Loss of mutation at the FMR1 locus through multiple exchanges between maternal X chromosomes.
The mutation observed in the fragile X syndrome, an X-linked inherited disorder causing mental retardation, is almost exclusively an expanded CGG repeat in the first exon of the FMR1 gene. Here we describe a daughter of a female carrier, who inherited the fragile X premutation chromosome based on haplotype analysis using flanking markers. However, the CGG repeat sequence and the intragenic polymorphic marker FMRb showed the normal maternal alleles, while two other intragenic markers, FMRa and FRAXAC2 and other, more distant markers, showed the risk haplotype. Since FMRa and FRAXAC2 are located in between the markers CGG and FMRb, this results in patches of normal and fragile X sequences in the FMR1 gene of the daughter. This observation is very likely due to gene conversion. As this daughter received a normal CGG repeat region, we expect that her risk to have affected offspring is the same as the population risk. The observed phenomenon would therefore represent a back mutation at the FMR1 locus.
Department of Clinical Genetics, University Hospital, Rotterdam, The Netherlands.
A M
AM
van den Ouweland
W H
WH
Deelen
C B
CB
Kunst
M L
ML
Uzielli
D L
DL
Nelson
S T
ST
Warren
B A
BA
Oostra
D J
DJ
Halley
D001483:Base Sequence; D017931:DNA Primers; D004276:DNA, Satellite / Q000097:blood / Q000235:genetics; D005091:Exons; D005260:Female; D051860:Fragile X Messenger Ribonucleoprotein 1; D005600:Fragile X Syndrome / Q000235:genetics*; D006580:Genetic Carrier Screening; D005819:Genetic Markers; D006801:Humans; D007962:Leukocytes; D008297:Male; D008969:Molecular Sequence Data; D009154:Mutation; D009419:Nerve Tissue Proteins / Q000235:genetics*; D010375:Pedigree; D016133:Polymerase Chain Reaction; D016601:RNA-Binding Proteins; D012016:Reference Values; D012091:Repetitive Sequences, Nucleic Acid; D014960:X Chromosome
D017931:DNA Primers; D004276:DNA, Satellite; D051860:Fragile X Messenger Ribonucleoprotein 1; D005819:Genetic Markers; D009419:Nerve Tissue Proteins; D016601:RNA-Binding Proteins; C497683:FMR1 protein, human