Homozygous Truncating Intragenic Duplication in TUSC3 Responsible for Rare Autosomal Recessive Nonsyndromic Intellectual Disability with No Clinical or Biochemical Metabolic Markers

Intellectual disability (ID), which affects around 2–3% of the general population, is classically divided into syndromic and nonsyndromic forms, with several modes of inheritance. Nonsyndromic autosomal recessive ID (NS-ARID) appears extremely heterogeneous with numerous genes identified to date, including inborn errors of metabolism. The TUSC3 gene encodes a subunit of the endoplasmic reticulum (ER)-bound oligosaccharyltransferase complex, which mediates a key step of N-glycosylation. To date, only five families with NS-ARID and TUSC3 mutations or rearrangements have been reported in the literature. All patients had speech delay, moderate-to-severe ID, and moderate facial dysmorphism. Micr…