Defective Protein Prenylation in a Spectrum of Patients With Mevalonate Kinase Deficiency

Abstract

The rare autoinflammatory disease mevalonate kinase deficiency (MKD, which includes HIDS and mevalonic aciduria) is caused by recessive, pathogenic variants in the MVK gene encoding mevalonate kinase. Deficiency of this enzyme decreases the synthesis of isoprenoid lipids and thus prevents the normal post-translational prenylation of small GTPase proteins, which then accumulate in their unprenylated form. We recently optimized a sensitive assay capable of detecting unprenylated Rab GTPase proteins in peripheral blood mononuclear cells (PBMCs) and showed that this assay distinguished MKD from other autoinflammatory diseases. We have now analyzed PBMCs from an additional six patients with genetically-confirmed MKD (with different compound heterozygous MVK genotypes), and compared these with PBMCs from three healthy volunteers and four unaffected control individuals heterozygous for the commonest pathogenic variant, MVK^V377I . We detected a clear accumulation of unprenylated Rab proteins, as well as unprenylated Rap1A by western blotting, in all six genetically-confirmed MKD patients compared to heterozygous controls and healthy volunteers. Furthermore, in the three subjects for whom measurements of residual mevalonate kinase activity was available, enzymatic activity inversely correlated with the extent of the defect in protein prenylation. Finally, a heterozygous MVK^V377I patient presenting with autoinflammatory symptoms did not have defective prenylation, indicating a different cause of disease. These findings support the notion that the extent of loss of enzyme function caused by biallelic MVK variants determines the severity of defective protein prenylation, and the accumulation of unprenylated proteins in PBMCs may be a sensitive and consistent biomarker that could be used to aid, or help rule out, diagnosis of MKD.

Keywords: HIDS; Rab GTPase; Rap1; autoinflammation; mevalonate kinase; prenylation.

Figure 1

Defective protein prenylation in PBMCs is detected consistently in patients with MKD. (A) PBMCs from four adult, compound heterozygous MKD patients (P1–P4) have clear accumulation of unprenylated Rab GTPases (uRabs) and unprenylated Rap1A (uRap1) compared to a healthy control (Cntrl) and 2 heterozygous individuals (Het1,2). (B) PBMCs from a young adult patient with MKD (P5) show accumulation of unprenylated Rab and Rap1A GTPases compared to 2 healthy controls (Cntrl) and a heterozygous (Het) individual (P6) with undiagnosed inflammatory disease. Patient P2 is also shown alongside for comparison. (C) A child (P7) with MKD shows a severe defective in Rab and Rap1A prenylation in PBMCs, compared to both unaffected, heterozygous parents (Prnt1,2). Seventy-five kilodalton endogenous biotinylated protein was used as a loading control in all of the blots.