The long road to diagnosis: recessive PMPCB deficiency hidden behind a dominant familial VCP defect.
Multiple mitochondrial dysfunctions syndrome 6 (MMDS6), caused by biallelic likely pathogenic variants in PMPCB, is an extremely rare autosomal recessive childhood-onset neurodegenerative disorder, with only six reported cases to date, most resulting in early mortality. Pathogenic variants in VCP cause multisystem proteinopathy 1 (MSP1), an autosomal dominant adult-onset disorder encompassing inclusion body myopathy (IBM), frontotemporal dementia (FTD), and amyotrophic lateral sclerosis (ALS), typically presenting in mid-adulthood. We describe a 23-year-old female with two likely pathogenic variants presumed to be in trans in PMPCB and a co-occurring pathogenic VCP variant. She was misdiagnosed for over 20 years with early-onset VCP-related neurodegeneration due to a maternal family history of ALS. Her disease began at birth with microcephaly and progressed throughout childhood, including developmental regression, cerebellar and cerebral atrophy, optic atrophy, seizures, spasticity, dysarthria, and loss of ambulation. Initial genetic testing identified only the familial VCP variant. Updated genomic sequencing at age 23 revealed two likely pathogenic PMPCB variants, strong supporting a diagnosis of MMDS6. Her clinical features closely align with previously reported MMDS6 cases and are inconsistent with the typical adult-onset phenotype of VCP-associated disorders. While she shares overlapping features with VCP-related disease (limb-girdle weakness, spasticity, FTD), the timing and severity of her neurodevelopmental findings support MMDS6 as the primary diagnosis. Early mitochondrial dysfunction may predispose her to an accelerated or more severe future VCP-associated phenotype. This is the first report of combined likely pathogenic and pathogenic variants in PMPCB and VCP respectively, expanding the phenotypic spectrum of both disorders. The case underscores the necessity of periodic re-evaluation with advanced genetic testing, highlights important ethical and familial implications, and informs future diagnosis and management of patients with overlapping rare genetic conditions.