Twin sisters, monozygotic with the fragile X mutation, but with a different phenotype.
The absence of the fragile X mental retardation protein (FMRP) results in fragile X syndrome. All males with a full mutation in the FMR1 gene and an inactive FMR1 gene are mentally retarded while 60% of the females with a full mutation are affected. Here we describe monozygotic twin sisters who both have a full mutation in their FMR1 gene, one of whom is normal while the other is affected. Using molecular and protein studies it was shown that owing to preferential X inactivation in the affected female a minority of the cells expressed the normal FMR1 gene, while in her sister most cells expressed the normal FMR1 gene. This shows that X inactivation took place in the female twins after separation of the embryos and that for a normal phenotype FMR1 expression is necessary in the majority of cells.
CBG Department of Clinical Genetics, Erasmus University, Rotterdam, The Netherlands.
R
R
Willemsen
R
R
Olmer
Y
Y
De Diego Otero
B A
BA
Oostra
D005260:Female; D051860:Fragile X Messenger Ribonucleoprotein 1; D005600:Fragile X Syndrome / Q000235:genetics*; D006801:Humans; D008607:Intellectual Disability / Q000235:genetics; D009419:Nerve Tissue Proteins / Q000235:genetics*; D010641:Phenotype; D016601:RNA-Binding Proteins; D014430:Twins, Monozygotic
D051860:Fragile X Messenger Ribonucleoprotein 1; D009419:Nerve Tissue Proteins; D016601:RNA-Binding Proteins; C497683:FMR1 protein, human