CF and populations

Question

Hello,
I would like to know why, when we do research on cystic fibrosis, it is very often said that this disease mainly affects Caucasian (European) populations. Outside, I come from Sub-Saharan Africa and my husband is a native of the Maghreb and our child suffers from cystic fibrosis. Likewise, I often see in the CF Cenre patients of African, Maghrebian or Asian origin.
Hence my great astonishment at the announcement of the diagnosis of my baby because when I was told about genetic disease I immediately thought about sickle cell anemia.
Is it because the statistics are based only on patients in Europe and North America, or is there really a genetic predisposition of Caucasians for cystic fibrosis?
I hope my question will not interfere.
Thank you.

Answer

Hello,

Your question is very relevant and has no reason to interfere. The frequency of cystic fibrosis actually varies in the ethnic populations of the globe even though these frequencies are not always known precisely.

The persistence over the centuries of an autosomal recessive genetic diseases, most often leading to an early death in the absence of treatment, is explained by a law of population genetics (Hardy Weinberg's law): a selective advantage of healthy carriers from a single gene mutation of these diseases. This is particularly the case for sickle-cell anemia and cystic fibrosis.

The most frequent occurrence of sickle cell anemia in African populations is thus probably related to the fact that healthy carriers of a mutation of this disease are more resistant to malaria, a particularly frequent and fatal disease in equatorial, tropical and subtropical regions, particularly in Africa. Thus, healthy carriers of a sickle cell mutation have enjoyed a selective advantage over the centuries in areas where malaria is particularly prevalent, leading to an increase in their proportion in the regions concerned and the frequency of couples at risk of transmitting the disease.

If cystic fibrosis is more frequent in Caucasian populations originating in Europe and particularly in Celtic populations, it is because healthy carriers of a mutation of the cystic fibrosis gene have been confronted with conditions showing a selective advantage. Several hypothesis have been advanced, none of which can be proved:
- existence of a greater fertility of the couples of healthy carriers?
- better survival of healthy carriers during the major epidemics of plague and cholera that have been particularly fatal in Europe?
- indeed, the existence of greater resistance to poisoning by heavy metals (lead, copper ...) whose artisanal use was one of the peculiarities of the Celtic populations?

Moreover, the frequency of cystic fibrosis is well known only in the countries where it is diagnosed. It varies according to the population, but the mixing of populations that has existed for several centuries and has increased strongly over several decades tends to attenuate the differences in frequency. The frequency of cystic fibrosis is certainly higher in Afro-American or Afro-European populations than in African populations.

Hope this answer can help.
Best wishes
Gilles RAULT, MD, Roscoff CF Center

24.02.2017