Mutations in PRPS1, which encodes the phosphoribosyl pyrophosphate synthetase enzyme critical for nucleotide biosynthesis, cause hereditary peripheral neuropathy with hearing loss and optic neuropathy (cmtx5)

Abstract

We have identified missense mutations at conserved amino acids in the PRPS1 gene on Xq22.3 in two families with a syndromic form of inherited peripheral neuropathy, one of Asian and one of European descent. The disease is inherited in an X-linked recessive manner, and the affected male patients invariably develop sensorineural hearing loss of prelingual type followed by gating disturbance and visual loss. The family of European descent was reported in 1967 as having Rosenberg-Chutorian syndrome, and recently a Korean family with the same symptom triad was identified with a novel disease locus CMTX5 on the chromosome band Xq21.32-q24. PRPS1 (phosphoribosyl pyrophosphate synthetase 1) is an isoform of the PRPS gene family and is ubiquitously expressed in human tissues, including cochlea. The enzyme mediates the biochemical step critical for purine metabolism and nucleotide biosynthesis. The mutations identified were E43D, in patients with Rosenberg-Chutorian syndrome, and M115T, in the Korean patients with CMTX5. We also showed decreased enzyme activity in patients with M115T. PRPS1 is the first CMT gene that encodes a metabolic enzyme, shedding a new light on the understanding of peripheral nerve-specific metabolism and also suggesting the potential of PRPS1 as a target for drugs in prevention and treatment of peripheral neuropathy by antimetabolite therapy.

Figure  1.

Pedigrees of the Korean family with CMTX5 (A) and the family of European descent with RCS (B). Affected male patients are indicated by blackened boxes and the obligate carrier females by circles with central dots. The individuals whose blood was drawn for the present study are marked with asterisks (*).

Figure  2.

Sural nerve biopsy findings of a patient with CMTX5. A, Histological examination of the sural nerve biopsy sample revealed loss of myelinated nerve fibers and interstitial fibrosis (Luxol-fast blue, ×400). B, Electron micrograph of the sural nerve showed myelinated axons surrounded by concentrically arranged cytoplasmic processes of Schwann cells forming an “onion bulb.” The arrow indicates a remyelinating axon with an abnormally thin myelin sheath.

Figure  3.

The transcript map of the region on chromosome band Xq22.3 within the CMTX5 locus and the genomic structure of PRPS1 with seven coding exons. Black bars represent coding sequences, and gray bars represent UTRs. All seven coding exons of PRPS1 were amplified and directly sequenced, according to standard procedures. The patients with CMTX5 had a missense mutation in exon 3 (blue circle), and the patients with RCS had a missense mutation in exon 2 (red circle). Tel = telomeric; Cen = centromeric.

Figure  4.

A, The chromatograms of the PRPS1 mutations detected by direct sequencing. M115T in patients with CMTX5 (left panel) and E43D in patients with RCS (right panel). B, The affected amino acids are perfectly conserved across different species.

Figure  5.

Multiple sequence alignment of PRPS1 by ClustalW, showing the degree of conservation of the amino acid sequences of the PRPS1 protein across different species. Homo = Homo sapiens; Canis = Canis familiaris; Rattus = Rattus norvegicus; Mus = Mus musculus; Danio = Danio rerio; Silurana = Silurana tropicalis. The reference amino acid sequences for alignment are same as in figure 4B.

Figure  6.

PRPS enzyme activity, determined by measuring the degree of generation of AMP, the end product of PRPS enzyme, in cultured fibroblasts. The activity was decreased in patients with CMTX5 (CMTX5:IV-1, IV-11, and III-18) compared with unaffected family members and unrelated control individuals (CMTX5:III-13, VI-2 and C-1, C-2).

Figure  7.

A, The locations of the mutations on the ribbon diagram of the PRPS1 monomer. M115T in CMTX5 on the α-helix of N-terminal domain (blue) and E43D in RCS on the “flag” region of the N-terminal domain (green). B, Hexameric structure and subunits of PRPS1. Reprinted with permission from Nature Publishing Group.