Microcephaly and the Asian community in Britain

Armand Leroi:

Microcephaly is a rare disorder in Britain. No one seems to know precisely how common it is in the Asian community of north England, but it was common enough to attract the attention of Geoff Woods, a geneticist working at Leeds University. He found that it ran in families. That implied that its cause was genetic; it was caused by a mutation. Or, more precisely, several. By the late 1990s, the disorder had been mapped to deficiencies in at least six different genes.

In the last few years, Woods and his collaborators have identified several of them. All seem to encode proteins that are needed if neuroblasts - the cells that give rise to the brain's neurons - are to divide and prosper. Should a child be born with an insufficiency of one of these proteins, the neuroblasts fail to divide. Or perhaps they divide slowly or die prematurely - the precise cellular defect is still obscure. In any event, the result is a brain that, in the extreme, grows to only one third of its normal size.

It is easy to see why peculiar theories of the origins of microcephaly have proliferated in Pakistan. To the untrained eye, the occurrence of the disorder is hard to explain. Healthy parents may have microcephalic children; microcephalic parents - there are a few - may have healthy children. To a geneticist, however, this merely speaks of recessive mutations. A child will only have microcephaly if it has inherited two copies of the mutant gene, one from each parent who are its carriers.

Disorders caused by recessive mutations are normally rather rare. But not in Lahore; nor in Leeds. That's because of the Pakistani way of marriage. Most of us marry people quite distantly related to ourselves and, as we travel ever further, our mates become ever more genetically remote.

In Pakistan, however, some 60 per cent of marriages are between first cousins; the frequency in Bradford and Leeds is thought to be comparable. The result is that clinical genetics units serving the British Pakistani community see a range and frequency of genetic disorders unknown elsewhere in the country.

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