Expanding Spectrum of <scp> <i>FIG4</i> </scp> ‐Related Neurological Disorders of Lysosomal Homeostasis: Case Report and Overview of the Potential Genotype–Phenotype Correlations

Biallelic loss-of-function variants in FIG4 are associated with Charcot-Marie-Tooth disease type 4J, a progressive peripheral sensorimotor demyelinating polyneuropathy. Biallelic null FIG4 variants cause Yunis-Varon syndrome, a severe neurological disorder characterized by global developmental delay, hypotonia, brain malformations, skeletal defects, dysmorphic facial features, and juvenile lethality. In the past few years, many individuals with combined central and peripheral nervous system disease associated with biallelic FIG4 variants have been described. In addition, certain heterozygous FIG4 variants are associated with amyotrophic lateral sclerosis. We describe an individual with global developmental delay, hypotonia, cerebral hypomyelination, peripheral hypomyelinating polyneuropathy, frequent fractures, and juvenile ossifying fibroma. The spectrum of clinical presentation of FIG4-related disorders is increasingly being recognized. Our observations expand the phenotypic spectrum of FIG4-related neurological disorders. In addition, we provide an overview of the potential genotype-phenotype correlations of this expanding group of disorders of lysosomal homeostasis.