Adult Diagnosis of ATN1-Related Neurodevelopmental Disorder: A Case Report of a Mild Phenotype with In-Frame Tandem Duplication in the HX Motif

ATN1-related neurodevelopmental disorder (ATN1-NDD) is a rare genetic condition typically diagnosed in infancy, characterized by profound developmental delay and hypotonia due to heterozygous pathogenic variants in the highly conserved HX motif of the ATN1 gene. We present a unique case of a 29-year-old male with mild intellectual delay, autism spectrum disorder, and microcephaly, diagnosed in adulthood through reclassification of a heterozygous in-frame tandem duplication variant (c.3188_3193dupTGCACC) within the HX motif. This case represents one of the mildest and latest diagnosed presentations of ATN1-NDD, expanding the known phenotypic spectrum. Notably, the patient’s genotype parallels the only other known mild case, suggesting a possible genotype-phenotype correlation distinct from the severe phenotypes typically observed. This report underscores the importance of genetic re-evaluation in adults with unexplained neurodevelopmental disorders and contributes valuable insight into the variability and natural history of ATN1-NDD.