blind sterile (bs) is a spontaneous autosomal-recessive mouse mutation discovered more than 30 years ago. Phenotypically, bs mice exhibit nuclear cataracts and male infertility; genetic analyses assigned the bs locus to mouse chromosome 2. In this study, we first positionally cloned the bs locus and identified a putative causative mutation in the Tbc1d20 gene. Functional analysis established the mouse TBC1D20 protein as a GTPase-activating protein (GAP) 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. Based on the function of TBC1D20 as a RABGAP and the bs cataract and testicular phenotypes, we hypothesized that mutations in TBC1D20 may contribute to Warburg micro syndrome (WARBM); WARBM constitutes a spectrum of disorders characterized by eye, brain, and endocrine abnormalities caused by mutations in RAB3GAP1, RAB3GAP2, and RAB18. Sequence analysis of a cohort of 77 families affected by WARBM identified five distinct TBC1D20 loss-of-function mutations, thereby 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. These findings collectively indicate that a defect in LD formation/metabolism may be a common cellular abnormality associated with WARBM, although it remains unclear whether abnormalities in LD metabolism are contributing to WARBM disease pathology.

Cone degeneration (cd ) is an autosomal recessive canine disease that occurs naturally in the Alaskan Malamute and German Shorthaired Pointer breeds. It is phenotypically similar to human achromatopsia, a heterogeneous autosomal recessive disorder associated with three distinct loci. Both the canine disease and its human counterparts are characterized by day-blindness and absence of retinal cone function in adults. We report linkage of the canine cd locus to marker C29.002 on canine chromosome 29 at recombination fraction theta = 0.0 with a maximum LOD score of 24.68 in a series of informative outbred pedigrees derived from cd-affected Alaskan Malamutes. Conserved gene order between CFA29 and the long arm of human chromosome 8 argued for homology between the cd locus and the human achromatopsia locus, ACHM3, at 8q21-22. The canine homolog of the cyclic nucleotide-gated channel beta-subunit gene (CNGB3), responsible for the human ACHM3 disease phenotype, was mapped within the zero-recombination interval for the cd locus. A deletion removing all exons of canine CNGB3 was identified in cd-affected Alaskan Malamute-derived dogs. A missense mutation in exon 6 (D262N, nucleotide 784) within a conserved region of the same gene was detected in German Shorthaired Pointers affected with an allelic disorder. Identification of these canine disorders as homologs of human ACHM3 underscores the power of recent developments in canine genomics, and provides a valuable system for exploring disease mechanisms and evaluating potential therapeutic measures in disorders of cone photoreceptors.

Galactokinase is an essential enzyme for the metabolism of galactose and its deficiency causes congenital cataracts during infancy and presenile cataracts in the adult population. We have cloned the human galactokinase cDNA, which maps to chromosome 17q24, and show that the isolated cDNA expresses galactokinase activity in bacteria and mammalian cells. We also describe two different mutations in this gene in unrelated families with galactokinase deficiency and cataracts. The availability of the cloned galactokinase gene provides an important reference to identify mutations in patients with galactokinase deficiency and cataracts.

Canine progressive rod-cone degeneration (prcd) is a retinal disease previously mapped to a broad, gene-rich centromeric region of canine chromosome 9. As allelic disorders are present in multiple breeds, we used linkage disequilibrium (LD) to narrow the approximately 6.4-Mb interval candidate region. Multiple dog breeds, each representing genetically isolated populations, were typed for SNPs and other polymorphisms identified from BACs. The candidate region was initially localized to a 1.5-Mb zero recombination interval between growth factor receptor-bound protein 2 (GRB2) and SEC14-like 1 (SEC14L). A fine-scale haplotype of the region was developed, which reduced the LD interval to 106 kb and identified a conserved haplotype of 98 polymorphisms present in all prcd-affected chromosomes from 14 different dog breeds. The findings strongly suggest that a common ancestor transmitted the prcd disease allele to many of the modern dog breeds and demonstrate the power of the LD approach in the canine model.

RAB18, RAB3GAP1, RAB3GAP2 and TBC1D20 are each mutated in Warburg Micro syndrome, a rare autosomal recessive multisystem disorder. RAB3GAP1 and RAB3GAP2 form a binary ‘RAB3GAP’ complex that functions as a guanine-nucleotide exchange factor (GEF) for RAB18, whereas TBC1D20 shows modest RAB18 GTPase-activating (GAP) activity in vitro. Here, we show that in the absence of functional RAB3GAP or TBC1D20, the level, localization and dynamics of cellular RAB18 is altered. In cell lines where TBC1D20 is absent from the endoplasmic reticulum (ER), RAB18 becomes more stably ER-associated and less cytosolic than in control cells. These data suggest that RAB18 is a physiological substrate of TBC1D20 and contribute to a model in which a Rab-GAP can be essential for the activity of a target Rab. Together with previous reports, this indicates that Warburg Micro syndrome can be caused directly by loss of RAB18, or indirectly through loss of RAB18 regulators RAB3GAP or TBC1D20.