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A small study in last weeks British Medical Journal (BMJ) suggests a new human genotype may be prone to variant Creutzfeldt-Jakob disease (vCJD). Although this new evidence may rekindle fears of a larger epidemic, others warn that it is important to be cautious in interpreting these results.
Since the initial discovery of vCJD in the United Kingdom a decade ago, there has been concern about the ultimate extent of the epidemic. Fortunately the magnitude of the epidemic at present seems to match the lower limit of the early estimates, with 161 definite or probable cases in the UK.
Researchers at the University of Edinburgh analyzed DNA from two tissue samples that harbored prion proteins (a marker for vCJD infection) to identify the genetic make-up (genotype) of the patients.
So far, all clinical cases of vCJD have occurred in individuals with the homozygous methionine (MM) genotype, and it was hoped that this was the only susceptible population group. But both these samples carried the homozygous valine (VV) genotype, suggesting that individuals with the VV genotype may also be susceptible to vCJD infection.
The fear is that individuals with this genotype may be at risk of developing the condition, possibly with longer incubation periods, say the authors. Alternatively, these people may be asymptomatic carriers who might transmit the condition to other susceptible individuals by blood transfusion or surgery.
Though they warn against over-interpreting data from only two positive cases, they conclude that these uncertainties further underline the need for continued surveillance of vCJD in the UK.
It is important to be cautious in interpreting the results of this study, warn experts from Canada in an accompanying editorial. The study shows the existence of the prion protein in two tissue samples, not clinical evidence of vCJD in two patients. The study also provides no evidence to suggest that tissue from these two people could transmit vCJD to others.
Studies such as this are essential to the continuing effort to control the extent of the epidemic and highlight the urgent need for ongoing surveillance for vCJD, they add. They also pose challenges to health officials who have to formulate policies comprising difficult trade-offs based on uncertain evidence.
Reference: Variant Creutzfeldt-Jakob disease: prion protein genotype analysis of positive appendix tissue samples from a retrospective prevalence study BMJ Volume 332, pp 1186-8
Source: British Medical Journal