Evolution and Spread of HbS

The gene for β globin (OMIM  is present on chromosome 11 (11p15.4) along with other globin genes (ε, γ, γ and δ). This is known as the β-globin cluster . Individuals carrying identical genes on the β-globin gene cluster may not have identical DNA sequences in non-codeing regions of the DNA of the cluster. The non-coding regions include segments of DNA between genes and introns within genes. . Differences in DNA exist between individuals every 1000-2000 bases in the form of single nucleotide polymorphisms (SNPs). Single nucleotide polymorphisms are variations in a single nucleotide that occurs at a specific position in the genome. Many of these differences have no consequences on gene expression because either they do not result in change in amino acid sequence or they occur in regions of DNA that neither code for the gene nor regulate the gene. SNPs evolve by spontaneous mutations over time. The lesser the number of such differences between two individuals closer the individuals are the each other genetically (and in terms of evolution). Fewer differences in SNPs between individuals mean a more recent common ancestor.

One of the meanings of the word haplotype is a pattern of SNPs. A haplotype may be considered as a DNA “environment” in which the gene(s) occurs. This “environment” is created by the sequence of single nucleotide polymorphisms in which the gene(s) exists. As mentioned above differences in SNPs (and hence the “environment” the gene(s) exist in) evolve by spontaneous mutations over period of time. Fewer the differences between the “environments” the genes occurs in the more the likelihood that they come from related individuals.

HbS results from a single base substitution in the codon 6 of the β-globin gene. GAG becomes GTA resulting in substitution of valine for glutamate. This change results in a haemoglobin that crystallizes in hypoxic conditions resulting in a haemolytic anaemia. HbS occurs in diverse population groups including African, Mediterranean, Middle-Eastern and Indian. Is the haplotype of the HbS gene in these regions similar?

The HbS mutation occurs on five different haplotypes four African and one Arab-Indian. The mutation is the same (GAG to GTA on codon 6) but the SNPs are different. The haplotypes are

  1. Senegal: The Senegal HbS haplotype is found in Atlantic West Africa and Portugal
  2. Benin: The Benin HbS haplotype is found Central West Africa, Northern Africa and Mediterranean Europe (Greece, Sicily)
  3. Central African Republic or Bantu: The Central African Republic or Bantu is found in South Central and Eastern Africa
  4. Cameroon: The Cameroon haplotype is found in the Eton ethnic group of eastern Cameroon
  5. Arab-Indian: The Arab-Indian haplotype is the only non-African phenotype of HbS found in the eastern oasis of Saudi Arabia and India.

Origin of Haplotypes

There are two theories about the origin of haplotypes. The first, and the more accepted one, states that the five haplotypes arose from five independent mutations. An alternative hypothesis states that HbS mutation occurred only once and spread to other haplotypes by gene conversion.

 

Haplotypes and Severity of Symptoms

Symptoms of sickle cell anaemia are a consequence of crystallisation of haemoglobin under hypoxic conditions. HbF inhibits sickling. Patients with high HbF have fewer symptoms. The Arab-Indian and the Senegal haplotype are associated with higher HbF levels (17% and 12.4% respectively). In general patients carrying these haplotypes have milder symptoms than the Bantu or Benin haplotypes (Blood 2014; 123: 481)

 

Haplotypes and Human Migrations

Trade, conquests and human migrations (voluntary and slave trade) have disseminated the African haplotypes beyond Africa.

  1. The Mediterranean: Most of the Mediterranean (Greece and Scilly) has the Benin haplotype. This reflects pre-historic migrations from Central West Africa along the then fertile Sahara to North Africa. From here it spread to the Mediterranean via the interactions (Trade and wars) between the two regions. The only exception is Portugal. Portugal has the Senegal haplotype which reflects the trading contacts between Portugal and Atlantic West Africa (Angola and Mozambique).
  2. Americas: Neither the native americans nor the original European settlers to the Americas carried the HbS gene. HbS was imported to the Americas with the slaves from Africa. Jamaica was an important slave import hub and records for where tthe slaves arrived from are available. Jamaica has 73% Benin haplotype, 17% Bantu and 10% Senegal haplotypes. These numbers are close to the actual number of slaves who arrived in Jamaica from regions of Africa where these haplotypes are prevalent. Similarly the distribution of haplotype correspond to the origins of slaves in Baltimore and South Carolina (Mariam Bloom. Understanding Sickle Cell Disease, Page 34).
  3. Arab or Indian: It is not clear if the Arab-Indian haplotype originated in India or Saudi Arabia. But considering that all of tribal India has only one haplotype but the East and West Arabian Peninsula have different haplotypes it is possible that the haplotype originated in India.
  4. Spread to Other Parts: As opposed to the era of slave trade modern migration of people in the recent past have been voluntary. These populations have spread across the world as have those form mediterranean but to a lesser extent. These migrations have introduced the HbS gene in areas where it was not indigenous.

 

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