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Transgenic fragile X knockout mice have been constructed to provide an animal model to study the physiologic function of the fragile X gene (FMR1) and to gain more insight into the clinical phenotype caused by the absence of the fragile X protein. Initial experiments suggested that the knockout mice show macroorchidism and cognitive and behavioral deficits, abnormalities comparable to those of human fragile X patients. In the present study, we have extended our experiments, and conclude that the Fmr1 knockout mouse is a reliable transgenic model to study the fragile X syndrome. 20 refs., 2 figs., 1 tab.

Fragile X syndrome is characterized at the molecular level by amplification of a (CGG)n repeat and hypermethylation of a CpG island preceeding the open reading frame of the fragile X gene (FMR-1) located in Xq27.3. Anticipation in this syndrome is associated with progressive amplification of the (CGG)n repeat from a premutation to a full mutation through consecutive generations. Remarkably, expansion of the premutation to the full mutation is strictly maternal. To clarify this parental influence we studied FMR-1 in sperm of four male fragile X patients. This showed that only the premutation was present in their sperm, although they had a full mutation in peripheral lymphocytes. This might suggest that expansion of the premutation to the full mutation in FMR-1 does not occur in meiosis but in a postzygotic stage.

We report here a family in which the fragile X mutation segregates from an affected grandfather through his normal daughter to an affected grandson. The grandson shows clinical and cytogenetic expression of fragile X syndrome due to a full mutation (large methylated insertion) in the fragile X gene (FMR-1). The mother shows a premutation (small unmethylated insertion) in her FMR-1 gene as the sole manifestation of the fragile X syndrome. The grandfather expresses the fragile X syndrome at the clinical and cytogenetic level, whereas he is mosaic for a methylated full mutation and an unmethylated premutation. The absence of expression of the fragile X mutation when transmitted through an expressing male might present further evidence for genomic imprinting of the FMR-1 gene. Alternatively, it is possible that the grandfather transmitted his premutation to his daughter due to germline mosaicism with both the premutation and the full mutation present in his sperm.

MASA syndrome is a recessive X-linked disorder characterized by mental retardation, adducted thumbs, shuffling gait, aphasia and, in some cases, hydrocephalus. Since it has been shown that X-linked hydrocephalus can be caused by mutations in L1CAM, a neuronal cell adhesion molecule, we performed an L1CAM mutation analysis in eight unrelated patients with MASA syndrome. Three different L1CAM mutations were identified: a deletion removing part of the open reading frame and two point mutations resulting in amino acid substitutions. L1CAM, therefore, harbours mutations leading to either MASA syndrome or HSAS, and might be frequently implicated in X-linked mental retardation with or without hydrocephalus.