rs41380347 — MCM6 G-13915T

MCM6 G-13915T — The East African Lactase Persistence Allele

Human populations that domesticated dairy animals faced a powerful selective pressure: adults who could digest lactose gained a calorie-dense, nutrient-rich food source that also served as a safer alternative to potentially contaminated water. Rather than a single evolutionary solution, natural selection produced at least five independent mutations on separate continents — all targeting the same regulatory switch in the MCM6 gene, all maintaining lactase enzyme expression into adulthood. The G-13915T variant (rs41380347) is one of these parallel solutions, found primarily among pastoralist peoples of the Horn of Africa and the Sudan: Oromo, Somali, Beja, and Sudanese Arab populations.

The Mechanism

The MCM6 gene sits immediately upstream of the LCT gene¹¹ LCT gene
LCT encodes lactase-phlorizin hydrolase, the intestinal brush-border enzyme that cleaves lactose into glucose and galactose for absorption on chromosome 2. Although MCM6 itself encodes a DNA replication helicase subunit²² DNA replication helicase subunit
Part of the MCM2-7 complex that unwinds DNA ahead of the replication fork, its intron 13 contains a long-range enhancer element that controls LCT expression roughly 13.9 kb away. In most mammals — and in most adult humans — this enhancer is progressively silenced after weaning, reducing lactase production to near-zero by early adulthood. A cluster of mutations in this enhancer region can override this silencing program.

The -13915 position sits within an approximately 300 bp region in MCM6 intron 13 that encompasses several transcription factor binding sites. The derived G allele at position -13915 (reported as C on the GRCh38 plus strand) alters interaction with the Oct-1 transcription factor³³ transcription factor
A ubiquitous Octamer-binding protein that regulates tissue-specific gene expression. Olds et al. (2011) used electrophoretic mobility shift assays and cell transfection experiments to demonstrate that Oct-1 binds the -13915*G sequence and enhances transcription of an LCT promoter-reporter construct — providing a molecular basis for how this single nucleotide change maintains lactase production through adulthood.

The Evidence

The variant was first identified by Ingram et al. (2007)⁴⁴ Ingram et al. (2007)
Ingram CJE et al. A novel polymorphism associated with lactose tolerance in Africa: multiple causes for lactase persistence? Hum Genet. 2007;120:779-88. through a cohort study of Sudanese pastoralists. The European diagnostic variant (C/T-13910, rs4988235) was absent in this population's lactase-persistent individuals, leading the authors to search for population-specific alleles. They identified -13915*G as significantly associated with lactose tolerance among Sudanese volunteers and confirmed its presence in other East African and Middle Eastern pastoral groups.

Tishkoff et al. (2007)⁵⁵ Tishkoff et al. (2007)
Tishkoff SA et al. Convergent adaptation of human lactase persistence in Africa and Europe. Nat Genet. 2007 Jan;39(1):31-40. genotyped 470 individuals from Tanzania, Kenya, and Sudan and identified T/G-13915 as one of three African persistence alleles, distinct from the European C/T-13910 and from each other at the haplotype level. Extended haplotype homozygosity analysis showed selection signatures consistent with a sweep beginning approximately 7,000 years ago — coinciding with the spread of cattle pastoralism across East Africa.

Jones et al. (2013)⁶⁶ Jones et al. (2013)
Jones BL et al. Diversity of lactase persistence alleles in Ethiopia: signature of a soft selective sweep. Am J Hum Genet. 2013;93(3):538-44. examined more than 350 Ethiopian individuals from multiple ethnic groups and confirmed -13915*G (rs41380347) as one of at least five independent functional persistence alleles. They documented greater genetic diversity among lactose digesters than non-digesters in Ethiopia — a pattern consistent with multiple independent mutations spreading simultaneously (a soft selective sweep), rather than the single dominant haplotype seen in the European lineage.

Hassan et al. (2016)⁷⁷ Hassan et al. (2016)
Hassan HY et al. Genetic diversity of lactase persistence in East African populations. BMC Res Notes. 2016;9:16. surveyed 12 East African populations and found that the -13915*G and -13907*G variants dominate among nomadic Arab and Beja communities in Sudan, reaching notably high frequencies in these pastoral groups.

Population Context

Unlike the European rs4988235, which spread from a single founder variant to high frequency across Northern Europe (70–90% in Scandinavia), the African persistence alleles each remain concentrated in the pastoral populations among whom they originated. The -13915*G allele is most common in the Beja of northeastern Sudan and Eritrea, and among Afro-Asiatic-speaking pastoralists including Somali and Oromo communities. It is essentially absent in East Asian, South Asian, and European populations, and rare in global genetic databases (like gnomAD) that are dominated by European and African American samples not drawn from East African pastoral groups.

Practical Actions

The practical implications of this variant exactly parallel the European lactase persistence system. Individuals who carry no persistence allele (AA genotype) in any of the five known variants are likely to be lactose non-persistent and may experience bloating, gas, cramps, or diarrhea after consuming lactose-containing foods. Note that individuals of East African descent may carry any combination of these five alleles — a comprehensive lactase persistence assessment requires testing all known variants (rs4988235, rs145946881, rs41525747, rs41380347, and rs869051967).

Interactions

This variant is one of at least five independent lactase persistence mutations operating through the same MCM6 intron 13 enhancer region. The companion variants rs145946881 (-14010C, East African), rs41525747 (-13907G, East African/Afro-Asiatic), and rs4988235 (-13910T, European) all act through the same Oct-1/HNF1α enhancer mechanism to maintain LCT expression. Since each allele is independently sufficient to confer lactase persistence, compound carriage of two persistence alleles does not add incremental benefit — one copy of any persistence allele on one chromosome is sufficient to maintain lactase production. For individuals of East African ancestry, the non-persistent call is only meaningful when all relevant alleles have been tested and found absent.