Increase of FMRP expression, raised levels of FMR1 mRNA, and clonal selection in proliferating cells with unmethylated fragile X repeat expansions: aclue to the sex bias in the transmission of full mutations?
U. Salat et al., Increase of FMRP expression, raised levels of FMR1 mRNA, and clonal selection in proliferating cells with unmethylated fragile X repeat expansions: aclue to the sex bias in the transmission of full mutations?, J MED GENET, 37(11), 2000, pp. 842-850
Citations number
66
Categorie Soggetti
Research/Laboratory Medicine & Medical Tecnology","Molecular Biology & Genetics
Fragile X syndrome is a triplet repeat disorder caused by expansions of a C
GG repeat in the fragile X mental retardation gene (FMR1) to more than 220
triplets (full mutation) that usually coincide with hypermethylation and tr
anscriptional silencing. The disease phenotype results from deficiency or l
oss of FMR1 protein (FMRP) and occurs in both sexes. The underlying full mu
tations arise exclusively on transmission from a mother who carries a premu
tation allele (60-200 CGGs). While the absolute requirement of female trans
mission could result from different mechanisms, current evidence favours se
lection or contraction processes acting at gametogenesis of pre- and full m
utation males. To address these questions experimentally, we used a model s
ystem of cultured fibroblasts from a male who presented heterogeneous unmet
hylated expansions in the pre- and full mutation size range. On continual c
ell proliferation to 30 doublings we reexamined the behaviour of the expand
ed repeats on Southern blots and also determined the expression of the FMR1
gene by FMRP immunocytochemistry, western analysis, and RT-PCR. With incre
asing population doublings, expansion patterns changed and showed accumulat
ion of shorter alleles. The FMRP levels were below normal but increased con
tinuously while the cells that were immunoreactive for FMRP accumulated. Th
e level of FMR1 mRNA was raised with even higher levels of mRNA measured at
higher passages. Current results support the theory of a selection advanta
ge of FMRP positive over FMRP deficient cells. During extensive proliferati
on of spermatogonia in fragile X males, this selection mechanism would even
tually replace all full mutations by shorter alleles allowing more efficien
t FMRP translation. At the proliferation of oogonia of carrier females, the
same mechanism would, in theory, favour transmission of any expanded FMR1
allele on inactive X chromosomes.