Cobalamin J disease detected on newborn screening: Novel variant and normal neurodevelopmental course

Cobalamin J disease (CblJ) is an ultra‐rare autosomal recessive disorder of intracellular cobalamin metabolism associated with combined methylmalonic acidemia and homocystinuria. It is caused by pathogenic variants in ABCD4, which encodes an ATP‐binding cassette (ABC) transporter that affects the lysosomal release of cobalamin (Cbl) into the cytoplasm. Only six cases of CblJ have been reported in the literature. Described clinical features include feeding difficulties, failure to thrive, hypotonia, seizures, developmental delay, and hematological abnormalities. Information on clinical outcomes is extremely limited, and no cases of presymptomatic diagnosis have been reported. We describe a now 17‐month‐old male with CblJ detected by newborn screening and confirmed by biochemical, molecular, and complementation studies. With early detection and initiation of treatment, this patient has remained asymptomatic with normal growth parameters and neurodevelopmental function. To the best of our knowledge, this report represents the first asymptomatic and neurotypical patient with CblJ.