Compound heterozygous variants in MAPK8IP3 were detected in severe congenital hypotonia mimicking lethal spinal muscular atrophy

Abstract Mitogen‐activated protein kinase 8‐interacting protein 3 gene ( MAPK8IP3) encodes the c‐Jun‐amino‐terminal kinase‐interacting protein 3 (JIP3) and is involved in retrograde axonal transport. Heterozygous de novo pathogenic variants in MAPK8IP3 result in a neurodevelopmental disorder with or without brain abnormalities and possible axonal peripheral neuropathy. Whole‐exome sequencing was performed on an individual presenting with severe congenital muscle hypotonia of neuronal origin mimicking lethal spinal muscular atrophy. Compound heterozygous rare variants (a splice and a missense) were detected in MAPK8IP3, inherited from the healthy parents. Western blot analysis in a muscle biopsy sample showed a more than 60% decrease in JIP3 expression. Here, we suggest a novel autosomal recessive phenotype of a lower motor neuron disease caused by JIP3 deficiency.