Guido Modiano

Haemoglobin C protects against clinical Plasmodium falciparum malaria.

Haemoglobin C (HbC; β6Glu → Lys) is common in malarious areas of West Africa, especially in Burkina Faso. Conclusive evidence exists on the protective role against severe malaria of haemoglobin S (HbS; β6Glu → Val) heterozygosity, whereas conflicting results for the HbC trait have been reported and no epidemiological data exist on the possible role of the HbCC genotype. In vitro studies suggested that HbCC erythrocytes fail to support the growth of P. falciparum but HbC homozygotes with high P. falciparum parasitaemias have been observed. Here we show, in a large case–control study performed in Burkina Faso on 4,348 Mossi subjects, that HbC is associated with a 29% reduction in risk of clinical malaria in HbAC heterozygotes (P = 0.0008) and of 93% in HbCC homozygotes (P = 0.0011). These findings, together with the limited pathology of HbAC and HbCC compared to the severely disadvantaged HbSS and HbSC genotypes and the low βS gene frequency in the geographic epicentre of βC, support the hypothesis that, in the long term and in the absence of malaria control, HbC would replace HbS in central West Africa.

The lower susceptibility to Plasmodium falciparum malaria of Fulani of Burkina Faso (west Africa) is associated with low frequencies of classic malaria-resistance genes.

The gene frequencies in 1993-94 for haemoglobin S, haemoglobin C, alpha-3.7 deletional thalassaemia, G6PDA-, HLAB*5301 were estimated in Fulani, Mossi and Rimaibé ethnic groups of Burkina Faso, West Africa. The aim of the study was to verify whether the previously reported Fulani lower susceptibility to Plasmodium falciparum malaria was associated with any of these malaria-resistance genes. Similar frequencies for haemoglobin S were recorded in the 3 ethnic groups (0.024 +/- 0.008, 0.030 +/- 0.011, 0.022 +/- 0.013; in Mossi, Rimaibé and Fulani, respectively). The Mossi and Rimaibé showed higher frequencies when compared to Fulani for haemoglobin C (0.117 +/- 0.018, 0.127 +/- 0.020, 0.059 +/- 0.020), alpha-3.7 deletional thalassaemia (0.227 +/- 0.040, 0.134 +/- 0.032, 0.103 +/- 0.028), G6PDA- (0.196 +/- 0.025, 0.187 +/- 0.044, 0.069 +/- 0.025) and HLA B*5301 (0.189 +/- 0.038, 0.202 +/- 0.041, 0.061 +/- 0.024). Among Fulani the proportion of individuals not having any of these protective alleles was more than 3-fold greater than in the Mossi-Rimaibé group (56.8% vs 16.7%; P < 0.001). These findings exclude the involvement of these genetic factors of resistance to P. falciparum in the lower susceptibility to malaria of Fulani. This evidence, in association with the previously reported higher immune reactivity to malaria of Fulani, further supports the existence in this ethnic group of unknown genetic factor(s) of resistance to malaria probably involved in the regulation of humoral immune responses.

Cystic fibrosis and lactase persistence: a possible correlation.

C ystic fibrosis (CF) is the most common lethal genetic disease in Europeans and is transmitted as an autosomal recessive disorder caused by loss of function of the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) gene. Since long it has been supposed that its quite high frequency (1/ 2500 implying an allele frequency equal to 0.02) is due to a Europe-restricted selective advantage of the heterozygous CF/þ individuals. Nevertheless, this hypothesis has never been directly demonstrated nor the hypothetical selective factor identified. Here, we provide compelling evidence that the CF/þ advantage hypothesis is right. As to the identification of the selective factor, as the CFTR protein is a Cl channel (and hence involved in water excretion), diarrhea appears a reasonable candidate for such a role. Cholera has, in fact, been suggested, but its area of diffusion had not been limited to Europe. Considering that initially all humans were lactose intolerant in the post-weaning life and Europeans became cattle breeders and adopted a dairy-milk diet, we propose that the hypothetical Europe-restricted CF/þ advantage consisted of a resistance to lactose-caused diarrhea in populations that adopted such a habit while they were still lactose intolerant. This initial emergency segregational load adaptation would have been followed by a not costly one consisting of the present European quasi-fixed post-weaning lactase persistence (due to the dominant P allele of the lactase, LCT, gene), resulting in lactose tolerance. This hypothesis is supported by the positive correlation between the proportion, among the CF alleles, of the F508del allele and the P allele – recently identified with the T allele 14 Kb upstream to the LCT gene – frequency and provides a possible explanation for the well-known north–south cline of the F508del allele frequency. Both adaptations would be examples of genetic responses to a culture-rooted adverse factor. CF is the most common (1/2500 newborns) severe autosomal recessive disease in Europe. The global frequency of the CFcausing alleles of the CFTR gene, estimated as the square root of the prevalence of the disease, turned out to be about 0.02 in Europe, 0.01 in Africa and 0.002 in FarEastern Asia. The high CF allele frequency in Europe could only be accounted for by a ‘Europerestricted’ high mutation rate or by a ‘Europe-restricted’ heterozygous CF/þ advantage. The most plausible explanation seems to be the latter, mainly because a ‘Europe-restricted’ high mutation rate was judged extremely unlikely, although it was never directly disproved. In addition, the biological basis of the hypothetical ‘Europe-restricted’ CF/þ heterozygous advantage is far from being identified because of the lack of a convincing candidate adverse factor: bacteria-driven diarrheas – proposed on the basis of the functional properties of the CFTR protein – do not seem to have had ‘Europe-restricted’ histories, a fundamental prerequisite for a good candidate. In this paper, first we definitively reject the ‘Europe-restricted’ high mutation rate (mþ-CF) hypothesis; then we propose a ‘Europe-restricted’ biological factor that could account for the CF/þ heterozygous advantage.

Haplotype block structure study of the CFTR gene. Most variants are associated with the M470 allele in several European populations.

An average of about 1700 CFTR (cystic fibrosis transmembrane conductance regulator) alleles from normal individuals from different European populations were extensively screened for DNA sequence variation. A total of 80 variants were observed: 61 coding SNSs (results already published), 13 noncoding SNSs, three STRs, two short deletions, and one nucleotide insertion. Eight DNA variants were classified as non-CF causing due to their high frequency of occurrence. Through this survey the CFTR has become the most exhaustively studied gene for its coding sequence variability and, though to a lesser extent, for its noncoding sequence variability as well. Interestingly, most variation was associated with the M470 allele, while the V470 allele showed an ‘extended haplotype homozygosity’ (EHH). These findings make us suggest a role for selection acting either on the M470V itself or through an hitchhiking mechanism involving a second site. The possible ancient origin of the V allele in an ‘out of Africa’ time frame is discussed.

A large-scale study of the random variability of a coding sequence: a study on the CFTR gene

Coding single nucleotide substitutions (cSNSs) have been studied on hundreds of genes using small samples (ng≈100–150 genes). In the present investigation, a large random European population sample (average ng≈1500) was studied for a single gene, the CFTR (Cystic Fibrosis Transmembrane conductance Regulator). The nonsynonymous (NS) substitutions exhibited, in accordance with previous reports, a mean probability of being polymorphic (q>0.005), much lower than that of the synonymous (S) substitutions, but they showed a similar rate of subpolymorphic (q<0.005) variability. This indicates that, in autosomal genes that may have harmful recessive alleles (nonduplicated genes with important functions), genetic drift overwhelms selection in the subpolymorphic range of variability, making disadvantageous alleles behave as neutral. These results imply that the majority of the subpolymorphic nonsynonymous alleles of these genes are selectively negative or even pathogenic.

Factors regulating Hb F synthesis in thalassemic diseases

BackgroundThe thalassemic syndromes originate from mutations of the globin genes that cause, besides the characteristic clinical picture, also an increased Hb F amount. It is not yet clear if there are more factors, besides the beta globin genotype, determining the Hb F production. We have tried to find out if there are relations between total Hb and Hb F, between erythropoietin (Epo) and Hb F, between Hb F and point mutations of the gamma gene promoters.Materials and MethodsHematologic parameters, iron status, alpha/non-alpha globin ratio, Epo level, and thalassemic defects of the alpha-, beta-, and gamma-globin genes were explored using standard methods in patients affected by thalassemic diseases. Ninety-five non thalassemic individuals have been examined as controls.ResultsTwo clinical variants of beta-thalassemia intermedia referred to as beta-thal int sub-silent and evident are associated with distinct sets of mutations of the beta-globin gene. Silent beta thal mutations are invariably associated with sub-silent beta thal int; beta° or severe beta+ thal mutations are associated with evident beta thal int (88%) and almost invariably (98%) with thalassemia major. A positive correlation was observed between the severity of the disease and the Hb F level, but no correlation was found between the Hb F and erythropoietin (Epo) level. The mutation Ggamma -158 C→T was detected in 26.9% of patients affected by beta-thal int sub-silent and evident, respectively, but only in 2% of patients with thalassemia major.ConclusionsThe severity of beta-thal int and the increased Hb F level are strictly dependent from the type of beta-globin gene mutations. No relation is found between Hb F synthesis and Epo secretion. The mutation Ggamma -158 C→T, common among patients affected by beta-thal int and very rare in thal major patients, does not seem, in this study, to influence the Hb F content in beta thal int patients.

Anthropological features of the CFTR gene: Its variability in an African population

Background: The CFTR gene (Cystic Fibrosis conductance Transmembrane Regulator) is the gene responsible for Cystic Fibrosis, the most common severe autosomal recessive disease in Europeans. It has been extensively explored in several European and European-derived populations, but poorly studied in the other major human groups. Aim: To characterize the variability of the CFTR gene in an African population. Subjects and methods: Using DGGE, all 27 exons (4443 bp) and 2184 bp of the flanking intronic regions of the CFTR gene were studied in a random sample of 45 Mossì from Burkina Faso (Western sub-Saharan Africa). Results: Sixteen variable sites were found: 13 SNPs (one in the promoter region, four non-synonymous and five synonymous in the exons and three in the introns) and three intronic STRs. Only the promoter site ( − 94 G/T), slightly polymorphic in the present survey, was not variable in different European populations. Comparison between Western Africans, Eastern Africans, Europeans and Eastern Asians showed that alleles at two intronic STRs (Tn and (TG)m in intron 8), four exonic (M470V, 2694 T/G, 4002 A/G and 4521 G/A) and one intronic (875+40 A/G) SNPs have very different frequencies among at least two major human groups. Moreover, the overall degree of non-synonymous variability in Mossì is much lower than that in Europeans. A possible interpretation of this finding is proposed. Conclusions: The CFTR gene has been since long hypothesized to have undergone selection in Europeans. The present study by comparing Africans and Europeans for the overall variability of the gene supports this hypothesis.