The genetic contribution to multiple sclerosis. Evidence from North-East England

The initiation of multiple sclerosis: a new infectious hypothesis

Both genetic and environmental factors cause multiple sclerosis (MS). Few genes have been identified, however, and environmental factors remain elusive. Some postulate an infectious cause, but no pathogens are reproducibly demonstrable in CNS lesions. I postulate that the CNS is not the infectious target in MS, but propose a two-hit infectious hypothesis focusing on nai;ve CD4 T-cells that initiate demyelination: (1) Various common viruses infect the thymus during childhood (first hit) and enhance nai;ve CD4 T-cell reactivity to CNS autoantigens; (2) Heterogeneous pathogens fully activate these T-cells during adulthood (second hit) to initiate myelin injury. The novel concept of thymic infection provides insight into the nature of some susceptibility genes, helps explain the high discordance rates in genetically susceptible individuals, and suggests it is futile to search for pathogens in MS lesions. Pathogen heterogeneity, i.e., the lack of a single infectious cause, implies there can be no simple therapies to prevent or treat MS.

Uses and limitations of twin studies

Series of twin pairs selected because one (or both) is ill are prone to biassed ascertainment, and great care has to be taken to avoid this. Such bias is absent if the primary source is a twin registry established at birth. In general, series of twin pairs have no advantage over studies on sibs in assessing the size of genetic contribution to disease. However, individual monozygotic twin pairs who are discordant for single gene or multifactorial conditions offer unique opportunities for investigating postzygotic mutations, for searching for factors that may precipitate disease or influence its course, and for assessing the effect of prophylactic measures.

Consanguinity and multiple sclerosis in Orkney

For all patients with multiple sclerosis in Orkney who were alive in 1974 or who had died during 1958 to 1974, pedigree data were analysed. The relationship between their parents, expressed by kinship coefficients, was compared with that between parents of matched controls. The closer relationship between parents of patients suggests that it is the genes that are shared by a patient's parents, and that he inherits from both, that influence his susceptibility to multiple sclerosis.

Identification of a locus on mouse chromosome 3 involved in differential susceptibility to Theiler's murine encephalomyelitis virus-induced demyelinating disease

Theiler's virus-induced demyelinating disease results from a chronic infection in the white matter of the central nervous system and provides an excellent model for human multiple sclerosis. Like multiple sclerosis, there are genetic risk factors in disease development, including genes associated with the major histocompatibility complex and with those encoding the beta chain of the T-cell receptor. Comparisons of the susceptible DBA/2 and resistant C57BL/6 strains have indicated an important role for the H-2D locus and for a non-H-2 gene (not involving the beta chain of the T-cell receptor) in differential susceptibility. In the present report, analysis of recombinant-inbred strains (BXD) between the DBA/2 and C57BL/6 strains indicated that this non-H-2 locus is located at the centromeric end of chromosome 3 near (4 +/- 4 centimorgans) the carbonic anhydrase-2 (Car-2) enzyme locus.

The inheritance of MS susceptibility

The object of this study was to determine, if possible, the mode of inheritance of the susceptibility to multiple sclerosis (MS). It was known that no single-gene model could fit the observations, so oligogenic models (models involving a small number of genes) were sought. Oligogenic hypotheses were tested against the available population data for MS until a reasonable level of agreement was found. The best-fitting simple hypothesis was this: MS occurs only in people who are homozygous for a recessive gene and carry a dominant X gene, and then only with reduced penetrance. The dangerous allele m- at the autosomal locus appears to be fairly uniformly distributed across England, Ireland and Canada, occurring in 10-30% of the gene pool. There are large variations in the frequency of the allele s- at the X-locus, ranging from 10% up to 72% of the gene pool. The penetrance varies significantly with geographical location, but nowhere approaches unity, so that the environmental factors are of great importance. The hypothesis explains the broad features of the population pattern of the occurrence of MS and it gives an outstanding fit to the best available data on MS in relatives. The result may assist attempts to map the genetic data on MS, opens the way for a reassessment of the attempts to identify the environmental factors, and it makes possible the completion of nonempirical risk tables for various countries. Similar techniques may be applied to other disorders with a genetic component in their aetiology.

Positive autoradiographic findings in brains of four MS patients

Autoradiography of brain slices from 4 multiple sclerosis (MS) and 9 control patients was performed. After 6 weeks of exposure the exact picture of the white matter appeared on the X-ray films in all cases with MS, but only in one of the controls. The high level of autoradiographic signal from MS white matter suggests that an abnormal accumulation of radioactive trace elements takes place within the brains of MS victims.

The characteristics of a national register of people with multiple sclerosis (MS): a comparison between the ARMS (Action for Research into Multiple Sclerosis) register and 10 British MS populations

Problems with case ascertainment in epidemiological research on multiple sclerosis (MS) make it necessary to use indirect sources. However, there is a lack of information about the characteristics of cases drawn from different sources and thus little basis on which sampling frames for large scale surveys may be constructed. The characteristics of a population drawn from the membership of Action for Research into Multiple Sclerosis (ARMS) were compared with those of 10 other British MS populations reported between 1980 and 1987. Demographic variables examined were geographical location, nationality, ethnicity, sex ratio and age. Diagnostic status, age at diagnosis and duration since diagnosis were considered together with data on the comparative populations based on date of onset. On all the variables investigated the ARMS population fell close to or within the range shown by the comparative populations. The relative youth of the ARMS population and its bias towards a high proportion of females were differences in keeping with existing knowledge about the membership of mutual support organisations. A population drawn from such an organisation may usefully complement other MS research populations provided that the likely biases in each are understood.

Multiple sclerosis: updated risks for relatives

Two important characteristics of multiple sclerosis (MS) are familial clustering and a variable age of onset. There is increasing evidence for a genetically influenced susceptibility in MS. Because of this, patients and their relatives are increasingly asking about the risk for relatives of developing MS. In the MS Clinic in Vancouver, genetic histories are taken routinely for all patients and are updated annually. Patients do not attend the clinic specifically to participate in genetic studies, which could result in over-representation of familial cases. Data were available for 815 MS index cases and 11,345 of their relatives. Age-specific MS risks were calculated for first-, second-, and third-degree relatives of probands and are presented in an easy-reference format. In general, first-degree relatives of probands have a risk that is 30-50 times greater than the 0.1% risk for the general population.

Genetic analysis of multiple sclerosis in Shetland

In a family study of all patients with multiple sclerosis in Shetland the number of inbred patients, although high for Britain and higher than in Orkney, is not higher than the number among controls, and the inbreeding coefficients suggest that there is no recessive involvement of rare genes in the aetiology. The kinship coefficients show close interweaving of ancestries of patients and controls and eliminate from the aetiology any involvement of recently introduced single genes dominant or codominant in effect. Family histories show that single locus inheritance is unlikely. Any genetic involvement is likely to be polygenic, but on a multifactorial hypothesis the estimates of heritability are very low. The findings suggest that the genetic contribution to the aetiology of the disease in Shetland is slight.