Functional characterization of novel deleterious MFSD8 mutations found by whole exome sequencing in early-onset isolated maculopathy

Abstract

Purpose : Non-syndromic autosomal recessive macular dystrophies are associated with mutations in a number of genes, of which ABCA4 is the most frequently mutated one. Sequencing of ABCA4 and whole exome sequencing (WES) was performed to identify the molecular cause of suspected atypical Stargardt disease in an eight-year-old girl.

Methods : The entire ABCA4 locus was enriched with a custom Haloplex panel followed by next-generation sequencing (NGS) (Miseq, Illumina). WES data was generated and analyzed (SureSelectXT HumanAllExon V5+UTRs, Agilent; NextSeq 500, Illumina; Ingenuity Variant Analysis, Qiagen). Mini-gene assays and cDNA sequencing on patient’s lymphocytes were used to study the effect of a donor splice variant in MFSD8. MFSD8 expression was assessed in lymphocytes with qPCR. Transmission electron microscopy (TEM) was performed on a patient's skin biopsy. The patient underwent a neurological examination and brain magnetic resonance imaging (MRI).

Results : Sequencing of the entire ABCA4 locus revealed one heterozygous variant p.(Ala1038Val). WES revealed two novel heterozygous MFSD8 variants: c.590del p.(Gly197Valfs*2) and c.439+3 A>C p.(=), occuring in trans. In vitro mini-gene assays and cDNA sequencing on patient’s lymphocytes demonstrated an out-of-frame skip of exon 5 p.(Ile67Glufs*3) resulting from c.439+3A>C. Expression of MFSD8 in patient’s lymphocytes was significantly reduced. TEM on a patient's skin biopsy showed lipopigment inclusions with characteristic and mixed lamellar profiles, as can be seen in cases with neuronal ceroid lipofuscinosis (NCL). The clinical neurological examination of the patient was normal, MRI of the brain showed slight cerebellar atrophy and discrete signs of cerebral cortical atrophy.

Conclusions : Two deleterious MFSD8 mutations were identified in a young patient with an isolated maculopathy. Roosing et al. (2015) reported a combination of mild and severe MFSD8 mutations in late-onset non-syndromic maculopathy. The combination of two severe MFSD8 mutations has only been seen in severe variant late-infantile NCL, unlike the presentation here. Through WES and downstream functional characterization we uncovered a potential syndromic maculopathy with poor outcome, despite the current absence of neurological manifestations. This study illustrates the power of WES to refine clinical diagnoses and to anticipate disease progression.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.