May 05, 2017 – This report nicely illustrates how, by both clever study design and selection of according phenotypic patients, genetic predispositions for even very rare serious adverse drug reactions (sARD) can be elucidated, which eventually may lead to theragenomics-based therapy approaches with the goal to minimise sARD-risks for treated patients.
The point in case here are atypical femoral fractures that have been associated with long-term bisphosphonate treatment. Bisphosphonates are a class of drugs that prevent the loss of bone mass, and are being used to treat osteoporosis and similar diseases. Evidence shows that they reduce the risk of fracture in post-menopausal women suffering from osteoporosis. Generally, bone undergoes constant turnover and is kept in balance by osteoblasts creating bone and osteoclasts destroying it. Bisphosphonates inhibit the
digestion of bone by encouraging osteoclasts to undergo apoptosis, or cell death, thereby slowing bone loss. Besides using for the treatment of osteoporosis, bisphosphonates are indicated in treatment of Paget’s disease of bone, bone metastasis (with or without hypercalcaemia), multiple myeloma, primary hyperparathyroidism, osteogenesis imperfecta, fibrous dysplasia, and other conditions that exhibit bone fragility.
There are two classes of bisphosphonate: the N-containing and non-N-containing bisphosphonates. The two types of bisphosphonates work differently in the killing of osteoclast cells. Non-nitrogenous bisphosphonates include Etidronate (Didronel), Clodronate (Bonefos, Loron), Tiludronate (Skelid). They are metabolised in the cell to compounds that replace the terminal pyrophosphate moiety of ATP, forming a non-functional molecule that competes with adenosine triphosphate (ATP) in the cellular energy metabolism. The osteoclast initiates apoptosis and dies, leading to an overall decrease in the breakdown of bone. This type of bisphosphonate it thought to have overall more negative effects than the nitrogen containing group, The N-containing bisphosphonates include Pamidronate (Aredia), Neridronate (Nerixia), Alendronate (Fosamax), Ibandronate (Boniva), Risedronate (Actonel), and Zoledronate (Zometa, Aclasta). Nitrogenous bisphosphonates act on bone metabolism by binding and blocking the enzyme farnesyl diphosphate synthase (FPPS) in the HMG-CoA reductase pathway (also known as the mevalonate pathway). Bisphosphonates that contain isoprene chains at the R1 or R2 position can impart specificity for inhibition of GGPS1.
The underlying mechanisms of atypical femoral fractures associated with long-term bisphosphonate treatment remain obscure. However, in a cleverly designed recent study published in the New England Journal of Medicine (NEJM), three sisters who had atypical femoral fractures after receiving various oral bisphosphonates for 6 years were studied. Two of the sisters had a single fracture (at the ages of 64 and 73 years), and one had bilateral fractures (one at the age of 60 years and the other at the age of 61 years). Given the low incidence of atypical femoral fractures in the general population (5.9 per 10,000 person-years), the researchers hypothesized that these sisters might have an underlying genetic background that contributed to these fractures.
The investigators performed whole-exome sequencing to detect possible shared genetic variants involved in the apparent increased risk of these sisters for atypical femoral fractures. In addition, they performed whole-exome sequencing in three unrelated patients with atypical femoral fractures who each had received bisphosphonates for more than 5 years. They prioritized rare nonsynonymous mutations in the variant filtering, and only mutations that were shared among the three sisters were considered. No mutation was found to be homozygous or in any gene containing mutations in both chromosomes (compound heterozygous). Assuming that a dominant model was involved, the investigators detected 37 rare mutations (in 34 genes), among them a novel p.Asp188Tyr substitution in the enzyme geranylgeranyl pyrophosphate synthase (GGPPS), which is a site of inhibition by bisphosphonates in the mevalonate pathway. The variant that is located in the genomic position g.235505746G→T on chromosome 1 (GRCh37/hg19) in GGPS1 had the best conservation score and was not described in any of the available databases. This variant would be expected to severely impair the enzyme activity. Furthermore, the gene encoding cytochrome P-450 family 1 subfamily A member 1 (CYP1A1), which is involved in steroid metabolism, was also mutated in all three sisters and in one of the unrelated patients, which suggests that it could be another potential susceptibility gene for bisphosphonate-related atypical femoral fractures. An additional mutation in the gene encoding the mevalonate diphosphate decarboxylase enzyme (MVD) was detected in one unrelated patient.
Pathway analysis of the mutated genes showed enrichment of the isoprenoid biosynthetic pathway (GO:0008299), which includes GGPS1, CYP1A1, and MVD (P<0.001). The investigators speculate that other variants that have been identified throughout this study might also be involved in susceptibility to bisphosphonate-related atypical femoral fractures. Such variants include missense changes in the gene encoding fibronectin 1 (FN1) and in the genes encoding synapse defective Rho GTPase homolog 2 (SYDE2) and neuronal guanine nucleotide exchange factor (NGEF); the latter two proteins are regulators of small GTPases. These results may support a model in which accumulation of susceptibility variants (including some in relevant genes, notably GGPS1) may lead to a genetic component of predisposition to atypical femoral fractures.
Eventually, the knowledge of predisposing genetic / allelic variants may lead to a diagnostic approach (i.e., companion test) to be used in combination with bisphosphonates in order to prospectively identify those rare individuals with a definitive risk to suffer from atypical femoral fractures. This would allow for informed targeted therapy option decisions, both for the treating physician and the patient concerned.