Co-inheritance of the membrane frizzled-related protein ocular phenotype and glycogen storage disease type Ib.

Aim: To report co-occurrence of two rare recessive conditions, the membrane frizzled-related protein (MFRP)-related ocular phenotype and glycogen storage disease type 1b (GSD-1b), in three siblings in an Omani family.

Background: Biallelic mutations in the MFRP gene (chromosome 11q23) result in a distinct ocular phenotype characterized by retinitis pigmentosa, foveoschisis, optic nerve head drusen, and posterior microphthalmos. GSD-1b is an autosomal-recessive disorder caused by mutations in SLC37A4 gene located in the same chromosomal region.

Methods: An Omani family with three siblings diagnosed with GSD-1b presented with ocular manifestations of progressive visual impairment and diminution of night vision. All siblings underwent a standard ophthalmic and clinical genetic evaluation. Full sequencing of the MFRP and SLC37A4 genes and haplotype analysis was carried out.

Results: The three children (2F:1M) aged 13, 17, and 18 years were born to consanguineous parents. Their best-corrected visual acuity ranged from 20/60 to 20/15. Ophthalmic exam revealed bilateral optic disc drusen, foveoschisis, and pigmentary retinopathy, hyperopia of +12 to +15.5 diopters, and decreased axial length (15.8-16.39 mm) in all affected siblings. Full-field electroretinography showed rod-cone dysfunction. Sequence analysis revealed two novel variants in a homozygous state in the SLC37A4 and MFRP genes in all the affected patients.

Conclusions: We report the MFRP-related ocular phenotype in three siblings with GSD-1b. Molecular genetic studies identified novel mutations in the MFRP and SLC37A4 genes. Co-inheritance of a haplotype harboring mutations in both loci on chromosome 11q23 resulted in co-occurrence of the MFRP-related ocular phenotype and GSD-1b. This has not been reported previously.