Speculation on the role of transposable elements in human genetic disease with particular attention to achondroplasia and the fragile X syndrome.

We suggest that mutations for fragile X-positive Martin-Bell syndrome, and perhaps also for achondroplasia, may result from the insertion of transposable elements (TE's). Loss of genetic function could result from either the insertion of TE's within or adjacent to a normal chromosomal gene or, in the case of fragile X, from the loss of genes distal to the site of TE insertion following subsequent TE excision without ligation of the resulting discontinuity. The phenotypically and often cytogenetically normal transmitting males in fragile X pedigrees are interpreted not as "nonpenetrant" transmitters of a fully formed fragile X but rather as transmitters of some or all of the factors necessary for TE insertion at Xq27. We consider it likely that such insertion frequently first occurs, both in soma and especially in the germline, in their daughters. Our models predict that father to son transmission of causative factors would be a common occurrence in fragile X pedigrees. The absence of documented father to son transmission either points to a flaw in the models or reflects systematic bias in the collection of pedigree information.