA3B, FRA4F, FRA6E, FRA6F, FRA7E, FRA7G, FRA7H, FRA7I, FRA8C, FRA9E, FRA16D and FRAXB) have been characterized at a molecular level and found to represent relatively AT-rich
DNA areas, but without any expanded repeat motifs. Analysis of structural characteristics of the
DNA at some of these sites (FRA2G, FRA3B, FRA6F, FRA7E, FRA7G, FRA7H, FRA7I, FRA16D and FRAXB) showed that they contained more areas of high
DNA torsional flexibility with more highly AT-dinucleotide-rich islands than neighbouring non-fragile regions. These islands were shown to have the potential to form secondary non-
B DNA structures and to interfere with higher-order
chromatin folding. Therefore, a common fragility mechanism, characterized by high flexibility and the potential to form secondary structures and interfere with
nucleosome assembly, is shared by all the cloned classes of fragile sites. From the clinical point of view, the
folate sensitive rare fragile site FRAXA is the most important fragile site as it is associated with the
fragile X syndrome, the most common form of familial
mental retardation, affecting about 1/4000 males and 1/6000 females.
Mental retardation in this syndrome is considered as resulting from the abolition of the FMR1 gene expression due to hypermethylation of the gene CpG islands adjacent to the expanded methylated trinucleotide repeat. FRAXE is associated with X-linked non-specific
mental retardation, and FRA11B with
Jacobsen syndrome. There is also some evidence that fragile sites, especially common fragile sites, are consistently involved in the in vivo chromosomal rearrangements related to
cancer, whereas the possible implication of common fragile sites in neuropsychiatric and developmental disorders is still poorly documented.