Lafora disease - from pathogenesis to treatment strategies.

Lafora disease is a severe, autosomal recessive, progressive myoclonus epilepsy. The disease usually manifests in previously healthy adolescents, and death commonly occurs within 10 years of symptom onset. Lafora disease is caused by loss-of-function mutations in EPM2A or NHLRC1, which encode laforin and malin, respectively. The absence of either protein results in poorly branched, hyperphosphorylated glycogen, which precipitates, aggregates and accumulates into Lafora bodies. Evidence from Lafora disease genetic mouse models indicates that these intracellular inclusions are a principal driver of neurodegeneration and neurological disease. The integration of current knowledge on the function of laforin-malin as an interacting complex suggests that laforin recruits malin to parts of glycogen molecules where overly long glucose chains are forming, so as to counteract further chain extension. In the absence of either laforin or malin function, long glucose chains in specific glycogen molecules extrude water, form double helices and drive precipitation of those molecules, which over time accumulate into Lafora bodies. In this article, we review the genetic, clinical, pathological and molecular aspects of Lafora disease. We also discuss traditional antiseizure treatments for this condition, as well as exciting therapeutic advances based on the downregulation of brain glycogen synthesis and disease gene replacement.

A novel missense mutation in ADAMTS10 in Norwegian Elkhound primary glaucoma.

Primary glaucoma is one of the most common causes of irreversible blindness both in humans and in dogs. Glaucoma is an optic neuropathy affecting the retinal ganglion cells and optic nerve, and elevated intraocular pressure is commonly associated with the disease. Glaucoma is broadly classified into primary open angle (POAG), primary closed angle (PCAG) and primary congenital glaucoma (PCG). Human glaucomas are genetically heterogeneous and multiple loci have been identified. Glaucoma affects several dog breeds but only three loci and one gene have been implicated so far. We have investigated the genetics of primary glaucoma in the Norwegian Elkhound (NE). We established a small pedigree around the affected NEs collected from Finland, US and UK and performed a genome-wide association study with 9 cases and 8 controls to map the glaucoma gene to 750 kb region on canine chromosome 20 (praw = 4.93×10-6, pgenome = 0.025). The associated region contains a previously identified glaucoma gene, ADAMTS10, which was subjected to mutation screening in the coding regions. A fully segregating missense mutation (p.A387T) in exon 9 was found in 14 cases and 572 unaffected NEs (pFisher = 3.5×10-27) with a high carrier frequency (25.3%). The mutation interrupts a highly conserved residue in the metalloprotease domain of ADAMTS10, likely affecting its functional capacity. Our study identifies the genetic cause of primary glaucoma in NEs and enables the development of a genetic test for breeding purposes. This study establishes also a new spontaneous canine model for glaucoma research to study the ADAMTS10 biology in optical neuropathy.

Increased expression of MERTK is associated with a unique form of canine retinopathy.

Progressive retinal degenerations are among the most common causes of blindness both in human and in dogs. Canine progressive retinal atrophy (PRA) resembles human retinitis pigmentosa (RP) and is typically characterized by a progressive loss of rod photoreceptors followed by a loss of cone function. The disease gradually progress from the loss of night and day vision to a complete blindness. We have recently described a unique form of retinopathy characterized by the multifocal gray/brown discoloration and thinning of the retina in the Swedish Vallhund (SV) breed. We aimed to identify the genetic cause by performing a genome wide association analysis in a cohort of 18 affected and 10 healthy control dogs using Illumina's canine 22k SNP array. We mapped the disease to canine chromosome 17 (p = 7.7×10(-5)) and found a 6.1 Mb shared homozygous region in the affected dogs. A combined analysis of the GWAS and replication data with additional 60 dogs confirmed the association (p = 4.3×10(-8), OR = 11.2 for homozygosity). A targeted resequencing of the entire associated region in four cases and four controls with opposite risk haplotypes identified several variants in the coding region of functional candidate genes, such as a known retinopathy gene, MERTK. However, none of the identified coding variants followed a compelling case- or breed-specific segregation pattern. The expression analyses of four candidate genes in the region, MERTK, NPHP1, ANAPC1 and KRCC1, revealed specific upregulation of MERTK in the retina of the affected dogs. Collectively, these results indicate that the retinopathy is associated with overexpression of MERTK, however further investigation is needed to discover the regulatory mutation for the better understanding of the disease pathogenesis. Our study establishes a novel gain-of-function model for the MERTK biology and provides a therapy model for retinopathy MERTK inhibitors. Meanwhile, a marker-based genetic counseling can be developed to revise breeding programs.

A CNGB1 frameshift mutation in Papillon and Phalène dogs with progressive retinal atrophy.

Progressive retinal degenerations are the most common causes of complete blindness both in human and in dogs. Canine progressive retinal atrophy (PRA) or degeneration resembles human retinitis pigmentosa (RP) and is characterized by a progressive loss of rod photoreceptor cells followed by a loss of cone function. The primary clinical signs are detected as vision impairment in a dim light. Although several genes have been associated with PRAs, there are still PRAs of unknown genetic cause in many breeds, including Papillons and Phalènes. We have performed a genome wide association and linkage studies in cohort of 6 affected Papillons and Phalènes and 14 healthy control dogs to map a novel PRA locus on canine chromosome 2, with a 1.9 Mb shared homozygous region in the affected dogs. Parallel exome sequencing of a trio identified an indel mutation, including a 1-bp deletion, followed by a 6-bp insertion in the CNGB1 gene. This mutation causes a frameshift and premature stop codon leading to probable nonsense mediated decay (NMD) of the CNGB1 mRNA. The mutation segregated with the disease and was confirmed in a larger cohort of 145 Papillons and Phalènes (PFisher = 1.4×10(-8)) with a carrier frequency of 17.2 %. This breed specific mutation was not present in 334 healthy dogs from 10 other breeds or 121 PRA affected dogs from 44 other breeds. CNGB1 is important for the photoreceptor cell function its defects have been previously associated with retinal degeneration in both human and mouse. Our study indicates that a frameshift mutation in CNGB1 is a cause of PRA in Papillons and Phalènes and establishes the breed as a large functional animal model for further characterization of retinal CNGB1 biology and possible retinal gene therapy trials. This study enables also the development of a genetic test for breeding purposes.

Genome-wide association study identifies a novel canine glaucoma locus.

Glaucoma is an optic neuropathy and one of the leading causes of blindness. Its hereditary forms are classified into primary closed-angle (PCAG), primary open-angle (POAG) and primary congenital glaucoma (PCG). Although many loci have been mapped in human, only a few genes have been identified that are associated with the development of glaucoma and the genetic basis of the disease remains poorly understood. Glaucoma has also been described in many dog breeds, including Dandie Dinmont Terriers (DDT) in which it is a late-onset (>7 years) disease. We designed clinical and genetic studies to better define the clinical features of glaucoma in the DDT and to identify the genetic cause. Clinical diagnosis was based on ophthalmic examinations of the affected dogs and 18 additionally investigated unaffected DDTs. We collected DNA from over 400 DTTs and a genome wide association study was performed in a cohort of 23 affected and 23 controls, followed by a fine mapping, a replication study and candidate gene sequencing. The clinical study suggested that ocular abnormalities including abnormal iridocorneal angles and pectinate ligament dysplasia are common (50% and 72%, respectively) in the breed and the disease resembles human PCAG. The genetic study identified a novel 9.5 Mb locus on canine chromosome 8 including the 1.6 Mb best associated region (p = 1.63 × 10(-10), OR = 32 for homozygosity). Mutation screening in five candidate genes did not reveal any causative variants. This study indicates that although ocular abnormalities are common in DDTs, the genetic risk for glaucoma is conferred by a novel locus on CFA8. The canine locus shares synteny to a region in human chromosome 14q, which harbors several loci associated with POAG and PCG. Our study reveals a new locus for canine glaucoma and ongoing molecular studies will likely help to understand the genetic etiology of the disease.