Abstract
The Maasai are a pastoral people in Kenya and Tanzania, whose traditional diet of milk, blood and meat is rich in lactose, fat and cholesterol. In spite of this, they have low levels of blood cholesterol, and seldom suffer from gallstones or cardiac diseases. Field studies in the 1970s suggested that the Maasai have a genetic adaptation for cholesterol homeostasis. Analysis of HapMap 3 data using Fixation Index (Fst) and two metrics of haplotype diversity: the integrated Haplotype Score (iHS) and the Cross Population Extended Haplotype Homozygosity (XP-EHH), identified genomic regions and single nucleotide polymorphisms (SNPs) as strong candidates for recent selection for lactase persistence and cholesterol regulation in 143–156 founder individuals from the Maasai population in Kinyawa, Kenya (MKK). The non-synonmous SNP with the highest genome-wide Fst was the TC polymorphism at rs2241883 in Fatty Acid Binding Protein 1(FABP1), known to reduce low density lipoprotein and tri-glyceride levels in Europeans. The strongest signal identified by all three metrics was a 1.7 Mb region on Chr2q21. This region contains the genes LCT (Lactase) and MCM6 (Minichromosome Maintenance Complex Component) involved in lactase persistence, and the gene Rab3GAP1 (Rab3 GTPase-activating Protein Catalytic Subunit), which contains polymorphisms associated with total cholesterol levels in a genome-wide association study of >100,000 individuals of European ancestry. Sanger sequencing of DNA from six MKK samples showed that the GC-14010 polymorphism in the MCM6 gene, known to be associated with lactase persistence in Africans, is segregating in MKK at high frequency (∼58%). The Cytochrome P450 Family 3 Subfamily A (CYP3A) cluster of genes, involved in cholesterol metabolism, was identified by Fst and iHS as candidate loci under selection. Overall, our study identified several specific genomic regions under selection in the Maasai which contain polymorphisms in genes associated with lactase persistence and cholesterol regulation.
Highlights
The Maasai are a pastoralist, Nilotic people living primarily in southern Kenya and northern Tanzania
A comparison of Fst among HapMap populations shows that the MKK and African-Americans from South-west USA (ASW) have the lowest average Fst (0.0145), followed by MKK and the Luhya in Webuye, Kenya (LWK) (0.017), while Fst between MKK and Yoruba from Nigeria (YRI) is significantly higher (0.027) (Table S6 in [29])
The LWK and ASW have a large admixture with YRI (66% and 76% respectively), while MKK have a smaller admixture with YRI (10%)
Summary
The Maasai are a pastoralist, Nilotic people living primarily in southern Kenya and northern Tanzania. An economy traditionally based on herds of cattle, sheep, and goats led to a diet rich in lactose, fat, and cholesterol consisting largely of milk, meat, and blood. Their cholesterol intake is high (600–2000 mg/ day), and 66% of their calories come from fat, their total serum cholesterol levels average 135 mg/100 ml [1,2,3,4]. The hypo-cholesterolaemic factor was never found, and the model of [10,11] could not explain the low frequencies of heart disease in older Maasai men who lead sedentary lives after age , 24, when their warrior (Murran/Moran) period ends [14,15]
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