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Resumen Objetivos: Las fracturas atípicas de fémur (FAF) son un tipo de fracturas poco frecuentes, a menudo relacionadas con un tratamiento prolongado con bisfosfonatos (BPs). Actualmente no se conocen con exactitud sus mecanismos patogénicos y no hay pruebas para identificar aquellos pacientes con un alto riesgo de sufrir una FAF. El objetivo de este trabajo es investigar las bases genéticas de las FAFs. Material y métodos: Se secuenció el exoma completo de 3 hermanas y de 3 pacientes adicionales no relacionadas, todas tratadas con BPs durante más de 5 años. Se seleccionaron variantes compartidas por las hermanas, de baja frecuencia y potencialmente patogénicas, y se construyó una red de interacciones de genes y proteínas con los datos hallados. Resultados: Identificamos 37 variantes raras (en 34 genes) compartidas por las 3 hermanas, algunas de ellas no descritas anteriormente. La variante más llamativa fue la mutación p.Asp188Tyr en el enzima geranilgeranil pirofosfato sintasa (codificada por el gen GGPS1), de la vía del mevalonato y esencial para la función del osteoclasto. Otro hallazgo interesante fueron dos mutaciones (una en las 3 hermanas y una en una paciente no relacionada) en el gen CYP1A1, implicado en el metabolismo de los esteroides. Identificamos otras variantes que también podrían estar involucradas en la susceptibilidad a las FAFs o en el fenotipo osteoporótico subyacente, tales como las presentes en los genes SYDE2, NGEF. COG4 y la FN1. Conclusiones: Nuestros datos son compatibles con un modelo donde la acumulación de variantes de susceptibilidad podría participar en la base genética de las FAFs. Abstract Objectives: Atypical femoral fractures (AFF) are rare, often related to long-term bisphosphonate (BPs) treatment. Their pathogenic mechanisms are not precisely known and there is no evidence to identify patients with a high risk of AFF. The aim of this work is to study the genetic bases of AFFs. Materials and methods: Whole-exome sequencing was carried out on 3 sisters and 3 unrelated additional patients, all treated with BPs for more than 5 years. Low frequency, potentially pathogenic variants shared by the 3 sisters, were selected, were selected and a network of gene and protein interactions was constructed with the data found. Results: We identified 37 rare variants (in 34 genes) shared by the 3 sisters, some not previously described. The most striking variant was the p.Asp188Tyr mutation in the enzyme geranylgeranyl pyrophosphate synthase (encoded by the GGPS1 gene), from the mevalonate pathway and essential for osteoclast function. Another noteworthy finding was two mutations (one in the 3 sisters and one in an unrelated patient) in the CYP1A1 gene, involved in the metabolism of steroids. We identified other variants that could also be involved in the susceptibility to AFFs or in the underlying osteoporotic phenotype, such as those present in the SYDE2, NGEF, COG4 and FN1 genes. Conclusions: Our data are compatible with a model where the accumulation of susceptibility variants could participate in the genetic basis of AFFs.

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.