The Principles of Clinical Cytogenetics - Extra Materials - Springer

500 Dana Crawford and Patricia Howard-Peebles
origin of replication associated with the location of the CGG repeat (46). No trans-acting factor has
been identified; however, several have been suggested, most of which involve proteins from the
DNA replication and repair systems (45). A “familial” factor has been proposed from the observation
that the size of the repeat expansion is more similar among siblings from the same family as compared
with siblings across families (36).
The Fragile X Gene and Its Product—FMR1 and FMRP
The fragile X syndrome mental retardation gene-1, or FMR1, was identified through positional
cloning (26–28). FMR1 encompasses 38 kb of Xq27.3 and consists of 17 exons (47). The polymorphic
CGG repeat exists in the 5' untranslated region (UTR) of FMR1. Among the general population,
the CGG repeat ranges from 6 to 55 repeats and usually does not change in size when passed from
parent to offspring (33). The most common form of the repeat size found in human populations
studied is 28–30 CGG repeats (48–51). Although the CGG repeat has no known function, it is found
in all species of mammals investigated (52,53). However, the repeat is found as a CCT in chickens
(54) and is not found in invertebrates such as Drosophila melanogaster (55).
The common CGG repeat sizes have not proven to be associated with a disease phenotype; however,
the consequence of an expanded CGG repeat (>230 repeats) in FMR1 is the fragile X syndrome.
The hyperexpanded CGG repeat signals the hypermethlyation (26,56) and deacetylation (57) of the
FMR1 promoter, the CGG repeat, and a nearby CpG island, which transcriptionally silences the gene
(58,59). Recent in vitro experiments demonstrated that it is methylation and chromatic modification
triggered by the expansion that are responsible for the transcriptional silencing of FMR1, rather than
the CGG repeat expansion itself (60,61).
Because the fragile X syndrome is essentially caused by the loss of the FMR1 gene product, there
is much interest in gathering information on the normal expression patterns of the gene and its
product’s function for the development of interventions or therapies. The FMR1 transcript is approximately
4.4 kb in size and is alternatively spliced at the 3' end, giving rise to various isoforms (47,62).
Expression studies in human and mouse tissues demonstrated that FMR1 is widely expressed, with
the highest levels localized to the brain, testes, ovaries, esophageal epithelium, thymus, spleen, and
eye (63–65). High expression of FMR1 in regions of the brain such as the neurons of the hippocampus
and the granular layer of the cerebellum (66,67) is consistent with the mental retardation phenotype
typical of the fragile X syndrome (see the section: Clinical Aspects of Fragile X Syndrome).
A search for genes similar to FMR1 within the human genome found two identified autosomal
homologs: fragile-X-related (FXR) genes 1 and 2, located at 3q28 and 17p13.1, respectively (68,69).
Analysis of mouse and human genomic sequences demonstrates similarities in gene structure among
FMR1, FXR1, and FXR2, suggesting an ancestral gene is common to the three genes (70). The function
of FXR1 and FXR2 is presently unclear; neither gene has been shown to be associated with
human disease. Many investigators have postulated that, because of their similarity to FMR1, the
FXR genes are somewhat redundant in function. Although there are similarities, significant differences
have been noted (71). Furthermore, FXR1 and FXR2 are not overexpressed in cells from persons
with the fragile X syndrome, suggesting that neither gene product compensates for the loss of
the FMR1 gene product (72,73).
The full-length protein product of FMR1 is 69 kDa in size and is known as the fragile X mental
retardation protein, or FMRP (74). At the protein level, FMR1 is highly conserved across humans
(27), mice (62), Xenopus laevis (75), and chickens (54). Although not as highly conserved as among
vertebrates, a homolog for the FMR1 coding sequence has also been identified in Drosophila
melanogaster (76).
In the last 10 years, much has been accomplished in elucidating the function of FMRP and how its
absence leads to the development of the fragile X syndrome phenotype. Several properties of FMRP
were the first clues to its function. First, FMRP contains two ribonucleoprotein K homology domains