FISH-detected delay in replication timing of mutated FMR1 alleles on both active and inactive X-chromosomes.
X-chromosome inactivation and the size of the CGG repeat number are assumed to play a role in the clinical, physical, and behavioral phenotype of female carriers of a mutated FMR1 allele. In view of the tight relationship between replication timing and the expression of a given DNA sequence, we have examined the replication timing of FMR1 alleles on active and inactive X-chromosomes in cell samples (lymphocytes or amniocytes) of 25 females: 17 heterozygous for a mutated FMR1 allele with a trinucleotide repeat number varying from 58 to a few hundred, and eight homozygous for a wild-type allele. We have applied two-color fluorescence in situ hybridization (FISH) with FMR1 and X-chromosome alpha-satellite probes to interphase cells of the various genotypes: the alpha-satellite probe was used to distinguish between early replicating (active) and late replicating (inactive) X-chromosomes, and the FMR1 probe revealed the replication pattern of this locus. All samples, except one with a large trinucleotide expansion, showed an early replicating FMR1 allele on the active X-chromosome and a late replicating allele on the inactive X-chromosome. In samples of mutation carriers, both the early and the late alleles showed delayed replication compared with normal alleles, regardless of repeat size. We conclude therefore that: (1) the FMR1 locus is subjected to X-inactivation; (2) mutated FMR1 alleles, regardless of repeat size, replicate later than wild-type alleles on both the active and inactive X-chromosomes; and (3) the delaying effect of the trinucleotide expansion, even with a low repeat size, is superimposed on the delay in replication associated with X-inactivation.
Yeshaya
J
J
Dept. of Human Gentics, Sackler School of Medicine, Tel-Aviv University, Israel.
Shalgi
R
R
Shohat
M
M
Avivi
L
L
D000293:Adolescent; D000328:Adult; D000483:Alleles; D000653:Amniotic Fluid / Q000166:cytology; D004261:DNA Replication; D004303:Dosage Compensation, Genetic; D005260:Female; D051860:Fragile X Messenger Ribonucleoprotein 1; D005820:Genetic Testing; D006579:Heterozygote; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D008214:Lymphocytes / Q000166:cytology; D008957:Models, Genetic; D009154:Mutation; D009419:Nerve Tissue Proteins / Q000235:genetics*; D016601:RNA-Binding Proteins; D014960:X Chromosome
D051860:Fragile X Messenger Ribonucleoprotein 1; D009419:Nerve Tissue Proteins; D016601:RNA-Binding Proteins; C497683:FMR1 protein, human