Bone Abstracts

Background: Paget’s disease of Bone (PDB) has a strong genetic component and a candidate locus for the disease has been identified on chromosome 14q32, tagged by rs10498635 located within RIN3 (Albagha et al, Nat Genet 2011). RIN3 encodes a protein that acts as a guanine nucleotide exchange factor for Rab5b and Rab31. Here we investigated the candidacy of RIN3 as a predisposing gene for PDB.

Methods: We studied expression of RIN3 by quantitative PCR, western blotting and immunohistochemistry in bone and bone derived cells and conducted DNA sequencing of the exons, intron–exon boundaries and promoter in 196 PDB cases where Q1STM1 mutations had been excluded. We then conducted an association study of the variants identified in a further study of 475 PDB cases and 475 controls.

Results: The mRNA for RIN3 was strongly expressed in lung, bone and liver and at lower levels in kidney, brain and muscle. Levels of RIN3 mRNA expression were higher in osteoclasts (P=0.02) but lower in bone marrow macrophages (P=0.006) than whole bone marrow. However, western blotting showed that RIN3 protein expression was highest in whole bone marrow, lower in bone marrow macrophages, and lower still in osteoclasts. The mutation screening identified 19 missense variants in RIN3 including 3 novel mutations that were not present in controls or publicly available databases including 1000 Genomes. Additionally, two common SNPs in the RIN3 promoter and 3’UTR were strongly associated with the disease (P=2.5×10⁻⁷ and 5.8×10^-6 respectively).

Conclusions: We conclude that RIN3 is expressed in the bone microenvironment and that the levels of expression vary during osteoclast differentiation. The mutation screening experiments raise the possibility that susceptibility to PDB may be influenced by rare protein coding mutations as well as more common variants in the gene promoter and 3′ UTR.

According to densitometry criteria 22% of the patients have osteoporosis and 22% are in the range of osteopenia.

1 Pathological fracture was registered (vertebral).

Conclusions: ERT prevents progression of bone abnormalities in GD. Vitamin D insufficiency is frequent in GD and almost half of the patients have decreased bone mass.