A consortium approach to molecular genetic services. Scottish Molecular Genetics Consortium

Genetics of neuromuscular disorders

Neuromuscular disorders affect the peripheral nervous system and muscle. The principle effect of neuromuscular disorders is therefore on the ability to perform voluntary movements. Neuromuscular disorders cause significant incapacity, including, at the most extreme, almost complete paralysis. Neuromuscular diseases include some of the most devastating disorders that afflict mankind, for example motor neuron disease. Neuromuscular diseases have onset any time from in utero until old age. They are most often genetic. The last 25 years has been the golden age of genetics, with the disease genes responsible for many genetic neuromuscular disorders now identified. Neuromuscular disorders may be inherited as autosomal dominant, autosomal recessive, or X-linked traits. They may also result from mutations in mitochondrial DNA or from de novo mutations not present in the peripheral blood DNA of either parent. The high incidence of de novo mutation has been one of the surprises of the recent increase in information about the genetics of neuromuscular disorders. The disease burden imposed on families is enormous including decision making in relation to presymptomatic diagnosis for late onset neurodegenerative disorders and reproductive choices. Diagnostic molecular neurogenetics laboratories have been faced with an ever-increasing range of disease genes that could be tested for and usually a finite budget with which to perform the possible testing. Neurogenetics has moved from one known disease gene, the Duchenne muscular dystrophy gene in July 1987, to hundreds of disease genes in 2011. It can be anticipated that with the advent of next generation sequencing (NGS), most, if not all, causative genes will be identified in the next few years. Any type of mutation possible in human DNA has been shown to cause genetic neuromuscular disorders, including point mutations, small insertions and deletions, large deletions and duplications, repeat expansions or contraction and somatic mosaicism. The diagnostic laboratory therefore has to be capable of a large number of techniques in order to identify the different mutation types and requires highly skilled staff. Mutations causing neuromuscular disorders affect the largest human proteins for example titin and nebulin. Successful molecular diagnosis can make invasive and expensive diagnostic procedures such as muscle biopsy unnecessary. Molecular diagnosis is currently largely based on Sanger sequencing, which at most can sequence a small number of exons in one gene at a time. NGS techniques will facilitate molecular diagnostics, but not for all types of mutations. For example, NGS is not good at identifying repeat expansions or copy number variations. Currently, diagnostic molecular neurogenetics is focused on identifying the causative mutation(s) in a patient. In the future, the focus might move to prevention, by identifying carriers of recessive diseases before they have affected children. The pathobiology of many of the diseases remains obscure, as do factors affecting disease severity. The aim of this review is to describe molecular diagnosis of genetic neuromuscular disorders in the past, the present and speculate on the future.

Newborn screening for cystic fibrosis: techniques and strategies

Newborn screening for cystic fibrosis has been carried out for over 25 years, and clinical and cost benefits have been documented. There is still much variation in the methods and strategies adopted. All current screening programmes use a measurement of immunoreactive trypsin as a primary screening test, and in most, a second tier test involves analysing DNA mutations. The choice of DNA mutations depends on the genetic background in the region, and considerations of cost. Using DNA analysis as part of a screening procedure has introduced unwanted carrier detection, and protocols have now been devised in an attempt to avoid this. There are at least seven distinct protocols in use, all of which have different advantages and disadvantages, and no method or strategy will suit every region. Further careful study of performance and costs of various strategies is needed.

Transfer of DNA tests from research to routine laboratories: some lessons from the French experience in the case of Duchenne muscular dystrophy

In the last few years, the activity of research laboratories has led to the emergence of new DNA diagnostic tests in France. They permit the origin of genetic diseases to be identified and provide an answer concerning the detection of carriers and prevention. Nevertheless, given this, new actors have emerged on the health care scene: the research workers who developed the tests and who work in public research laboratories. The economic question of the transfer of the test practice from research to hospital laboratories is the main topic of this paper, taking Duchenne Muscular Dystrophy (DMD) DNA diagnostic tests as the example. After a presentation of the complexity of DNA tests for DMD, the fact that financial and human constraints do not allow the actors to continue to produce the DNA tests is discussed. The financial role of the non-profit-making associations is then explained and leads to the conclusion that a more global policy on DNA tests, such as carried out in the UK and the Netherlands, should be adopted in France by the Social Security if it wants DNA testing activity to be pursued.

UK clinicians' knowledge of and attitudes to the prenatal diagnosis of single gene disorders

Postal questionnaires were sent to 308 clinicians in the UK (general practitioners, obstetricians, clinical geneticists, neurologists, paediatricians, and paediatric neurologists) to assess their knowledge of, and attitudes to, the prenatal diagnosis of three common single gene disorders, Huntington's disease (HD), cystic fibrosis (CF), and Duchenne muscular dystrophy (DMD). Replies received numbered 213, a response rate of 69%. Overall, 95% of responding clinicians thought that offering prenatal diagnosis for the three test conditions was often or always appropriate. There was a correlation between the clinicians' estimates of life expectancy and their willingness to offer prenatal diagnosis (p less than 0.01). Among the non-geneticists questioned, fewer than 50% of general practitioners answered correctly regarding the availability of prenatal tests.

Protocol for genetic testing in Huntington disease: three years of experience in Minnesota

Molecular genetic testing for Huntington disease (HD) by linkage analysis of DNA markers close to the HD gene has been possible since the mid-1980s. Because of ethical and practical concerns about this kind of testing, most groups performing the test in the past have operated under lengthy research protocols designed to assess the psychological morbidity of the presymptomatic diagnosis of a fatal disease. Our approach to HD testing is service-oriented, and our testing process has been designed to be flexible, to meet the varying needs of our patients. Between 1988 and 1990, 87 inquiries about the test have been received; 22 inquiries had family structures which were unsuitable for linkage analysis. Eleven of the 37 individuals who entered the testing program have not completed it. Of 19 patients who have received DNA results, seven received an increased risk of carrying the HD gene, and ten, a decreased risk. For two additional individuals, nonpaternity resulted in a negligible risk for HD. Several of those consulted, or their spouses, have had continuing outpatient counseling since completing the test; none have required hospitalization. Our short-term results indicate that molecular genetic testing for HD can be performed safely in a clinical setting using our protocol. As molecular genetic testing for HD and other diseases moves out of research centers and into clinics, clinicians must devise practical strategies for providing the medical, genetic, and psychological services needed for the growing number of individuals who will seek such testing.

The incidence of different cystic fibrosis mutations in the Scottish population: effects on prenatal diagnosis and genetic counselling

We present an analysis of the frequency of 16 different cystic fibrosis (CF) mutant alleles in the Scottish population. Each allele was detected in DNA amplified by the polymerase chain reaction (PCR) either directly on polyacrylamide gels, on agarose gels after restriction enzyme digestion, or by using allele specific oligonucleotides. Among 506 CF chromosomes, of predominantly Scottish origin, the frequencies of the different mutations were delta F508 0.71, G551D 0.05, G542X 0.04, R117H 0.01, 1717-1G----A 0.01, A455E + delta I507 + R553X + R560T + W1282X + 621 + 1G----T combined 0.03, unpublished 0.01, and unknown 0.13. No examples of D110H, R347P, S549N, S549I, or 2566ins AT mutations were found. The relevance of this type of analysis for both prenatal diagnosis and heterozygote screening is discussed.