Ataxia Syndrome With Hearing Loss and Nephronophthisis Associated With a Novel Homozygous Variant in XPNPEP3 .

Objectives: Biallelic variants in XPNPEP3 are associated with a rare mitochondrial syndrome characterized by nephronophthisis leading to kidney failure, essential tremor, hearing loss, seizures, and intellectual disability. Only 2 publications on this condition are available. We report a man with a complex ataxia syndrome, hearing loss, and kidney failure associated with a new biallelic variant in XPNPEP3 .
Methods: Clinical evaluation, neuroimaging studies, a kidney biopsy, and whole genome sequencing (WGS) were applied. Since the phenotype was compatible with a mitochondrial disease, a muscle biopsy with morphological and mitochondrial biochemical investigations was performed.
Results: Axial ataxia, cerebellar atrophy, hearing loss, myopathy, ptosis, supranuclear palsy, and kidney failure because of nephronophthisis were the prominent features in this case. WGS revealed the novel biallelic variant c.766C>T (p.Gln256*) in XPNPEP3 . A muscle biopsy revealed COX negative fibers, a few ragged red fibers, and ultrastructural mitochondrial changes. Enzyme activity in respiratory chain complex IV was reduced in muscle and fibroblasts.
Discussion: This is the first report of a slowly progressive cerebellar ataxia associated with a novel biallelic variant in XPNPEP3 . Abnormalities typical for mitochondrial disease and the slow progression of kidney disease are also striking. Our report expands the spectrum of XPNPEP3 -related diseases.
Competing Interests: The authors report no relevant disclosures. Go to Neurology.org/NG for full disclosures.
(Copyright © 2023 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American Academy of Neurology.)