Deciphering the phenotypic spectrum associated with MIA3-related odontochondrodysplasia

Odontochondrodysplasia (ODCD) is a rare skeletal dysplasia characterized by short stature, skeletal deformities, and dentinogenesis imperfecta (DI). Although the majority of cases were associated with biallelic variants in TRIP11, one study described a homozygous truncating variant in MIA3, encoding TANGO1, in four sibs with ODCD in association with insulin-dependent diabetes, hearing loss, obesity, and intellectual disability. Subsequently, a homozygous truncating variant in the luminal domain of TANGO1 was identified in a fetus with a lethal skeletal dysplasia and fetal hydrops. Herein, we describe two unrelated patients with a distinct phenotype including severe short limbs, short stature, metaphyseal dysplasia, dysmorphic facies, lax joints, and DI. Other variable features were scoliosis, squint, and cardiac problems. Exome sequencing revealed two homozygous MIA3 variants in the luminal domain of TANGO1, c.354+2T>G and p.Cys38Phe. The c.354+2T>G variant was confirmed by investigating the patient's mRNA to result in exon 3 skipping and an inframe deletion of 29 amino acids. Our patients lacked the extra-skeletal manifestations noted in the four sibs with MIA3 variant. However, they had more severe skeletal deformities closely resembling those observed in patients with TRIP11 variants. Our study suggests the presence of a phenotypic spectrum associated with MIA3 variants including ODCD with milder skeletal deformities, a classic ODCD with severe skeletal deformities, and a lethal skeletal dysplasia at the severe end of the spectrum. Although the striking phenotypic variability appears to be related to the type and or the location of the MIA3 variants, the influence of other factors cannot be ruled out.