Mutation analysis of candidate genes within the 2q33.3 linkage area for familial early-onset generalised osteoarthritis

In a genome-wide linkage scan of seven families with familial early-onset osteoarthritis (FOA), we mapped a FOA locus to a 5 cM region on chromosome 2q33.3–2q34 with a maximum LOD score of 6.05. To identify causal variants, 17 positional candidate genes and FRZB were sequenced for coding, splice sites, and 5′ and 3′ untranslated regions. The pathogenicity of possible disease-causing variants was evaluated using predicted effects on protein structure and function, splicing enhancers, degree of conservation and frequency in 790 unrelated subjects from the population-based Rotterdam study scored for the presence of radiographic signs of OA (ROA). Nine novel variants, identified in NRP2 , XM_371590 , ADAM23 , IDH1 , PIP5K3 and PTHR2 , cosegregated with FOA, of which two were promising. IDH1 Y183C cosegregated in one family, involved a conserved amino-acid change and showed a damaging effect predicted by PolyPhen and SIFT. In the Rotterdam sample, carriers of IDH1 Y183C (0.02) had an increased but insignificant risk for generalised ROA. The second variant, NRP2 c.1938-21T>C cosegregated in three families. In the Rotterdam sample, carriers conferred an increased risk of 2.1 (95% confidence interval, 1.1–4.1, P =0.032) to have generalised ROA. Furthermore, two variants ( NRP2 c.1938–21T>C and IDH1 c.933-28C>T) occurred together on the haplotypes segregating with FOA in two out seven families. This haplotype was rare in the Rotterdam sample (0.0013). Two promising variants in or near NRP2 and IDH1 may not be sufficient for the onset of FOA alone but might have a modulating role with FOA. Confirmation in other OA populations is required.