Pathogenesis of multiple sclerosis: genetic, environmental and random mechanisms.

Background: The pathogenesis of multiple sclerosis (MS) requires both genetic factors and environmental events. The question remains, however, whether these factors and events completely describe the MS disease process. This question was addressed using the Canadian MS data, which includes 29 478 individuals, estimated to represent 65-83% of all Canadian patients with MS.

Method: The 'genetically-susceptible' subset of the population, (G), includes everyone who has any non-zero life-time chance of developing MS, under some environmental conditions. A 'sufficient' environmental exposure, for any genetically-susceptible individual, includes every set of environmental conditions, each of which is 'sufficient', by itself, to cause MS in that person. This analysis incorporates many epidemiological parameters, involved in MS pathogenesis, only some of which are directly observable, and establishes 'plausible' value ranges for each parameter. Those parameter value combinations (ie, solutions) that fall within these plausible ranges are then determined.

Results: Only a small proportion of the population (≤52%) has any possibility of developing MS, regardless of any environmental conditions that they could experience. Moreover, some of these genetically-susceptible individuals, despite their experiencing a 'sufficient' environmental exposure, will still not develop disease.

Conclusions: This analysis explicitly includes all of those genetic factors and environmental events (including their interactions), which are necessary for MS pathogenesis, regardless of whether these factors, events and interactions are known, suspected or as yet unrecognised. Nevertheless, in addition, a 'truly' random mechanism also seems to play a critical role in disease pathogenesis. This observation provides empirical evidence, which undermines the widely-held deterministic view of nature. Moreover, both sexes seem to share a similar genetic and environmental disease basis. If so, then it is this random mechanism, which is primarily responsible for the currently-observed differences in MS disease expression between susceptible women and susceptible men.