Mutation Analysis in a Cohort of CSNB Patients: Novel Mutations in TRPM1 Gene

Abstract

Introduction: : Congenital stationary night blindness (CSNB) represents a group of low vision disorders with abnormal retinal neurotransmission in which patients show a negative ERG, reduced visual acuity, impaired night vision, myopia, nystagmus, strabismus and different modes of inheritance. In a cohort of 199 patients diagnosed with CSNB, we have identified 101 unique mutations in 142 patients; 65 were found in CACNA1F (15 in splice sites), 29 in NYX (17 single-nucleotide substitutions or small in-frame deletions or insertions), 7 in GRM6, and none in CAPB4. No mutations were identified in 53 patients with CSNB (27%), suggesting that additional genetic heterogeneity exists in CSNB. Recent findings that night blindness in the Appaloosa horse involved a lowered expressed of the TRPM1 gene and that TRPM1 is a binding partner of NYX strongly suggest that the TRPM1 gene a candidate for mutations in some patients with autosomal recessive complete CSNB.

Purpose: : We have undertaken a mutation analysis of the TRPM1 gene in 22 patients with complete CSNB (cCSNB), normal NYX and GRM6 genes and a family history not inconsistent with autosomal recessive inheritance.

Methods: : To detect DNA sequence changes, we PCR-amplified and sequenced genomic fragments that together span the exons and intronic flanking regions of TRPM1, similar to the methods we used in the analysis of CACNA1F, NYX, GRM6 and CAPB4.

Results: : We detected TRPM1 mutations in 5 of 22 cCSNB patients (4 missense and 1 splice site mutations; in comparison with recent observations from other centers, four of our TRPM1 mutations were novel). Each of the five patients with TRPM1 mutations had negative ERGs with unrecordable scotopic b-waves and normal flicker, impaired visual acuity and myopia, (mild to severe), nyctalopia, nystagmus and strabismus.

Conclusions: : The finding of mutations in the TRPM1 gene confirms and extends the basis of genetic heterogeneity in patients with complete CSNB. Overall, definitive DNA diagnosis of CSNB can now be established in over 75% of cases. For CSNB patients in which the genetic causes are still to be discovered, mutations are likely to be found among genes for other proteins that function in photoreceptor pre- and post-synaptic processes that affect retinal neurotransmission.

Keywords: retinal degenerations: hereditary • gene screening • mutations