Uniparental disomy of chromosome 16 unmasks recessive mutations of FA2H/SPG35 in 4 families

Objective: Identifying an intriguing mechanism for unmasking recessive hereditary spastic paraplegias. Method: Herein, we describe 4 novel homozygous FA2H mutations in 4 nonconsanguineous families detected by whole-exome sequencing or a targeted gene panel analysis providing high coverage of all known hereditary spastic paraplegia genes. Results: Segregation analysis revealed in all cases only one parent as a heterozygous mutation carrier whereas the other parent did not carry FA2H mutations. A macro deletion within FA2H, which could have caused a hemizygous genotype, was excluded by multiplex ligation-dependent probe amplification in all cases. Finally, a microsatellite array revealed uniparental disomy (UPD) in all 4 families leading to homozygous FA2H mutations. UPD was confirmed by microarray analyses and methylation profiling. Conclusion: UPD has rarely been described as causative mechanism in neurodegenerative diseases. Of note, we identified this mode of inheritance in 4 families with the rare diagnosis of spastic paraplegia type 35 (SPG35). Since UPD seems to be a relevant factor in SPG35 and probably additional autosomal recessive diseases, we recommend segregation analysis especially in nonconsanguineous homozygous index cases to unravel UPD as mutational mechanism. This finding may bear major repercussion for genetic counseling, given the markedly reduced risk of recurrence for affected families

Biallelic Loss‐of‐Function NDUFA12 Variants Cause a Wide Phenotypic Spectrum from Leigh/Leigh‐Like Syndrome to Isolated Optic Atrophy

Abstract: Background: Biallelic loss‐of‐function NDUFA12 variants have hitherto been linked to mitochondrial complex I deficiency presenting with heterogeneous clinical and radiological features in nine cases only. Objectives: To fully characterize, both phenotypically and genotypically, NDUFA12‐related mitochondrial disease. Methods: We collected data from cases identified by screening genetic databases of several laboratories worldwide and systematically reviewed the literature. Results: Nine unreported NDUFA12 cases from six pedigrees were identified, with presentation ranging from movement disorder phenotypes (dystonia and/or spasticity) to isolated optic atrophy. MRI showed basal ganglia abnormalities (n = 6), optic atrophy (n = 2), or was unremarkable (n = 1). All carried homozygous truncating NDUFA12 variants, three of which are novel. Conclusions: Our case series expands phenotype–genotype correlations in NDUFA12‐associated mitochondrial disease, providing evidence of intra‐ and inter‐familial clinical heterogeneity for the same variant. It confirms NDUFA12 variants should be included in the diagnostic workup of Leigh/Leigh‐like syndromes – particularly with dystonia – as well as isolated optic atrophy