Effect of Cyclosporine and Prednisone Treatment in Golden Retriever Muscular Dystrophy
Duchenne Muscular Dystrophy (DMD) is a lethal X-linked disorder induced by a mutated dystrophin gene producing a deficient protein. A canine relevant homologous disease is termed Golden Retriever Muscular Dystrophy (GRMD). To date, no curative treatment exists although different therapeutic modalities such as drugs, gene and cell therapies have been developed to amend the dystrophic phenotypes. However several of these protocols require immunosuppressive regimens such as cyclosporine (CsA) combined with prednisone to prevent rejection of the cells or the transgene.
The aim of the study was to determine the effects of this regimen regarding the clinical evolution of dystrophic dogs, the muscle pathology as well as the muscle force.
Eight dogs were obtained from a GRMD colony at the Veterinary School of Alfort and were cared for according to the European Guide for Care and Use of Laboratory Animals: 4 GRMD dogs were treated since 2 months of age with CsA (20 mg/kg/d) and prednisone (2 mg/kg/d), and 4 GRMD dogs were untreated controls. A clinical locomotion status was evaluated once a month since 2 months of age until 9 months of age, using a specific scale based on the intensity of splaying the digits, palmigrade/plantigrade stance, stiffness of the gait, joint ankylosis, muscular contractures and ability to stand up and to get over a fence. At 6 and 9 months of age, a biopsy from both tibialis cranialis muscles was stained for HE and alizarin red and by immunohistochemistry to evaluate the distribution of slow and fast-twitch fibres. At 4 and 6 months of age, the maximal isometric force generated by tarsal flexor muscle of the tibialis cranialis compartment was evaluated by supramaximal percutaneous stimulation of the peroneal nerve. The average of 6 tetanic contractions was measured.
Our results show a similar evolution of the locomotion but a predominance of fast fibres, an increase of calcified myofibers and a lower force measurement for treated dogs.
In literature, despite increase in calcified myofibers in GRMD dogs under prednisone treatment, functional benefit has been shown in DMD and GRMD. Additionally, administration of CsA (25 mg/kg/d) in mice induces a slow-to fast phenotype without affecting animal health or ambulation. However, fast fibres require dystrophin to produce force, whereas slow fibres seem not. Fast fibres are also known to be less resistant to fatigue therefore unable to withstand sustained tetanic contractions. Collectively, these two conditions may contribute to the reduced force in CsA/prednisone treated GRMD dogs due to the observed predominance of fast dystrophic fibres.
Other effects of this immunosuppressive regimen were constant such as papillomas, overweight and hepatic overload. In conclusion, our results indicate that CsA/prednisone treatment in GRMD dogs suggest possible detrimental consequences.