Case Report of Friedreich's Ataxia and <scp><i>ALG1</i></scp>‐Related Biochemical Abnormalities in a Patient With Progressive Spastic Paraplegia

Frataxin 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.