Paternal transmission of a FMR1 full mutation allele

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability (ID) and autism. In most of cases, the molecular basis of this syndrome is a CGG repeat expansion in the 5' untranslated region of the FMR1 gene. It is inherited as an X linked dominant trait, with a reduced penetrance (80% for males and 30% for females). Full mutation (FM) expansion from premutated alleles (PM) is only acquired via maternal meiosis, while paternal transmission always remains in the PM range. We present a 16-year-old girl with a mild fragile X syndrome phenotype. FMR1 gene study showed that the patient inherited a mosaic premutation-full mutation with an unmethylated uninterrupted allele (175, >200 CGG) from her father. The father showed an 88 CGG uninterrupted unmethylated allele in blood and sperm cells. To our knowledge, this is the first case of a FMR1 mosaic premutation-full mutation allele inherited from a PM father. In our opinion, the most likely explanation could be a postzygotic somatic expansion. We can conclude that in rare cases of a child with a full mutation whose mother does not carry a premutation, the possibility of paternal transmission should be considered.