Genetic predisposition for femoral neck stress fractures in military conscripts

Stress fractures are a significant problem among athletes and soldiers and may result in devastating complications or even permanent handicap. Genetic factors may increase the risk, but no major susceptibility genes have been identified. The purpose of this study was to search for possible genetic factors predisposing military conscripts to femoral neck stress fractures. Eight genes involved in bone metabolism or pathology (COL1A1, COL1A2, OPG, ESR1, VDR, CTR, LRP5, IL-6) were examined in 72 military conscripts with a femoral neck stress fracture and 120 controls. The risk of femoral neck stress fracture was significantly higher in subjects with low weight and body mass index (BMI). An interaction between the CTR (rs1801197) minor allele C and the VDR C-A haplotype was observed, and subjects lacking the C allele in CTR and/or the C-A haplotype in VDR had a 3-fold higher risk of stress fracture than subjects carrying both (OR = 3.22, 95% CI 1.38-7.49, p = 0.007). In addition, the LRP5 haplotype A-G-G-C alone and in combination with the VDR haplotype C-A was associated with stress fractures through reduced body weight and BMI. Our findings suggest that genetic factors play a role in the development of stress fractures in individuals subjected to heavy exercise and mechanical loading. The present results can be applied to the design of future studies that will further elucidate the genetics of stress fractures

Mutations in LRP5 cause primary osteoporosis without features of OI by reducing Wnt signaling activity

Peer reviewe

Genetic factors in bone disorders:osteogenesis imperfecta, juvenile osteoporosis and stress fractures

Abstract Genetic factors and their resulting phenotypes were evaluated in three different bone disorders: osteogenesis imperfecta (OI), juvenile idiopathic osteoporosis (JIO), and stress fractures. The spectrum of the OI phenotypes caused by mutations in the COL1A1 and COL1A2 genes is well defined, but the mechanisms by which the variations affect the hearing phenotype are not well-known. A total of 54 Finnish OI patients with previously diagnosed hearing loss, or aged 35 or more years, were analyzed here for mutations in COL1A1, or COL1A2. Altogether, 49 mutations were identified, of which 41 were novel. No correlation was observed between the mutated gene, or the mutation type, and the hearing pattern. This indicates that the basis of hearing loss in OI is complex, and is a result of multifactorial, still unknown genetic effects, or of variable expressions of the COL1A1 and COL1A2 genes. JIO presents peri-pubertally as an acute symptomatic osteoporosis (bone pain and fractures) in otherwise healthy children, and no underlying cause has yet been identified for this disorder. Here, the analysis of the low-density lipoprotein receptor-related protein 5 gene (LRP5) in 20 patients with JIO revealed two missense mutations (A29T and R1036Q) and one frameshift mutation (C913fs) in 3 of the patients. The LRP5 gene has recently been shown to be also involved in osteoporosis-pseudoglioma syndrome and a high-bone-mass phenotype. Stress fractures are a significant problem among athletes and soldiers. Genetic factors may increase the fracture risk, but no susceptibility genes have yet been identified. Seven genes involved in bone metabolism, or pathology, were studied in terms of their roles in stress fracture. No disease-causing, or predisposing variations were found in the candidate gene, or association analyses, but a highly significant association was found between the phenotype and a vitamin D receptor (VDR) haplotype, TGT, which is composed of three polymorphic sites, FokI, BsmI and TaqI. We showed that femoral neck stress fractures are associated with a certain VDR haplotype, accounting for a five-fold increase in the risk of developing stress fractures, with an associated attributable risk of 12%. The results of this study show that genetic factors play a role in different pathological bone phenotypes. These findings provide new information on the pathogenesis of the disorders and for the development of genetic testing and targeted treatment for the disorders

Mutations in LRP5 cause primary osteoporosis without features of OI by reducing Wnt signaling activity

Abstract Background Primary osteoporosis is a rare childhood-onset skeletal condition whose pathogenesis has been largely unknown. We have previously shown that primary osteoporosis can be caused by heterozygous missense mutations in the Low-density lipoprotein receptor-related protein 5 (LRP5) gene, and the role of LRP5 is further investigated here. Methods LRP5 was analyzed in 18 otherwise healthy children and adolescents who had evidence of osteoporosis (manifested as reduced bone mineral density i.e. BMD, recurrent peripheral fractures and/or vertebral compression fractures) but who lacked the clinical features of osteogenesis imperfecta (OI) or other known syndromes linked to low BMD. Also 51 controls were analyzed. Methods used in the genetic analyses included direct sequencing and multiplex ligation-dependent probe amplification (MLPA). In vitro studies were performed using luciferase assay and quantitative real-time polymerase chain reaction (qPCR) to examine the effect of two novel and three previously identified mutations on the activity of canonical Wnt signaling and on expression of tryptophan hydroxylase 1 (Tph1) and 5-hydroxytryptamine (5-Htr1b). Results Two novel LRP5 mutations (c.3446 T > A; p.L1149Q and c.3553 G > A; p.G1185R) were identified in two patients and their affected family members. In vitro analyses showed that one of these novel mutations together with two previously reported mutations (p.C913fs, p.R1036Q) significantly reduced the activity of the canonical Wnt signaling pathway. Such reductions may lead to decreased bone formation, and could explain the bone phenotype. Gut-derived Lrp5 has been shown to regulate serotonin synthesis by controlling the production of serotonin rate-limiting enzyme, Tph1. LRP5 mutations did not affect Tph1 expression, and only one mutant (p.L1149Q) reduced expression of serotonin receptor 5-Htr1b (p < 0.002). Conclusions Our results provide additional information on the role of LRP5 mutations and their effects on the development of juvenile-onset primary osteoporosis, and hence the pathogenesis of the disorder. The mutations causing primary osteoporosis reduce the signaling activity of the canonical Wnt signaling pathway and may therefore result in decreased bone formation. The specific mechanism affecting signaling activity remains to be resolved in future studies

Consortium for osteogenesis imperfecta mutations in the helical domain of type I collagen: regions rich in lethal mutations align with collagen binding sites for integrins and proteoglycans.

Osteogenesis imperfecta (OI) is a generalized disorder of connective tissue characterized by fragile bones and easy susceptibility to fracture. Most cases of OI are caused by mutations in type I collagen. We have identified and assembled structural mutations in type I collagen genes (COL1A1 and COL1A2, encoding the proalpha1(I) and proalpha2(I) chains, respectively) that result in OI. Quantitative defects causing type I OI were not included. Of these 832 independent mutations, 682 result in substitution for glycine residues in the triple helical domain of the encoded protein and 150 alter splice sites. Distinct genotype-phenotype relationships emerge for each chain. One-third of the mutations that result in glycine substitutions in alpha1(I) are lethal, especially when the substituting residues are charged or have a branched side chain. Substitutions in the first 200 residues are nonlethal and have variable outcome thereafter, unrelated to folding or helix stability domains. Two exclusively lethal regions (helix positions 691-823 and 910-964) align with major ligand binding regions (MLBRs), suggesting crucial interactions of collagen monomers or fibrils with integrins, matrix metalloproteinases (MMPs), fibronectin, and cartilage oligomeric matrix protein (COMP). Mutations in COL1A2 are predominantly nonlethal (80%). Lethal substitutions are located in eight regularly spaced clusters along the chain, supporting a regional model. The lethal regions align with proteoglycan binding sites along the fibril, suggesting a role in fibril-matrix interactions. Recurrences at the same site in alpha2(I) are generally concordant for outcome, unlike alpha1(I). Splice site mutations comprise 20% of helical mutations identified in OI patients, and may lead to exon skipping, intron inclusion, or the activation of cryptic splice sites. Splice site mutations in COL1A1 are rarely lethal; they often lead to frameshifts and the mild type I phenotype. In alpha2(I), lethal exon skipping events are located in the carboxyl half of the chain. Our data on genotype-phenotype relationships indicate that the two collagen chains play very different roles in matrix integrity and that phenotype depends on intracellular and extracellular events