Fragile X prenatal analyses show full mutation females at high risk for mosaic Turner syndrome: fragile X leads to chromosome loss.
The fragile X mutation is an expansion of a CGG triplet repeat in the 5' untranslated region of the FMR1 gene. Expansion to >200 repeats (the "full mutation") silences FMR1 transcription and leads to the fragile X mental retardation syndrome in males and in some females. It also affects the structure of the mitotic chromosome as evidenced by a folate sensitive fragile site. Isolated cases of 45,X/46,XX (mosaic Turner syndrome) in full mutation females have been reported but an increased prevalence was not apparent from these reports. PCR and Southern analysis of the CGG repeat in 423 prenatal female samples identified 106 full mutation cases. Surprisingly five of these had 45,X/4,6XX mosaicism while none of the other 317 female fetuses did. In two of the five cases >or=50% of the cells were reported to be 45,X and in the other three, <or=14%. We investigated the association of the full mutation and mosaic Turner syndrome using polymorphisms and single cell cloning to identify the X chromosome that was lost. Our analysis indicates that at least four of the five full mutation mosaicism cases were due to loss of the maternal, full mutation chromosome. These data indicate that 45,X/46,XX mosaicism is much more common than expected in fragile X full mutation females. Moreover, they suggest that the presence of the fragile X full mutation on a chromosome may predispose it to loss during mitosis, possibly due to the altered structure of the metaphase fragile X chromosome.
Dobkin
Carl
C
Department of Human Genetics, New York State Institute for Basic Research in Developmental Disabilities, Staten Island, New York 10314, USA. carl.dobkin@omr.state.ny.us
Radu
Gabriel
G
Ding
Xiao-Hua
XH
Brown
W Ted
WT
Nolin
Sarah L
SL
D002478:Cells, Cultured; D041321:Chromosomes, Human, X / Q000235:genetics*; D004252:DNA Mutational Analysis; D005260:Female; D051860:Fragile X Messenger Ribonucleoprotein 1 / Q000032:analysis / Q000235:genetics*; D005600:Fragile X Syndrome / Q000175:diagnosis / Q000235:genetics; D017353:Gene Deletion; D005787:Gene Frequency; D006060:Gonadal Dysgenesis, Mixed / Q000235:genetics; D006801:Humans; D008297:Male; D009030:Mosaicism; D009154:Mutation / Q000502:physiology; D011247:Pregnancy; D011296:Prenatal Diagnosis; D012307:Risk Factors; D012729:Sex Chromosome Aberrations; D014424:Turner Syndrome / Q000175:diagnosis* / Q000209:etiology / Q000235:genetics
D051860:Fragile X Messenger Ribonucleoprotein 1; C497683:FMR1 protein, human