Biallelic ATG9B Variants Define a Novel Autophagy-Related Neurodevelopmental Disorder with Cerebellar Ataxia

Background/Objectives: Autophagy is a highly conserved eukaryotic cellular process whose dysfunction results in human pathologies including cancer and neurodegenerative disease. First identified in yeast, ATG genes are central players in autophagy. Mutations in core autophagy genes ATG5 and ATG7 have been previously reported to cause rare genetic disorders with autosomal recessive inheritance. Methods: Here we report, for the first time, variants in human ATG9B gene as causative factors for a rare neurodevelopmental disease with autosomal recessive inheritance. Three distinct mutations were detected in three independent families with consanguinity, five patients affected in total. Results: The first variant is an 11-nucleotide deletion resulting in a frameshift. A premature stop codon is added and the C-terminal cytosolic domain of ATG9B protein is truncated. The second one is a point mutation that changes a critical amino acid in the transmembrane domain. The third variant is a 2-nucleotide deletion causing a different truncation product. Patients presented with diverse neurodevelopmental anomalies including intellectual disability, behavioral abnormalities, congenital cerebellar ataxia, mild cerebellar atrophy, and microcephaly. Since human ATG9B is expressed specifically in the placenta, we hypothesized that the disease pathology originates during placental development. To characterize the effects of the first frameshift mutation and gain insight into the specific functions of ATG9B in a physiological setting, we used mammalian cells and a knock-in mouse model. Truncated ATG9B was not stable when expressed in cells. It was localized to perinuclear vesicles like the WT protein, but not to peripheral vesicles. Homozygous knock-in mice were viable, fertile, and displayed no gross phenotypical abnormalities. Histomorphometry analysis of the placenta layers did not reveal a significant difference between mutant and control embryos. The assessments of neurobehavioral tests were similar in wild-type and homozygous knock-in mice. However, knock-in mice had a reduced fear memory trend, which is an amygdala-involved response. Conclusions: In this study, we describe a new rare disease linked to ATG9, including cerebellar ataxia and atrophy, as described for ATG5 and ATG7.