Characterisation of skeletal development and the use of anabolic agents in murine models of Duchenne muscular dystrophy

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Duchenne muscular dystrophy (DMD) is a severe and ultimately fatal disease. It affects up to 1 in 4000 males and there is no cure. Steroids are the only treatment that can help slow down muscle weakness but they have many side effects including growth failure and osteoporosis. It is unclear exactly why, but even boys who are not treated with steroids tend to be short and have weak bones. We do not know whether the bone and growth problems are just caused by the muscle weakness (our bones need to be 'loaded' by muscle activity in order to strengthen and grow efficiently), or whether there is another primary problem. It may be that because DMD is a chronic inflammatory process, this disrupts the growth hormone (GH)/insulin-like growth factor (IGF-1) pathway. Growth hormone is produced by the anterior pituitary gland. It can act directly on the skeleton, but mainly acts indirectly, through IGF-1. Together the GH/IGF-1 pathway plays a crucial role in regulating normal bone growth and bone mass during childhood, and children with either GH or IGF-1 deficiency are short and have osteoporosis. There are no treatments available to help bone growth in DMD. We will investigate the role of GH and IGF-1 in bone and growth of mice models of DMD. The mouse model commonly used in DMD is the X-linked Muscular Dystrophy (mdx) mouse, but it has significant limitations because the disease process is not as severe and muscle regeneration occurs in the mdx mice, unlike in patients with Duchenne. Very few medications that are effective in the mdx mouse have shown to be useful in patients. Therefore we will use a new mouse model, the Cmah-/-mdx mouse. This mouse is bred from the mdx mouse, but also has an additional gene deletion to make it more 'human' and therefore we hope will behave in more similar way to patients with Duchenne. I will look, for the first time, at bone and growth in the Cmah-/-mdx and compare it to the mdx and a healthy 'wildtype' mouse. I will then give the mice steroids to determine the effects on their growth and skeleton. I will also see if the Cmah-/mdx mouse responds to GH and IGF-1, both alone and in combination. To do this I will use a range of techniques, including micro CT and labeling of the bones and growth plate to monitor the rate of growth. I will also culture metatarsals (feet) bones from the embryos of these mice models to see if they also display reduced growth and bone fragility. If they do this will show us that there is a primary problem relating to the skeleton in addition to the problems caused by steroids and muscle weakness. This work is very important because both GH and IGF-1 are readily available and are already used in children. If they are found to help bone development and encourage growth, either alone or acting together, in mice models, then this work could be carried out as a clinical trial in patients with DMD. It is also important to further describe the new Cmah-/-mdx mouse model, so that it can be used in other DMD experiments. The data that we collect may also help in the design of new therapies that are directed specifically at the mechanism of the problem that leads to poor bone health and growth in DMD.