Rodan, Lance H.

ABSTRACTFrataxin is an evolutionarily conserved mitochondrial protein responsible for iron homeostasis and metabolism. A deficiency of frataxin (encoded by FXN) leads to Friedreich's ataxia (FRDA), a progressive disorder that affects both the central and peripheral nervous systems, most commonly via a pathogenic GAA trinucleotide expansion. In contrast, pathogenic variants in ALG1 in humans cause a form of congenital disorder of glycosylation. Here, we present a 15‐year‐old boy with a clinical presentation that raised concern for complex hereditary spastic paraplegia (HSP), with motor features including progressive spastic paraparesis, cervical dystonia, cerebellar dysfunction, and diminished lower extremity reflexes. The proband was initially found to have a novel compound heterozygous variant in ALG1 on exome sequencing, along with N‐glycan profiling revealing evidence of defective mannosylation and Western blot analysis demonstrating an 84% reduction in ALG1 expression. Although several of his clinical features could be explained by the ALG1 variant specifically or considered as part of the presentation of CDGs in general, there were additional phenotypes that suggested an alternative, or additional, genetic diagnosis. Subsequently, he was found to have biallelic pathogenic GAA repeat expansions in FXN on genome sequencing, leading to a diagnosis of FRDA. Given that FRDA explained all his clinical features, the ALG1 variant may have been a hypomorphic form and/or a biochemical phenotype. Our findings underscore the importance of considering FRDA as a differential diagnosis in cases of complex HSP and demonstrate the utility of unbiased genome sequencing approaches that include detection of trinucleotide repeat expansions for progressive motor disorders.

Sonic hedgehog signaling regulates processes of embryonic development across multiple tissues, yet factors regulating context-specific Shh signaling remain poorly understood. Exome sequencing of families with polymicrogyria (disordered cortical folding) revealed multiple individuals with biallelic deleterious variants in TMEM161B , which encodes a multi-pass transmembrane protein of unknown function. Tmem161b null mice demonstrated holoprosencephaly, craniofacial midline defects, eye defects, and spinal cord patterning changes consistent with impaired Shh signaling, but were without limb defects, suggesting a CNS-specific role of Tmem161b. Tmem161b depletion impaired the response to Smoothened activation in vitro and disrupted cortical histogenesis in vivo in both mouse and ferret models, including leading to abnormal gyration in the ferret model. Tmem161b localizes non-exclusively to the primary cilium, and scanning electron microscopy revealed shortened, dysmorphic, and ballooned ventricular zone cilia in the Tmem161b null mouse, suggesting that the Shh-related phenotypes may reflect ciliary dysfunction. Our data identify TMEM161B as a regulator of cerebral cortical gyration, as involved in primary ciliary structure, as a regulator of Shh signaling, and further implicate Shh signaling in human gyral development.

Abstract The pre‐mRNA‐processing factor 8, encoded by PRPF8, is a scaffolding component of a spliceosome complex involved in the removal of introns from mRNA precursors. Previously, heterozygous pathogenic variants in PRPF8 have been associated with autosomal dominant retinitis pigmentosa. More recently, PRPF8 was suggested as a candidate gene for autism spectrum disorder due to the enrichment of sequence variants in this gene in individuals with neurodevelopmental disorders. We report 14 individuals with various forms of neurodevelopmental conditions, found to have heterozygous, predominantly de novo, missense, and loss‐of‐function variants in PRPF8 . These individuals have clinical features that may represent a new neurodevelopmental syndrome.