Clinical and genetic heterogeneity of hypochondroplasia

Hypochondroplasia (HCH) is an autosomal dominant condition characterised by short stature, micromelia, and lumbar lordosis. In a series of 29 HCH probands (13 sporadic cases, 16 familial cases), we tested their DNA for the N540K recurrent mutation previously described in the proximal tyrosine kinase domain of the FGFR3 gene on chromosome 4p16.3, and we detected this mutation in 21/29 HCH patients. Interestingly, three familial cases were clearly unlinked to chromosome 4p16.3. Reviewing the clinical and radiological manifestations of the disease a posteriori, we observed that the N540K mutation was associated with relative macrocrania with a high and large forehead and short hands. By contrast, in the three pedigrees inconsistent with linkage to chromosome 4p16.3, the clinical phenotype was milder, macrocephaly and shortening of the long bones was less obvious, the hands were normal, and no metaphyseal flaring was noted. This study supports the view that HCH is a clinically and genetically heterogeneous condition.