Animal Abstracts - December 2005
Unstable DNA and human disease
Date: Mon Feb 27 15:48:54 GMT 2006
A number of inherited human disorders, including myotonic dystrophy and Huntington disease, are associated with highly mutable genes. We are using genetically modified mice to understand why the genes in these disorders are so unstable and how we might use this information to provide effective therapies.
A growing number of inherited human disorders, including myotonic dystrophy, fragile X syndrome, Huntington disease, Friedreich ataxia and several of the spinocerebellar ataxias are associated with the expansion of a repeated segment of DNA within the gene. All of these disorders are associated with extreme clinical variability and unusual inheritance patterns within families. For each disorder the associated gene contains a repeated segment of DNA within the gene. The number of copies of this repeat varies between people in the general population, but is of restricted size (usually <35 repeats). In contrast, patients have expanded genes with usually greater than 50 repeats. Patients with more repeats have an earlier age of onset and more severe symptoms. Normally, genes are passed stably from one cell to another and from one generation to the next. In contrast however, the expanded disease-causing repeats are very unstable. They nearly always get bigger when passed from parent to child. Because of this the age of onset of these disorders decreases from one generation to the next. The expanded disease associated repeats also get bigger in all the other cells in the body throughout the lifetime of the patient. These properties contribute toward the tissue specificity and progressive nature of the symptoms.
These disorders currently remain incurable with little in the way of effective therapies available and little knowledge of the environmental factors (e.g. diet, smoking, chemical exposure etc) that modify disease progression. Moreover, although it is now possible to give a precise 'yes' or 'no' genetic diagnosis, it is not possible to provide accurate prognostic information to patients or risk assessments to future generations. This is at least in part associated with our current lack of understanding of how the repeat length changes in patients and the genetic and environmental factors that modify these processes. The knowledge gained from these experiments should lead to a better understanding of the genetic and environmental factors that underlie the incidence of human diseases associated with unstable DNA, their unusual inheritance patterns and the progressive nature of the symptoms. In the short to medium term this will allow us to provide more accurate prognostic information to individuals and more accurate risk assessments for future generations. In the long run a thorough grasp of the underlying genetics will open up new avenues for therapeutic intervention.
Much of the ongoing research in our laboratory involves the use of human samples, but confounding effects of age, genetic differences between people, uncontrolled environmental effects etc, in addition to the non-availability of certain tissues, developmental time points and the inability to perform specific breeding experiments or environmental manipulation means that the experimental objectives proposed here cannot yet be achieved using human patients. We have used cells grown in test-tubes, both human and mouse, in many of our experiments and will continue to do so. Indeed, many of the objectives proposed here build upon work that has been piloted in test-tube experiments, therefore reducing the number of animals we have used to date and optimising the experiments we have proposed to take forward. However, human and mouse cells grown in the laboratory do not always faithfully reproduce the effects observed in whole organisms and cannot be used to address phenomena associated with mammalian development or inheritance from one generation to the next.
The unstable DNA associated with disorders such as myotonic dystrophy and Huntington disease has not yet been observed in any species other than humans. A great many laboratories have tried to model these phenomena in simple organisms such as bacteria, yeast, the nematode worm and the fruit fly. Unfortunately, in none of these model organisms has the genetic instability observed in humans yet been reproduced. In contrast, a number of genetically modified mouse models have been produced in which the instability seen in myotonic dystrophy and Huntington disease is faithfully reproduced. The mouse thus remains the most appropriate organism in which to understand these phenomena, in particular the complex factors associated with mammalian development and inheritance.
In order to identify cellular factors that modify the genetic stability of expanded repeats we will study the tissues from genetically modified mice that we have generated previously in which the genetic instability is replicated (~500 animals per year). These experiments will provide direct insights into the dynamics and molecular mechanisms mediating genetic instability in humans providing candidate genetic and environmental modifiers and targets for the rational design of therapeutic agents. In this part of the research all of the experiments will be performed on tissues taken from mice post mortem and will not involve any invasive manipulations. Moreover, many experiments will actually be performed entirely in the laboratory using cells taken from the mice grown in test-tubes.
In order to identify other genes that modify the expansion of expanded repeats we will breed our mice with other genetically modified mouse strains that have defined mutations in genes involved in the repair of DNA (~500 animals per year). These experiments will provide direct insights into the molecular mechanisms mediating genetic instability suggesting candidate genes natural variation in which may modify genetic instability in humans and providing targets for the rational design of therapeutic agents. Once again, analyses will be performed on tissues taken from mice post mortem and will not involve any invasive manipulations. In addition, some experiments will actually be performed entirely in the laboratory using cells taken from the mice grown in test-tubes.
The reason that the genetic instability observed in disorders such as myotonic dystrophy and Huntington disease occurs predominantly in only certain tissues is not known. It is widely assumed that the genetic instability occurs when the DNA is being copied from one cell to another and it is possible that the rate of growth in cell numbers may affect the tissue specificity of genetic instability. The liver is one of the tissues in which genetic instability is observed in our genetically modified mice. The liver is an unusual organ in having a very high capacity for self repair. Indeed it is possible to surgically remove part of the liver which then grows back over a period of a few weeks. This re-growth is mediated by a large growth in the number of cells in the liver and hence the copying of DNA in many cells. To test if this re-growth effects the genetic instability, we will use a relatively simple surgical procedure to remove part of the liver in a small number of our mice (<30) and measure the amount of genetic instability observed in the regenerated liver.
One of the main observations from humans with myotonic dystrophy and Huntington disease is that the amount of genetic instability increases with age. However it is unclear if this effect is a true effect of age, or more simply a time dependent process. Experimental mice are usually provided with as much food as they care to eat. Consequently, many such mice overeat, becoming very large and succumbing to the same diseases of ageing with which humans are becoming increasingly accustomed. It has previously been demonstrated that by putting mice on a strict calorie controlled diet their rate of ageing is dramatically reduced and the mice remain fitter and healthier for much longer i.e. their rate of ageing is reduced. Therefore, using small numbers of animals (<100) will determine if changing the rate of ageing by caloric restriction results in a difference in the levels of genetic instability observed. If this is the case, then it may have immediate implications for the diets of patients affected with disorders such as myotonic dystrophy and Huntington disease.
Because the genetic instability is causal in mediating the progressive nature and tissue specificity of the symptoms in Huntington disease and myotonic dystrophy, any drug that affected genetic instability would be predicted to be therapeutically beneficial. No such drugs have yet been identified. However, using mouse cells grown in a test-tube we have identified several drugs that appear to have such an effect. Therefore, we will test these drugs in our genetically modified mice to determine if they are also effective in whole animals. Groups of mice (<10) will be treated with a variety of drugs via their incorporation into their normal diet. The drugs we will test are not expected to have any adverse effects on mouse health but we hope will be effective in modulating the genetic instability. Obviously, if this is successful it will have important implications for treating human patients with these disorders.