British researchers say they've completed the largest sequencing study of human disease to date, focusing on the genetic basis of six autoimmune diseases.

Scientists from Queen Mary, University of London, said the sequencing is important because a complex combination of genetic and environmental factors are thought to cause the diseases studied -- autoimmune thyroid disease, celiac disease, Crohn's disease, psoriasis, multiple sclerosis and type 1 diabetes.

In the study, a global team of scientists led by the British university sought to identify new variants in 25 previously identified risk genes associated with the diseases.

Some scientists had suggested a small number of rare variants in risk genes are likely to be a major cause of the heritability of the conditions, but the new study, published in the journal Nature, suggests the genetic risk of the diseases more likely involves a complex combination of hundreds of common weak-effect variants.

"For each disease there are probably hundreds of such variants and the genetic risk is likely to come from inheriting a large number of these variants from both parents," researcher David van Heel said in a university release Wednesday.

"If this is the case then it may never be possible to accurately predict an individual's genetic risk of these common autoimmune diseases," he said. "However, the [study] results do provide important information about the biological basis of these conditions and the pathways involved, which could lead to the identification of new drug targets."