I write in response to the article by Rasinperä and colleagues (Gut 2004;53:1571–6) in which a DNA test was proposed for “adult‐type hypolactasia”.
The ability to digest the milk sugar lactose as an adult (lactase persistence) is a variable genetic trait in human populations, lactase persistence being the most frequent phenotype in Northern Europe, while lactase non‐persistence or “adult‐type hypolactasia” is more frequent in most other populations.1 In sub‐Saharan Africa for example, lactase persistence is found only at low frequency in the majority of populations that have been tested, but in some populations, particularly pastoralist groups, it is significantly more frequent.
A CT polymorphism located 13.9 kb upstream of exon 1 of the lactase gene (LCT) was previously shown in a Finnish population to be tightly associated with the lactase persistence phenotype2 and it is this change that is proposed as a DNA test for both Europeans and Africans. We agree that presence of a T at this polymorphic site is indeed a fairly good predictor of lactase persistence in Northern Europeans,3 and there is evidence that this nucleotide resides in a functional element.4,5 However, the presence of the alternative allele C at this site is not a good predictor of lactase non‐persistence or “adult hypolactasia” in many non‐Northern Europeans.6,7
I particularly draw readers' attention to our recent study.6 We typed this polymorphism in 1671 individuals from seven African countries, which included 20 distinct cultural groups. In seven cases it was possible to match the groups tested with groups from the literature for whom phenotypic information was available. In five of these groups the published frequencies of lactase persistence were ⩾25%. We found the T allele in Cameroon but it was so rare elsewhere that it cannot explain the frequency of the lactase persistence phenotype throughout Africa and we devised a statistical test to show that these results were unlikely to have been obtained by chance.
Our ongoing results support this published information and we urge the community to refrain from using DNA tests on Africans and probably other non‐Northern Europeans until an appropriate DNA change has been identified.
Footnotes
Conflict of interest: None declared.
References
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