Mild feline mucopolysaccharidosis type VI. Identification of an N-acetylgalactosamine-4-sulfatase mutation causing instability and increased specific activity

Mucopolysaccharidosis VI in a European Shorthair cat: Neurological presentation, computed tomography findings and genetic investigation

The present case report describes the clinical signs of a 10-month-old, intact female, Domestic Shorthair cat presented with a history of chronic progressive difficulty to walk with the four limbs. The physical and neurological examinations revealed skeletal deformities, corneal opacity and a severe spastic non-ambulatory tetraparesis. Complete blood count and biochemistry profiles were unremarkable. Diffuse bone rarefaction, hyperostosis and an apparent fusion of the vertebral bodies were observed on spinal radiographs. A non-contrast computed tomography (CT) exam of the whole body of the patient was performed. Based on the medical history, clinical findings, laboratory analysis, spinal radiographs and CT findings, a lysosomal storage disorder was suspected. Genetic testing for mucopolysaccharidosis VI and VII revealed a genetic mutation, ARSB variant L476P, confirming the diagnosis of mucopolysaccharidosis type VI.

BMN 250, a fusion of lysosomal alpha-N-acetylglucosaminidase with IGF2, exhibits different patterns of cellular uptake into critical cell types of Sanfilippo syndrome B disease pathogenesis

Sanfilippo syndrome type B (Sanfilippo B; Mucopolysaccharidosis type IIIB) occurs due to genetic deficiency of lysosomal alpha-N-acetylglucosaminidase (NAGLU) and subsequent lysosomal accumulation of heparan sulfate (HS), which coincides with devastating neurodegenerative disease. Because NAGLU expressed in Chinese hamster ovary cells is not mannose-6-phosphorylated, we developed an insulin-like growth factor 2 (IGF2)-tagged NAGLU molecule (BMN 250; tralesinidase alfa) that binds avidly to the IGF2 / cation-independent mannose 6-phosphate receptor (CI-MPR) for glycosylation independent lysosomal targeting. BMN 250 is currently being developed as an investigational enzyme replacement therapy for Sanfilippo B. Here we distinguish two cellular uptake mechanisms by which BMN 250 is targeted to lysosomes. In normal rodent-derived neurons and astrocytes, the majority of BMN250 uptake over 24 hours reaches saturation, which can be competitively inhibited with IGF2, suggestive of CI-MPR-mediated uptake. K_uptake, defined as the concentration of enzyme at half-maximal uptake, is 5 nM and 3 nM in neurons and astrocytes, with a maximal uptake capacity (V_max) corresponding to 764 nmol/hr/mg and 5380 nmol/hr/mg, respectively. Similar to neurons and astrocytes, BMN 250 uptake in Sanfilippo B patient fibroblasts is predominantly CI-MPR-mediated, resulting in augmentation of NAGLU activity with doses of enzyme that fall well below the K_uptake (5 nM), which are sufficient to prevent HS accumulation. In contrast, uptake of the untagged recombinant human NAGLU (rhNAGLU) enzyme in neurons, astrocytes and fibroblasts is negligible at the same doses tested. In microglia, receptor-independent uptake, defined as enzyme uptake resistant to competition with excess IGF2, results in appreciable lysosomal delivery of BMN 250 and rhNAGLU (V_max = 12,336 nmol/hr/mg and 5469 nmol/hr/mg, respectively). These results suggest that while receptor-independent mechanisms exist for lysosomal targeting of rhNAGLU in microglia, BMN 250, by its IGF2 tag moiety, confers increased CI-MPR-mediated lysosomal targeting to neurons and astrocytes, two additional critical cell types of Sanfilippo B disease pathogenesis.

Mucopolysaccharidosis Type VI in a Great Dane Caused by a Nonsense Mutation in the ARSB Gene

Mucopolysaccharidoses are inherited metabolic disorders that result from a deficiency of lysosomal enzymes required for the catabolism of glycosaminoglycans. Lysosomal glycosaminoglycan accumulation results in cell and organ dysfunction. This study characterized the phenotype and genotype of mucopolysaccharidosis VI in a Great Dane puppy with clinical signs of stunted growth, facial dysmorphia, skeletal deformities, corneal opacities, and increased respiratory sounds. Clinical and pathologic evaluations, urine glycosaminoglycan analyses, lysosomal enzyme assays, and ARSB sequencing were performed. The urine mucopolysaccharide spot test was strongly positive predominantly due to the accumulation of dermatan sulfate. Enzyme assays in leukocytes and tissues indicated a deficiency of arylsulfatase B (ARSB) activity. Histologic examination revealed cytoplasmic vacuoles in many tissues. Analysis of the exonic ARSB DNA sequences from the affected puppy compared to the published canine genome sequence revealed a homozygous nonsense mutation (c.295C>T) in exon 1, replacing glutamine with a premature stop codon (p.Gln99*), predicting no enzyme synthesis. A polymerase chain reaction-based restriction fragment length polymorphism test was established to assist with the clinical diagnosis and breeding of Great Danes. This genotyping test revealed that the clinically healthy parents and some other relatives of the puppy were heterozygous for the mutant allele, but all 200 clinically healthy dogs screened including 15 Great Danes were homozygous for the normal allele. This ARSB mutation is the fourth identified genetic variant causing canine mucopolysaccharidosis VI. Mucopolysaccharidosis VI is the first lysosomal storage disorder described in Great Danes but does not appear to be widespread in this breed.

Mucopolysaccharidosis VI in cats - clarification regarding genetic testing

The release of new DNA-based diagnostic tools has increased tremendously in companion animals. Over 70 different DNA variants are now known for the cat, including DNA variants in disease-associated genes and genes causing aesthetically interesting traits. The impact genetic tests have on animal breeding and health management is significant because of the ability to control the breeding of domestic cats, especially breed cats. If used properly, genetic testing can prevent the production of diseased animals, causing the reduction of the frequency of the causal variant in the population, and, potentially, the eventual eradication of the disease. However, testing of some identified DNA variants may be unwarranted and cause undo strife within the cat breeding community and unnecessary reduction of gene pools and availability of breeding animals. Testing for mucopolysaccharidosis Type VI (MPS VI) in cats, specifically the genetic testing of the L476P (c.1427T>C) and the D520N (c.1558G>A) variants in arylsulfatase B (ARSB), has come under scrutiny. No health problems are associated with the D520N (c.1558G>A) variant, however, breeders that obtain positive results for this variant are speculating as to possible correlation with health concerns. Birman cats already have a markedly reduced gene pool and have a high frequency of the MPS VI D520N variant. Further reduction of the gene pool by eliminating cats that are heterozygous or homozygous for only the MPS VI D520N variant could lead to more inbreeding depression effects on the breed population. Herein is debated the genetic testing of the MPS VI D520N variant in cats. Surveys from different laboratories suggest the L476P (c.1427T>C) disease-associated variant should be monitored in the cat breed populations, particularly breeds with Siamese derivations and outcrosses. However, the D520N has no evidence of association with disease in cats and testing is not recommended in the absence of L476P genotyping. Selection against the D520N is not warranted in cat populations. More rigorous guidelines may be required to support the genetic testing of DNA variants in all animal species.

DNA mutations of the cat: the good, the bad and the ugly

PRACTICAL RELEVANCE

The health of the cat is a complex interaction between its environment (nurture) and its genetics (nature). Over 70 genetic mutations (variants) have been defined in the cat, many involving diseases, structural abnormalities and clinically relevant health concerns. As more of the cat's genome is deciphered, less commonly will the term 'idiopathic' be used regarding the diagnosis of diseases and unique health conditions. State-of-the-art health care will include DNA profiling of the individual cat, and perhaps its tumor, to establish the best treatment approaches. Genetic testing and eventually whole genome sequencing should become routine diagnostics for feline health care.

GLOBAL IMPORTANCE

Cat breeds have disseminated around the world. Thus, practitioners should be aware of the breeds common to their region and the mutations found in those regional populations. Specific random-bred populations can also have defined genetic characteristics and mutations.

AUDIENCE

This review of 'the good, the bad and the ugly' DNA variants provides the current state of knowledge for genetic testing and genetic health management for cats. It is aimed at feline and general practitioners wanting to update and review the basics of genetics, what tests are available for cats and sources for genetic testing. The tables are intended to be used as references in the clinic. Practitioners with a high proportion of cat breeder clientele will especially benefit from the review.

EVIDENCE BASE

The data presented is extracted from peer-reviewed publications pertaining to mutation identification, and relevant articles concerning the heritable trait and/or disease. The author also draws upon personal experience and expertise in feline genetics.

Hypervitaminosis A-induced hepatic fibrosis in a cat

RATIONALE

The excessive intake of vitamin A in the form of vitamin concentrate, supplement or vitamin-rich liver can result in hypervitaminosis A in man and animals. Although osteopathologies resulting from chronic vitamin A intoxication in cats are well characterized, no information is available concerning feline hypervitaminosis A-induced liver disease.

CLINICAL SUMMARY

We report the first case of hepatic stellate cell lipidosis and hepatic fibrosis in a domestic cat that had been fed a diet based on raw beef liver. Radiographic examination revealed exostoses and ankylosis between vertebrae C1 and T7, compatible with deforming cervical spondylosis. Necropsy showed a slightly enlarged and light yellow to bronze liver. Microscopic and ultrastructural analyses of liver tissues revealed diffuse and severe liver fibrosis associated with hepatic stellate cell hyperplasia and hypertrophy. These cells showed immunopositive staining for α-smooth muscle actin and desmin markers. The necropsy findings of chronic liver disease coupled with osteopathology supported the diagnosis of hypervitaminosis A.

PRACTICAL RELEVANCE

As in human hepatology, if there is dietary evidence to support increased intake of vitamin A, then hypervitaminosis A should be considered in the differential diagnosis of chronic liver disease in cats.

Genetic testing in domestic cats

Varieties of genetic tests are currently available for the domestic cat that support veterinary health care, breed management, species identification, and forensic investigations. Approximately thirty-five genes contain over fifty mutations that cause feline health problems or alterations in the cat's appearance. Specific genes, such as sweet and drug receptors, have been knocked-out of Felidae during evolution and can be used along with mtDNA markers for species identification. Both STR and SNP panels differentiate cat race, breed, and individual identity, as well as gender-specific markers to determine sex of an individual. Cat genetic tests are common offerings for commercial laboratories, allowing both the veterinary clinician and the private owner to obtain DNA test results. This article will review the genetic tests for the domestic cat, and their various applications in different fields of science. Highlighted are genetic tests specific to the individual cat, which are a part of the cat's genome.

Feline genetics: clinical applications and genetic testing

DNA testing for domestic cat diseases and appearance traits is a rapidly growing asset for veterinary medicine. Approximately 33 genes contain 50 mutations that cause feline health problems or alterations in the cat's appearance. A variety of commercial laboratories can now perform cat genetic diagnostics, allowing both the veterinary clinician and the private owner to obtain DNA test results. DNA is easily obtained from a cat via a buccal swab with a standard cotton bud or cytological brush, allowing DNA samples to be easily sent to any laboratory in the world. The DNA test results identify carriers of the traits, predict the incidence of traits from breeding programs, and influence medical prognoses and treatments. An overall goal of identifying these genetic mutations is the correction of the defect via gene therapies and designer drug therapies. Thus, genetic testing is an effective preventative medicine and a potential ultimate cure. However, genetic diagnostic tests may still be novel for many veterinary practitioners and their application in the clinical setting needs to have the same scrutiny as any other diagnostic procedure. This article will review the genetic tests for the domestic cat, potential sources of error for genetic testing, and the pros and cons of DNA results in veterinary medicine. Highlighted are genetic tests specific to the individual cat, which are a part of the cat's internal genome.

An index case for the attenuated end of the mucopolysaccharidosis type VI clinical spectrum

Mucopolysaccharidosis type VI (MPS VI, Maroteaux-Lamy syndrome, McKusick #253200) is a lysosomal storage disorder that is caused by a deficiency in the lysosomal exohydrolase N-acetylgalactosamine-4-sulphatase (4-sulphatase, EC 3.1.6.1). We report a patient with no obvious clinical signs of MPS VI that has 5% of normal 4-sulphatase catalytic capacity. This patient represents an index case for the attenuated end of the MPS VI clinical spectrum.

Prevalence of mucopolysaccharidosis type VI mutations in Siamese cats

Mucopolysaccharidosis type VI (MPS VI), a lysosomal storage disease, is one of the more prevalent inherited diseases in cats and is commonly found in cats with Siamese ancestry. The prevalence of 2 known MPS VI mutations in cats was investigated in 101 clinically normal Siamese cats, in 2 cats with clinical signs of MPS VI, and in 202 cats from 4 research colonies. The mutation L476P which causes a severe clinical phenotype, was present on both alleles in the known MPS VI cats from Italy and North America and was present in all research colonies that originated from North America. However, LA76P was not detected in the Siamese population screened. In contrast, the mutation D520N, which causes a mild clinical phenotype, was identified in 23 of 202 (11.4%) alleles tested in Siamese cats from 3 continents, 2 of which were homozygous for D520N. Thus, the D520N mutation was widespread, and it is likely that cats inheriting both mutations (LA76P/D520N compound heterozygotes) would be in the general Siamese population, particularly in North America. Practitioners should note the high incidence of degenerative joint disease in these animals.

Mucopolysaccharidosis type VI: Structural and clinical implications of mutations in N-acetylgalactosamine-4-sulfatase

Mucopolysaccharidosis type VI (MPS-VI) is an autosomal recessive lysosomal storage disorder caused by the deficiency of N-acetylgalactosamine-4-sulfatase (4S; or ARSB). Mutations in the 4S gene are responsible for 4S deficiency, which leads to the intralysosomal storage of partially degraded glycosaminoglycans, dermatan sulfate, and chondroitin 4-sulfate. To date, a total of 45 clinically relevant mutations have been identified in the human 4S gene. Missense mutations are the largest group, with 31 identified mutations. Nonsense mutations and small insertions or deletions comprise the remainder, with seven mutations each. Six polymorphisms have also been reported: two amino acid substitutions and four silent transitions. Mapping of the missense mutations onto the 4S structure shows that they are distributed throughout the three subunits of the mature 4S polypeptide. Mutations have been identified in active site residues, in residues adjacent to the active site, in potential substrate binding residues, in residues exposed on the surface, and in residues buried within the protein core. Missense mutations have also been identified in disulfide crosslinks. Molecular modeling of MPS-VI mutations onto the 4S structure suggests that the majority cause 4S deficiency via destabilization and the consequent reduction of 4S protein concentration. The vast majority of MPS-VI mutant alleles are either unique to a patient or are present in a small number of patients. So far, no common mutations have been described. Therefore, screening of the general population for MPS-VI alleles will be difficult.

Mucopolysaccharidosis type IIIB: characterisation and expression of wild-type and mutant recombinant alpha-N-acetylglucosaminidase and relationship with sanfilippo phenotype in an attenuated patient

Mucopolysaccharidosis type IIIB (MPS-IIB) is a lysosomal storage disorder characterised by the defective degradation of heparan sulfate due to a deficiency of alpha-N-acetylglucosaminidase (NAG). The clinical severity of MPS-IIIB ranges from an attenuated to severely affected Sanfilippo phenotype. This paper describes the expression and characterisation of wild-type recombinant NAG and the molecular characterisation of a previously identified R297X/F48L compound heterozygous MPS-IIIB patient with attenuated Sanfilippo syndrome. We have previously shown R297X to be the most common mutation in a cohort of Dutch and Australian patients, occurring at a frequency of approximately 12.5%. To date F48L has only been described in the proband. To determine the contribution of each mutation to the overall clinical phenotype of the patient, both mutant alleles were engineered into the wild-type NAG cDNA and expressed in Chinese hamster ovary cells. The wild-type NAG and F48L mutant alleles were also retrovirally expressed in MPS-IIIB skin fibroblasts. Residual NAG activity and the stability and maturation of immunoprecipitated NAG were determined for wild-type NAG and mutant NAG. The combined biochemical phenotypes of the two NAG mutant alleles demonstrated a good correspondence with the observed attenuated Sanfilippo phenotype of the patient.

Evaluation of fibroblast-mediated gene therapy in a feline model of mucopolysaccharidosis type VI

Fibroblast-mediated ex vivo gene therapy was evaluated in the N-acetylgalactosamine 4-sulfatase (4S) deficient mucopolysaccharidosis type VI (MPS VI) cat. Skin biopsies were obtained at birth from severely affected MPS VI kittens and used to initiate fibroblast outgrowths for retroviral transduction with the 4S cDNA. 4S gene expression in transduced cells was under the transcriptional control of the MoMLV long terminal repeat promoter or the cytomegalovirus (CMV) immediate-early promoter. Characterisation of gene-transduced fibroblasts demonstrated the cells to be over-expressing 4S activity. Twenty-four to forty million autologous, gene-corrected fibroblasts were implanted under the renal capsule of three MPS VI kittens at 8-16 weeks of age. Transient, low levels of 4S activity were detected in peripheral blood leukocytes shortly after implantation but were not detectable within 3-8 weeks' post-implantation. Long-term biochemical and clinical evaluation of these cats demonstrated identical disease progression to that previously described in untreated, clinically severe MPS VI cats.