A canine orthologue of the human GFAP c.716G>A (p.Arg239His) variant causes Alexander disease in a Labrador retriever

A comparative medical genomics approach may facilitate the interpretation of rare missense variation

PURPOSE

To determine the degree to which likely causal missense variants of single-locus traits in domesticated species have features suggestive of pathogenicity in a human genomic context.

METHODS

We extracted missense variants from the Online Mendelian Inheritance in Animals database for nine animals (cat, cattle, chicken, dog, goat, horse, pig, rabbit and sheep), mapped coordinates to the human reference genome and annotated variants using genome analysis tools. We also searched a private commercial laboratory database of genetic testing results from >400 000 individuals with suspected rare disorders.

RESULTS

Of 339 variants that were mappable to the same residue and gene in the human genome, 56 had been previously classified with respect to pathogenicity: 31 (55.4%) pathogenic/likely pathogenic, 1 (1.8%) benign/likely benign and 24 (42.9%) uncertain/other. The odds ratio for a pathogenic/likely pathogenic classification in ClinVar was 7.0 (95% CI 4.1 to 12.0, p<0.0001), compared with all other germline missense variants in these same 220 genes. The remaining 283 variants disproportionately had allele frequencies and REVEL scores that supported pathogenicity.

CONCLUSION

Cross-species comparisons could facilitate the interpretation of rare missense variation. These results provide further support for comparative medical genomics approaches that connect big data initiatives in human and veterinary genetics.

An Overview of Canine Inherited Neurological Disorders with Known Causal Variants

Hereditary neurological conditions documented in dogs encompass congenital, neonatal, and late-onset disorders, along with both progressive and non-progressive forms. In order to identify the causal variant of a disease, the main two approaches are genome-wide investigations and candidate gene investigation. Online Mendelian Inheritance in Animals currently lists 418 Mendelian disorders specific to dogs, of which 355 have their likely causal genetic variant identified. This review aims to summarize the current knowledge on the canine nervous system phenes and their genetic causal variant. It has been noted that the majority of these diseases have an autosomal recessive pattern of inheritance. Additionally, the dog breeds that are more prone to develop such diseases are the Golden Retriever, in which six inherited neurological disorders with a known causal variant have been documented, and the Belgian Shepherd, in which five such disorders have been documented. DNA tests can play a vital role in effectively managing and ultimately eradicating inherited diseases.

Diagnostic features of type II fibrinoid leukodystrophy (Alexander disease) in a juvenile Beagle dog

A 3-month-old female entire Beagle presented with a progressive history of caudotentorial encephalopathy. Reactive encephalopathies were ruled out and tests for the most common infectious diseases agents were negative. Magnetic resonance imaging of the brain using a 1.5 Tesla scanner showed diffuse, bilateral, T2-weighted and T2-weighted-FLAIR hyperintense, T1-weighted hypointense, noncontrast-enhancing lesions involving the white matter of the cerebellum, brainstem, spinal cord, and forebrain to a lesser extent. There was cerebellar enlargement. Abnormalities were not detected on cerebrospinal fluid examination. Given the progressive nature of the disease and suspected poor prognosis the dog was euthanized. Histopathological analysis of the brain was consistent with fibrinoid leukodystrophy, also known as Alexander disease. Based on the classification used in humans, this is a description of MRI of a case of type II Alexander disease in veterinary medicine, with characteristics different to other described leukoencephalopathies in dogs.

Genetic prevalence and clinical relevance of canine Mendelian disease variants in over one million dogs

Hundreds of genetic variants implicated in Mendelian disease have been characterized in dogs and commercial screening is being offered for most of them worldwide. There is typically limited information available regarding the broader population frequency of variants and uncertainty regarding their functional and clinical impact in ancestry backgrounds beyond the discovery breed. Genetic panel screening of disease-associated variants, commercially offered directly to the consumer or via a veterinary clinician, provides an opportunity to establish large-scale cohorts with phenotype data available to address open questions related to variant prevalence and relevance. We screened the largest canine cohort examined in a single study to date (1,054,293 representative dogs from our existing cohort of 3.5 million; a total of 811,628 mixed breed dogs and 242,665 purebreds from more than 150 countries) to examine the prevalence and distribution of a total of 250 genetic disease-associated variants in the general population. Electronic medical records from veterinary clinics were available for 43.5% of the genotyped dogs, enabling the clinical impact of variants to be investigated. We provide detailed frequencies for all tested variants across breeds and find that 57% of dogs carry at least one copy of a studied Mendelian disease-associated variant. Focusing on a subset of variants, we provide evidence of full penetrance for 10 variants, and plausible evidence for clinical significance of 22 variants, on diverse breed backgrounds. Specifically, we report that inherited hypocatalasia is a notable oral health condition, confirm that factor VII deficiency presents as subclinical bleeding propensity and verify two genetic causes of reduced leg length. We further assess genome-wide heterozygosity levels in over 100 breeds, and show that a reduction in genome-wide heterozygosity is associated with an increased Mendelian disease variant load. The accumulated knowledge represents a resource to guide discussions on genetic test relevance by breed.

A Nonsense Variant in the DMD Gene Causes X-Linked Muscular Dystrophy in the Maine Coon Cat

(1) Feline dystrophin-deficient muscular dystrophy (ddMD) is a fatal disease characterized by progressive weakness and degeneration of skeletal muscles and is caused by variants in the DMD gene. To date, only two feline causal variants have been identified. This study reports two cases of male Maine coon siblings that presented with muscular hypertrophy, growth retardation, weight loss, and vomiting. (2) Both cats were clinically examined and histopathology and immunofluorescent staining of the affected muscle was performed. DMD mRNA was sequenced to identify putative causal variants. (3) Both cats showed a significant increase in serum creatine kinase activity. Electromyography and histopathological examination of the muscle samples revealed abnormalities consistent with a dystrophic phenotype. Immunohistochemical testing revealed the absence of dystrophin, confirming the diagnosis of dystrophin-deficient muscular dystrophy. mRNA sequencing revealed a nonsense variant in exon 11 of the feline DMD gene, NC_058386.1 (XM_045050794.1): c.1180C > T (p.(Arg394*)), which results in the loss of the majority of the dystrophin protein. Perfect X-linked segregation of the variant was established in the pedigree. (4) ddMD was described for the first time in the Maine coon and the c.1180C>T variant was confirmed as the causal variant.

Symptomatic care of late-onset Alexander disease presenting with area postrema-like syndrome with prednisolone; a case report

Background

Alexander disease (AxD) is classified into AxD type I (infantile) and AxD type II (juvenile and adult form). We aimed to determine the potential genetic cause(s) contributing to the AxD type II manifestations in a 9-year-old male who presented area postrema-like syndrome and his vomiting and weight loss improved after taking prednisolone.

Case presentation

A normal cognitive 9-year-old boy with persistent nausea, vomiting, and a significant weight loss at the age of 6 years was noticed. He also experienced an episode of status epilepticus with generalized atonic seizures. He showed non-febrile infrequent multifocal motor seizures at the age of 40 days which were treated with phenobarbital. He exhibited normal physical growth and neurologic developmental milestones by the age of six. Occasionally vomiting unrelated to feeding was reported. Upon examination at 9 years, a weak gag reflex, prominent drooling, exaggerated knee-deep tendon reflexes (3+), and nasal tone speech was detected. All gastroenterological, biochemical, and metabolic assessments were normal. Brain magnetic resonance imaging (MRI) revealed bifrontal confluent deep and periventricular white matter signal changes, fine symmetric frontal white matter and bilateral caudate nucleus involvements with garland changes, and a hyperintense tumefactive-like lesion in the brain stem around the floor of the fourth ventricle and area postrema with contrast uptake in post-contrast T1-W images. Latter MRI at the age of 8 years showed enlarged area postrema lesion and bilateral middle cerebellar peduncles and dentate nuclei involvements. Due to clinical and genetic heterogeneities, whole-exome sequencing was performed and the candidate variant was confirmed by Sanger sequencing. A de novo heterozygous mutation, NM_001242376.1:c.262 C > T;R88C in exon 1 of the GFAP (OMIM: 137,780) was verified. Because of persistent vomiting and weight loss of 6.0 kg, prednisolone was prescribed which brought about ceasing vomiting and led to weight gaining of 3.0 kg over the next 3 months after treatment. Occasional attempts to discontinue prednisolone had been resulting in the reappearance of vomiting.

Conclusions

This study broadens the spectrum of symptomatic treatment in leukodystrophies and also shows that R88C mutation may lead to a broad range of phenotypes in AxD type II patients.

GFAP variants leading to infantile Alexander disease: Phenotype and genotype analysis of 135 cases and report of a de novo variant

OBJECTIVES

Alexander disease (AxD) is a rare autosomal dominant disorder due to GFAP mutations; infantile AxD is the most common severe form which usually results in death. In this study, phenotype and genotype analysis of all reported cases with IAxD are reported as well as a de novo variant.

METHODS

We conduct a comprehensive review on all reported Infantile AxD due to GFAP mutation. Clinical data and genetics of the reported patients were analyzed. Clinical evaluations, pedigree drawing, MRI and sequencing of GFAP were performed.

RESULTS

135 patients clinically diagnosed with IAxD had GFAP mutations. A total of fifty three variants of GFAP were determined; 19 of them were located at 1A domain. The four common prevalent variants (c 0.715C>T, c 0.236G˃A, c 0.716G˃A, and c 0.235C˃T) were responsible for 64/135 (47.4%) of the patients. Seizure was the dominant clinical symptom (62.3%) followed by macrocephaly (41%), developmental delay (23.9%) and spasticity (23.9%). A de novo variant c 0.715C˃T was found in the presented Iranian case.

DISCUSSION

The majority of GFAP variant are located in a specific domain of the protein. Seizure as the most common symptom of IAxD could be considered. This study highlighted the role of genetic testing for diagnosing AxD.

Astrocytes in the Ventrolateral Preoptic Area Promote Sleep

Although ventrolateral preoptic (VLPO) nucleus is regarded as a center for sleep promotion, the exact mechanisms underlying the sleep regulation are unknown. Here, we used optogenetic tools to identify the key roles of VLPO astrocytes in sleep promotion. Optogenetic stimulation of VLPO astrocytes increased sleep duration in the active phase in naturally sleep-waking adult male rats (n = 6); it also increased the extracellular ATP concentration (n = 3) and c-Fos expression (n = 3-4) in neurons within the VLPO. In vivo microdialysis analyses revealed an increase in the activity of VLPO astrocytes and ATP levels during sleep states (n = 4). Moreover, metabolic inhibition of VLPO astrocytes reduced ATP levels (n = 4) and diminished sleep duration (n = 4). We further show that tissue-nonspecific alkaline phosphatase (TNAP), an ATP-degrading enzyme, plays a key role in mediating the somnogenic effects of ATP released from astrocytes (n = 5). An appropriate sample size for all experiments was based on statistical power calculations. Our results, taken together, indicate that astrocyte-derived ATP may be hydrolyzed into adenosine by TNAP, which may in turn act on VLPO neurons to promote sleep.SIGNIFICANCE STATEMENT Glia have recently been at the forefront of neuroscience research. Emerging evidence illustrates that astrocytes, the most abundant glial cell type, are the functional determinants for fates of neurons and other glial cells in the central nervous system. In this study, we newly identified the pivotal role of hypothalamic ventrolateral preoptic (VLPO) astrocytes in the sleep regulation, and provide novel insights into the mechanisms underlying the astrocyte-mediated sleep regulation.

Long-term survival of a dog with Alexander disease

A 1-year- and 11-month-old spayed female toy poodle had showed progressive ataxia and paresis in the hindlimbs since 11 months old. Magnetic resonance imaging revealed high signal intensity on T2-weighted and fluid-attenuated inversion recovery images at the thoracic and lumbar spinal cord. The dog’s neurological condition slowly deteriorated and flaccid tetraparesis was exhibited. At 4 years and 11 months old, the dog died of respiratory failure. On postmortem examination, eosinophilic corkscrew bundles (Rosenthal fibers) were observed mainly in the thoracic and lumbar spinal cord. Histological features were comparable to previously reported cases with Alexander disease. This is a first case report to describe the clinical course and long-term prognosis of a dog with Alexander disease.

TSEN54 missense variant in Standard Schnauzers with leukodystrophy

We report a hereditary leukodystrophy in Standard Schnauzer puppies. Clinical signs occurred shortly after birth or started at an age of under 4 weeks and included apathy, dysphoric vocalization, hypermetric ataxia, intension tremor, head tilt, circling, proprioceptive deficits, seizures and ventral strabismus consistent with a diffuse intracranial lesion. Magnetic resonance imaging revealed a diffuse white matter disease without mass effect. Macroscopically, the cerebral white matter showed a gelatinous texture in the centrum semiovale. A mild hydrocephalus internus was noted. Histopathologically, a severe multifocal reduction of myelin formation and moderate diffuse edema without inflammation was detected leading to the diagnosis of leukodystrophy. Combined linkage analysis and homozygosity mapping in two related families delineated critical intervals of approximately 29 Mb. The comparison of whole genome sequence data of one affected Standard Schnauzer to 221 control genomes revealed a single private homozygous protein changing variant in the critical intervals, TSEN54:c.371G>A or p.(Gly124Asp). TSEN54 encodes the tRNA splicing endonuclease subunit 54. In humans, several variants in TSEN54 were reported to cause different types of pontocerebellar hypoplasia. The genotypes at the c.371G>A variant were perfectly associated with the leukodystrophy phenotype in 12 affected Standard Schnauzers and almost 1000 control dogs from different breeds. These results suggest that TSEN54:c.371G>A causes the leukodystrophy. The identification of a candidate causative variant enables genetic testing so that the unintentional breeding of affected Standard Schnauzers can be avoided in the future. Our findings extend the known genotype-phenotype correlation for TSEN54 variants.

Various hereditary diseases of the cerebral white matter occur in humans and dogs. We describe a new leukodystrophy in Standard Schnauzers. Genetic mapping and whole genome sequence analysis identified a likely candidate causative variant in the TSEN54 gene encoding tRNA splicing endonuclease 54. These results provide new information about the role of TSEN54 in cell metabolism and the development of the central nervous system in the late gestational and early post-natal period. The affected dogs potentially represent a translational large animal model for similar leukoencephalopathies in human medicine. The clinical phenotype in Schnauzers included multifocal central nervous system signs. A holistic pathogenically driven understanding of disease initiation and perpetuation requires a solid analysis of the underlying genetics and characterization of the disease phenotype at the clinical and cellular as well as sub-cellular level. In contrast to the canine phenotype with a predominant manifestation in the cerebrum white matter, other TSEN54 variants in humans have been reported to result in a different pathological phenotype characterized by pontocerebellar hypoplasia. The differences between humans and dogs underscore the need for comparative analysis at the clinical, pathological and molecular level to understand species-specific protein mediated pathways, interactions and outcomes.

Truncating SLC12A6 variants cause different clinical phenotypes in humans and dogs

Clinical, pathological, and genetic findings of a primary hereditary ataxia found in a Malinois dog family are described and compared with its human counterpart. Based on the family history and the phenotype/genotype relationships already described in humans and dogs, a causal variant was expected to be found in KCNJ10. Rather surprisingly, whole-exome sequencing identified the SLC12A6 NC_006612.3(XM_014109414.2): c.178_181delinsCATCTCACTCAT (p.(Met60Hisfs*14)) truncating variant. This loss-of-function variant perfectly segregated within the affected Malinois family in an autosomal recessive way and was not found in 562 additional reference dogs from 18 different breeds, including Malinois. In humans, SLC12A6 variants cause "agenesis of the corpus callosum with peripheral neuropathy" (ACCPN, alias Andermann syndrome), owing to a dysfunction of this K⁺-Cl^- cotransporter. However, depending on the variant (including truncating variants), different clinical features are observed within ACCPN. The variant in dogs encodes the shortest isoform described so far and its resultant phenotype is quite different from humans, as no signs of peripheral neuropathy, agenesis of the corpus callosum nor obvious mental retardation have been observed in dogs. On the other hand, progressive spinocerebellar ataxia, which is the most important feature of the canine phenotype, hindlimb paresis, and myokymia-like muscle contractions have not been described in humans with ACCPN so far. As this is the first report of a naturally occurring disease-causing SLC12A6 variant in a non-human species, the canine model will be highly valuable to better understand the complex molecular pathophysiology of SLC12A6-related neurological disorders and to evaluate novel treatment strategies.

The novel homozygous KCNJ10 c.986T>C (p.(Leu329Pro)) variant is pathogenic for the SeSAME/EAST homologue in Malinois dogs

SeSAME/EAST syndrome is a multisystemic disorder in humans, characterised by seizures, sensorineural deafness, ataxia, developmental delay and electrolyte imbalance. It is exclusively caused by homozygous or compound heterozygous variations in the KCNJ10 gene. Here we describe a similar syndrome in two families belonging to the Malinois dog breed, based on clinical, neurological, electrodiagnostic and histopathological examination. Genetic analysis detected a novel pathogenic KCNJ10 c.986T>C (p.(Leu329Pro)) variant that is inherited in an autosomal recessive way. This variant has an allele frequency of 2.9% in the Belgian Malinois population, but is not found in closely related dog breeds or in dog breeds where similar symptoms have been already described. The canine phenotype is remarkably similar to humans, including ataxia and seizures. In addition, in half of the dogs clinical and electrophysiological signs of neuromyotonia were observed. Because there is currently no cure and treatment is nonspecific and unsatisfactory, this canine translational model could be used for further elucidating the genotype/phenotype correlation of this monogenic multisystem disorder and as an excellent intermediate step for drug safety testing and efficacy evaluations before initiating human studies.