PDXK Mutations Cause Polyneuropathy Responsive to Pyridoxal 5 0 -Phosphate Supplementation

Objective: To identify disease-causing variants in autosomal recessive axonal polyneuropathy with optic atrophy and provide targeted replacement therapy. Methods: We performed genome-wide sequencing, homozygosity mapping, and segregation analysis for novel disease-causing gene discovery. We used circular dichroism to show secondary structure changes and isothermal titration calorimetry to investigate the impact of variants on adenosine triphosphate (ATP) binding. Pathogenicity was further supported by enzymatic assays and mass spectroscopy on recombinant protein, patient-derived ﬁ broblasts, plasma, and erythrocytes. Response to supplementation was measured with clinical validated rating scales, electrophys-iology, and biochemical quanti ﬁ cation. Results: We identi ﬁ ed biallelic mutations in PDXK in 5 individuals from 2 unrelated families with primary axonal polyneuropathy and optic atrophy. The natural history of this disorder suggests that untreated, affected individuals become wheelchair-bound and blind. We identi ﬁ ed conformational rearrangement in the mutant enzyme around the ATP-binding pocket. Low PDXK ATP binding resulted in decreased erythrocyte PDXK activity and low pyridoxal 5 0 phosphate (PLP) concentrations. We rescued the clinical and biochemical pro ﬁ le with PLP supplementation in 1 family, improvement in power, pain, and fatigue contributing to patients regaining their ability to walk independently during the ﬁ rst year of PLP normalization. Interpretation: We show that mutations in PDXK cause autosomal recessive axonal peripheral polyneuropathy leading to disease via reduced PDXK enzymatic activity and low PLP. We show that the biochemical pro ﬁ le can be rescued with PLP supplementation associated with clinical improvement. As B 6 is a cofactor in diverse essential biological pathways, our ﬁ ndings may have direct implications for neuropathies of unknown etiology characterized by reduced PLP levels. B 6 vitamers (pyridoxamine 5 0 -phosphate [PMP]; pyridoxine 5 0 phosphate [PNP]; pyridoxal 5 0 -phosphate [PLP]) present in the diet are hydrolyzed to pyridoxal (PL), pyridoxamine (PM), and pyridoxine (PN) by intestinal phosphatases prior to absorption and then converted to their 5 0 -phosphate derivatives in the liver by PL kinase (PDXK). PNP and PMP are then converted to PLP by pyridox(am)ine 5 0 -phosphate oxidase (PNPO). PLP re-enters the circulation bound to a lysine residue of albumin. Homeostatic regulation of tissue levels of PLP is achieved by various mechanisms, including feedback inhibition of PNPO and PL kinase by PLP. 62 Albumin in plasma, hemoglobin in erythrocytes, and glycogen phosphorylase in muscle also play a role, binding to PLP and helping to keep concentrations of this very reactive aldehyde low 63 so as to avoid any unwanted reactions with biologically important molecules. Subsequent delivery of PLP to the tissues requires hydrolysis of circulating PLP to PL by the ectoenzyme tissue nonspeci ﬁ c alkaline phosphatase (TNSALP). The resulting pyridoxal is able to enter cells prior to being rephosphorylated by PL kinase to produce the active cofactor, PLP, required by B 6 -dependent apoenzymes. 64 Within cells, recycling pathways also exist, with PMP being oxidized by PNPO to form PLP. 63 AOX = aldehyde oxidase; GPI = glycosylphosphatidylinositol; PA = pyridoxic acid.