Ringo: Discordance between the molecular and clinical manifestation in a Golden Retriever Muscular Dystrophy dog

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Abstract

Of the various genetic homologues to Duchenne Muscular Dystrophy (DMD), the Golden Retriever Muscular Dystrophy (GRMD) dog, which presents a variable but usually severe and progressive muscle weakness, has the closest relevance to DMD in both clinical severity and histopathological change. Among 77 GRMD dogs born in our colony in Brazil, we have identified a very mildly affected dog, Ringo, born July 2003. Among his descendants, at least one male, Suflair, is also showing a mild course. In an attempt to better characterize these two dogs, we studied the pattern of muscle proteins expression in Ringo and Suflair, as compared to severely affected and normal control dogs. Dystrophin was absent in both and utrophin was overexpressed in a pattern similar to the observed in severely affected dogs. Understanding the mechanism that is protecting Ringo and Suflair from the deleterious effect of the dystrophin gene mutation is of utmost interest. In addition it points out that the clinical impact of therapeutic trials should be interpreted with caution.

Introduction

Duchenne Muscular Dystrophy (DMD), a recessive lethal X-linked disease, is the most frequent and severe form of muscular dystrophy. It is caused by a complete absence of dystrophin in skeletal muscle due to mutations in the dystrophin gene [1]. DMD is manifested in young boys and the clinical course is severe and progressive. Affected individuals usually are confined to a wheelchair by 10–12 years of age and without special care and assisted ventilation death owing to respiratory or cardiac failure occurs usually before the third decade [2]. The Golden Retriever Muscular Dystrophy (GRMD) dogs represent an excellent genetic homologue for human DMD. Affected animals have a frameshift point mutation within the splice acceptor site in intron 6 of the dystrophin gene, which results in the absence of the muscle protein. GRMD dogs and DMD patients have many phenotypic and biochemical similarities, including early progressive muscle degeneration and atrophy, fibrosis, contractures and raised serum creatine kinase levels [3]. However, differently from humans, GRMD may have difficulties in swallowing and loss of ambulation is uncommon. Death may occur within the first weeks but is most frequent around 1 or 2 years of age as a result of failure of respiratory or cardiac muscles.

There is a great clinical variability among GRMD dogs. Perinatal death is frequent but some dogs may have a longer survival and be less severely affected. Since we have established the Brazilian GRMD colony, which was initiated in April 2000 with the female carrier Beth, donated by Dr. Joe Kornegay (University of North Carolina), 77 affected dogs were born. Among them, we identified Ringo (Fig. 1A), born in July 2003 presenting a mild phenotype clearly distinguishable from other affected dogs, despite complete muscle dystrophin deficiency [4].

Ringo is able to run, to jump and stay on his hindlegs more easily than the other affected dogs. Furthermore, Ringo is, to our knowledge, the only reported affected GRMD dog that has sufficient muscular strength to open doors (Fig. 1C). He was able to breed naturally, originating an offspring of 26 affected dogs (18 males and 8 females), 12 female carriers and 11 normal males (Fig. 2). Interestingly, one of his male descendant named Suflair (Fig. 1B), currently aged 3 years, is showing a milder course, although less than his father, which rules out the possibility of an unknown X-linked modifier gene responsible for this milder phenotype.

Understanding the mechanism that is protecting these two dogs of the deleterious effect of the dystrophin gene mutation is of utmost interest aiming future therapies. On the other hand, the great phenotypic variability observed among GRMD dogs turns very difficult to analyze the clinical success of therapeutic trials since the existence of animals like Ringo and Suflair could lead to misleading or inconclusive results in preclinical studies using GRMD dogs.

Section snippets

Animals

All animals were housed and cared for in the University of São Paulo and genotyped at birth as previously described [5]. Studies were done after approval by the Ethical Committee of Animal Use of the Institute of Biosciences, University of São Paulo.

Functional assessment

After being weighed, the dogs were submitted to several tests reported in previous studies [6], [7]: (a) the ambulation test to determine the mean length of a step measured in hind paw ink prints; (b) the jumping test to evaluate the maximum height

Body weight, serum CK and functional assessment

Severely affected dogs weighed on average 19.6 ± 2.3 kg (ranging from 14.3 to 22.9). GRMD carrier females weighed on average 27.4 ± 0.5 kg (ranging from 26.6 to 28.3), which was comparable to Ringo (29.1 kg) and Suflair (25.7 kg). Normal males weight about 32 kg.

Serum CK assessed August 2009 showed that Ringo’s CK was increased about 57-fold above normal levels (up to 170 U/L) Suflair’s CK was 105-fold above normal, while the average serum CK in severely affected animals was 96-fold above average.

Discussion

A mild phenotype despite the complete absence of muscle dystrophin has been reported in the mdx mouse [10] and more recently in a 7-year-old DMD boy [11]. To our knowledge it was never reported in GRMD dogs although mildly affected dogs were also observed in the National Center for Canine Models of Duchenne Muscular Dystrophy at the University of North Carolina-Chapel Hill (Joe Kornegay, personal communication). While the phenotype is highly variable, even among dogs from the same litter, most

Acknowledgements

We gratefully acknowledge Dr. Joe Kornegay (University of North Carolina-Chapel Hill) who donated our first GRMD carrier female and Janet Bogan for unvaluable support. We thank Dr. Glenn Morris and Dr. Nguyen thi Man (The North East Wales Institute, Wrexham, UK) for kindly providing the MANCHO3 antibody. We are also grateful to Dr. Ronaldo Jun Yamato for cardiological evaluation of GRMD dogs. We additionally thank Constância Urbani, Marta Canovas and Maria Denise Fernandes Carvalho. This work

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