Suchergebnisse

Osteoporosis is the most common metabolic bone disorder and nitrogen-containing bisphosphonates (BP) are a first line treatment for it. Yet, atypical femoral fractures (AFF), a rare adverse effect, may appear after prolonged BP administration. Given the low incidence of AFF, an underlying genetic cause that increases the susceptibility to these fractures is suspected. Previous studies uncovered rare CYP1A1 mutations in osteoporosis patients who suffered AFF after long-term BP treatment. CYP1A1 is involved in drug metabolism and steroid catabolism, making it an interesting candidate. However, a functional validation for the AFF-associated CYP1A1 mutations was lacking. Here we tested the enzymatic activity of four such CYP1A1 variants, by transfecting them into Saos-2 cells. We also tested the effect of commonly used BPs on the enzymatic activity of the CYP1A1 forms. We demonstrated that the p.Arg98Trp and p.Arg136His CYP1A1 variants have a significant negative effect on enzymatic activity. Moreover, all the BP treatments decreased CYP1A1 activity, although no specific interaction with CYP1A1 variants was found. Our results provide functional support to the hypothesis that an additive effect between CYP1A1 heterozygous mutations p.Arg98Trp and p.Arg136His, other rare mutations and long-term BP exposure might generate susceptibility to AFF. ; Funding was available from Fundación Española de Investigación Ósea y Metabolismo Mineral ("Beca FEIOMM 2019 de Investigación Básica") and grant PID2019-107188RB-C21 from the Spanish Ministerio de Ciencia e Innovación. N.M.G. and N.R.A. were recipients of Ph.D. fellowships from AGAUR (Catalan Government) and the Spanish Ministerio de Universidades.

Atypical femoral fractures (AFFs) are a rare but potentially devastating event, often but not always linked to bisphosphonate (BP) therapy. The pathogenic mechanisms underlying AFFs remain obscure, and there are no tests available that might assist in identifying those at high risk of AFF. We previously used exome sequencing to explore the genetic background of three sisters with AFFs and three additional unrelated AFF cases, all previously treated with BPs. We detected 37 rare mutations (in 34 genes) shared by the three sisters. Notably, we found a p.Asp188Tyr mutation in the enzyme geranylgeranyl pyrophosphate synthase, a component of the mevalonate pathway, which is critical to osteoclast function and is inhibited by N-BPs. In addition, the CYP1A1 gene, responsible for the hydroxylation of 17β-estradiol, estrone, and vitamin D, was also mutated in all three sisters and one unrelated patient. Here we present a detailed list of the variants found and report functional analyses of the GGPS1 p.Asp188Tyr mutation, which showed a severe reduction in enzyme activity together with oligomerization defects. Unlike BP treatment, this genetic mutation will affect all cells in the carriers. RNAi knockdown of GGPS1 in osteoblasts produced a strong mineralization reduction and a reduced expression of osteocalcin, osterix, and RANKL, whereas in osteoclasts, it led to a lower resorption activity. Taken together, the impact of the mutated GGPPS and the relevance of the downstream effects in bone cells make it a strong candidate for AFF susceptibility. We speculate that other genes such as CYP1A1 might be involved in AFF pathogenesis, which remains to be functionally proved. The identification of the genetic background for AFFs provides new insights for future development of novel risk assessment tools. ; We thank the patients for their kind participation. Funds for the study include grants SAF2014‐56562R, SAF2016‐75948‐R (Spanish MINECO), PI12/02315 (FIS, ISCII), 2014SGR932 (Catalan Government), and CIBERER (U720). This work was also supported by the Centro de Investigación Biomédica en Red en Fragilidad y Envejecimiento Saludable (CIBERFES; CB16/10/00245) and FEDER funds. JED was supported by the NIHR Biomedical Research Centre, Oxford, UK. NR is recipient of an FPU predoctoral fellowship from the Spanish Ministerio de Educación Cultura y Deporte. The work was also supported by a grant from the US government, NIH, NIAMS (R01 AR062054) to RB.