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D J Sidjanin, Adam Ronchetti; Mutations in TBC1D20 cause cataracts in blind sterile (bs) mice and Warburg Micro syndrome in humans. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5040.
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© ARVO (1962-2015); The Authors (2016-present)
Blind sterile (bs) is a spontaneous autosomal recessive mouse mutation discovered over 30 years ago. Phenotypically, bs mice exhibit nuclear cataracts and male infertility; genetic analyses assigned the bs locus to mouse chromosome 2. The goal of this study was to identify the mutation responsible for cataracts in bs mice. Additionally, we wanted to determine if mutations in the same gene contribute to human cataracts.
1177 F2 progeny were generated phenotyped for cataracts, euthanized, and genotyped. Linkage data was analyzed with MapManager QTX software. cDNA and genomic DNA sequencing from bs/bs and bs/+ mice was utilized to screen open reading frames and intron/exon junctions of candidate genes. Our cohort consisted of 77 individuals with a spectrum of WARBM disorders and was screened for mutations in TBC1D20 using direct sequence analysis of the genomic DNA extracted from venous blood according to standard procedures. Mouse and human fibroblasts were isolated, maintained and evaluated using standard protocols.
We positionally cloned the bs locus and identified a mutation in the Tbc1d20 gene. Functional analysis established TBC1D20 as a GTPase activating protein for RAB1 and RAB2, and bs as a TBC1D20 loss-of-function mutation. Evaluation of bs mouse embryonic fibroblasts (mEFs) identified enlarged Golgi morphology and aberrant lipid droplet (LD) formation. We hypothesized that mutations in TBC1D20 may contribute to Warburg Micro syndrome (WARBM). Sequence analysis of a cohort of 77 WARBM families identified five distinct TBC1D20 loss-of-function mutations, thus establishing these mutations as causative of WARBM. Evaluation of human fibroblasts deficient in TBC1D20 function identified aberrant LDs similar to those identified in the bs mEFs. Additionally, our results show that human fibroblasts deficient in RAB18 and RAB3GAP1 function also exhibit aberrant LD formation.
Results from this study established that loss-of-function mutations in TBC1D20 cause cataracts and male infertility in blind sterile (bs) mice and Warburg Micro syndrome in humans. Aberrant LDs seem to be the only overlapping cell abnormality suggesting that a defect in LD metabolism is associated with WARBM. TBC1D20 is an essential protein for lens transparency in both humans and mice although the function of TBC1D20 in the lens remains unknown and requires further investigation.
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