Whole Exome Sequencing Identifies Mutations in GNB3 to Cause a Novel Congenital Stationary Night Blindness Phenotype

Abstract

Purpose : To identify the genetic cause underlying autosomal recessive congenital stationary night blindness (CSNB) in a family.

Methods : Ten of the 11 members in a three generation pedigree including 3 affected (2 siblings & maternal aunt) underwent detailed eye evaluation. Full-field electroretinogram (ERG) was done in 8 members. DNA was extracted from 11 members using classical methods. The proband (7 year old male) had been screened negative for mutations in the known CSNB genes by targeted next generation sequencing. Whole exome sequencing (WES) was performed on 2 trios in the family using Illumina Hi-Seq 2500 platform. On WES analysis, genes containing 2 rare non-synonymous coding or splicing variants (allele frequency <0.01) were prioritized in two affected; 13 genes were shared amongst the two fulfilling this criterion. Candidate genes were prioritized and variant validation & segregation analysis was performed by Sanger sequencing.

Results : The proband was compound heterozygous for an in-frame deletion (p.K57del) and a stop mutation (p.W338*) in GNB3. The affected aunt was homozygous for stop mutation (p.W338*). Both variants segregated with the disease phenotype in the family. Distance visual acuity was 20/30 or better in all affected; fundus was normal in all affected. Dim flash scotopic ERG (.01 cd.s.m^-2) showed reduced (n=2; sib-ship) or absent (n=1; aunt) rod b-wave. Scotopic ERG to a bright flash (10 cd.s.m^-2) showed electronegative response in all affected, consistent with rod ON-bipolar dysfunction. Photopic ERG to standard flash (3 cd.s.m^-2) and long flash (150ms) showed selective cone ON-bipolar involvement in the sib-ship, and cone ON- & OFF-bipolar involvement in the aunt. All single heterozygous carriers of GNB3 (n=5) had normal eye exam & normal ERG. Forty cases of CSNB previously excluded for mutations in known genes, did not reveal pathogenic variations in GNB3.

Conclusions : This is the first study to implicate GNB3 in a retinal disease. The CSNB phenotype in the family is variable but novel, and show 2 subtypes: the milder subtype shows partial but selective ON-bipolar abnormality & the severe subtype shows complete ON-bipolar involvement with additional OFF-bipolar involvement. Phenotype variability noted in the family may be mutation specific. The milder human phenotype resemble Gnb3 knock out mice models that show partial ON-bipolar pathway abnormality.

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