The IFITM5 mutation c.-14C > T results in an elongated transcript expressed in human bone; and causes varying phenotypic severity of osteogenesis imperfecta type V

Background

The genetic mutation resulting in osteogenesis imperfecta (OI) type V was recently characterised as a single point mutation (c.-14C > T) in the 5’ untranslated region (UTR) of IFITM5, a gene encoding a transmembrane protein with expression restricted to skeletal tissue. This mutation creates an alternative start codon and has been shown in a eukaryotic cell line to result in a longer variant of IFITM5, but its expression has not previously been demonstrated in bone from a patient with OI type V.

Methods

Sanger sequencing of the IFITM5 5’ UTR was performed in our cohort of subjects with a clinical diagnosis of OI type V. Clinical data was collated from referring clinicians. RNA was extracted from a bone sample from one patient and Sanger sequenced to determine expression of wild-type and mutant IFITM5.

Results

All nine subjects with OI type V were heterozygous for the c.-14C > T IFITM5 mutation. Clinically, there was heterogeneity in phenotype, particularly in the manifestation of bone fragility amongst subjects. Both wild-type and mutant IFITM5 mRNA transcripts were present in bone.

Conclusions

The c.-14C > T IFITM5 mutation does not result in an RNA-null allele but is expressed in bone. Individuals with identical mutations in IFITM5 have highly variable phenotypic expression, even within the same family.

A previously undescribed form of congenital disorder of glycosylation with variable presentation in siblings: Early fetal loss with hydrops fetalis, and infant death with hypoproteinemia

We present two siblings with a previously undescribed congenital disorder of glycosylation (CDG). The first child died in utero with severe hydrops fetalis and the second presented following preterm delivery with respiratory insufficiency, generalised edema and a protein-losing enteropathy. Both had a similar pattern of facial dysmorphism and joint contractures. The diagnosis of CDG-I was made following the birth of the second child based on the serum transferrin isoform pattern. CDG-Ia and -Ib were excluded by specific enzyme analysis. Joint contractures are a relatively uncommon finding in CDG, although fetal hydrops (CDG-Ia) and protein-losing enteropathy (CDG-Ib) are well recognized. CDG must be considered in the differential diagnosis of hydrops fetalis, congenital hypoproteinemia and death in early infancy, particularly when associated with dysmorphic features.