Mevalonate kinase deficiency (MKD) is one of a group of periodic fever syndromes including familial Mediterranean fever (FMF), cryopyrin-associated periodic syndrome (CAPS) and TNF receptor-associated periodic syndrome (TRAPS). The systemic inflammatory symptoms of MKD, usually appearing in early childhood, include bone and joint pain, abdominal swelling, rashes and high fever. MKD is caused by hypomorphic mutations in mevalonate kinase (MVK), an enzyme of the mevalonate pathway required for the synthesis of isoprenoid lipids that are essential for the post-translational prenylation of proteins such as Rab GTPases. This is the same metabolic pathway that is inhibited by bisphosphonate and statin drugs, which inhibit enzymes downstream or upstream of MVK. It has long been assumed, but never proven, that protein prenylation is defective in MKD. We have optimized a highly sensitive in vitro prenylation assay to detect unprenylated Rab proteins as a marker of bisphosphonate action. We applied the same assay to determine whether Rab prenylation is altered in MKD.

MKD patient-derived lymphoblast cell lines did not show a defect in Rab prenylation unless cultured at 40^oC to mimic a febrile episode. Under these conditions, cell lines from patients with more severe genotypes showed a greater defect in protein prenylation. These data suggest that mutations in MVK cause a temperature-sensitive decrease in enzyme activity, but also that patient cell lines adapt in culture and are not a suitable model to study MKD. However, for the first time we also detected an accumulation of unprenylated Rab proteins in freshly-isolated PBMCs from three MKD patients with compound heterozygous or homozygous V377I mutations in MVK. Importantly, this prenylation defect was absent in patients with FMF, CAPS, TRAPS or healthy controls, or in patients treated with bisphosphonates or statins. These findings suggest that defective Rab prenylation is a sensitive and specific diagnostic biomarker of MKD.