Mutations in the FGFR3 gene cause the phenotypic spectrum of FGFR3 chondrodysplasias ranging from lethal forms to the milder phenotype seen in hypochondroplasia (Hch). The p.N540K mutation in the FGFR3 gene occurs in ~70% of individuals with Hch, and nearly 30% of individuals with the Hch phenotype have no mutations in the FGFR3, which suggests genetic heterogeneity. The identification of a severe case of Hch associated with the typical mutation c.1620C > A and the occurrence of a c.1150T > C change that resulted in a p.F384L in exon 10, together with the suspicion that this second change could be a modulator of the phenotype, prompted us to investigate this hypothesis in a cohort of patients. An analysis of 48 patients with FGFR3 chondrodysplasia phenotypes and 330 healthy (control) individuals revealed no significant difference in the frequency of the C allele at the c.1150 position (p = 0.34). One patient carrying the combination `pathogenic mutation plus the allelic variant c.1150T > C' had a typical achondroplasia (Ach) phenotype. In addition, three other patients with atypical phenotypes showed no association with the allelic variant. Together, these results do not support the hypothesis of a modulatory role for the c.1150T > C change in the FGFR3 gene.

Turner syndrome (TS) is an interesting model for investigating the association between methylenetetrahydrofolate reductase (MTHFR) gene polymorphisms and non-disjunction because of the high frequency of chromosomal mosaicism among patients with this syndrome. We determined the frequencies of MTHFR 677C -> T and 1298A -> C polymorphic mutations in 49 patients with TS and 200 control individuals. The frequency of the 677C -> T allele was 0.39 for patients and 0.29 for controls while that of the 1298A -> C allele was 0.28 for patients and 0.25 for controls. Genotype frequencies were shown to be different in patients and controls (chi2 = 12.143; p = 0.033), and this was attributable to the higher frequency of the C677C -> T /677C -> T genotype among TS patients. In homozygotes, this mutation might have an effect on somatic chromosome disjunction by decreasing MTHFR activity.