Thursday, January 31, 2008

Sickle Cell Disease by the HbS haplotype

HbS of African derivation...some parties almost always associate its presence in non-African locations, or else what is perceived to be "non-Sub-Saharan" Africa locations, with historic slavery. How accurate is this observation?

Before one gets into the details about the issue of legitimacy of the above mentioned perception, it is perhaps necessary to know some basic things about HbS:

It is essentially an altered beta-globin (b-globin) cluster of the Hemoglobin A, a cluster which is located at position 15.5 away from the centromere of chromosome 11 on its short arm; the specific alteration that gives rise to HbS instead of the "normal" HbA, is a "point mutation" [*1] in GAC codon at position 6 of the b-globin cluster, which converts GAC to GTG, resulting in the replacement of glutamic acid amino acid by valine. Several haplotypes are associated with this mutation; there are supposed to be some 475 identified allele variants of the b-globin gene, of which the HbS haplotypes are a subset. This includes the African derivatives, namely; the Senegalese, Benin and Bantu haplotypes. Outside of African derivatives, there is the Asian haplotype, mainly found in Arabia and the Indian sub-continent.

Now, since it is difficult to age HbS variants with respect to one another, due to selective pressure acting on the extent of its availability, the distribution pattern becomes worthy of noting, as well as geographical origin. Apparently, the African haplotypes have the highest prevalence among African subjects and recent African descendants. Of the African variants, the Benin haplotype appears to have wider geographical reach, perhaps because it originated in the vicinity of the Niger River Valley, and spread through ancient contact in the Sahara. So, treating African HbS haplotypes unanimously as the product of slave trade is not only simplistic, but also erroneous. Consider the fact for example, that there are locations where African slaves had historically been brought as "pick up points" for trade between the trading parties, after having been picked up from their home region in some other geographical location, but are nonetheless virtually free from the sickle cell disease; the African Horn comes to mind as an example, and yet HbS has hardly penetrated that region. Some may be tempted to attribute this phenomenon to the environment of that region, but that assessment would be oblivous to the fact that Bantu-speaking populations neighbouring the populations of these regions, have high prevalence cases of HbS variants [i.e. Bantu haplotype]. This point is especially relevant, when its presence in Egypt is merely explained off as the product of that region being a historic corridor for slave trade. Additionally, it is necessary to note documented cases of HbS outside of sub-Saharan Africa, in order to realize that the antiquity of its heritage in those regions precedes historic slave trade involving slaves brought from sub-Saharan Africa. For examples, we have the following:

Haplotypes of the beta-globin gene as prognostic factors in sickle-cell disease.

el-Hazmi MA, Warsy AS, Bashir N, Beshlawi A, Hussain IR, Temtamy S, Qubaili F.Medical

Biochemistry Department, World Health Organization Collaborating Centre for Haemoglobinopathies, Thalassaemias and Enzymopathies, College of Medicine, King Khalid University Hospital, Riyadh, Saudi Arabia.

This study was conducted with two objectives: to determine the b-globin gene haplotypes associated with Hb S in Arabic-speaking populations from different countries (Egypt, Syrian Arab Republic and Jordan), and to compare the results with those of Saudi SCD patients from different regions of Saudi Arabia, where both mild and severe forms of the disease exist.

Patients and methods
The study group of 126 SCD patients comprised 14 Egyptians, 9 Syrians, 10 Jordanians and 93 Saudis. Some of the Egyptians, Syrians and Jordanians were living in Riyadh, while others were living in their own countries, from where buffy coats were received. The Saudis were from three areas of Saudi Arabia where the sickle-cell gene has been reported at a high frequency, these being eastern (22 patients), south-western (67) and north-western (4) regions...

Buffy coat was used to extract DNA [18]. Portions of DNA were first subjected to polymerase chain reaction (PCR) amplification, and the PCR product was restricted using Ava II, HindIII, HincII, Hpa I and Xmn I, following the procedure published earlier [19,20]. The presence or absence of each site (shown in Figure 1) was identified, based on the size of the fragments produced (Table 1).

"We collaborated with researchers from Egypt, Syrian Arab Republic and Jordan in a study of patients with sickle-cell disease from those countries, and from various parts of Saudi Arabia, in order to investigate the influence of genetics on the clinical presentation of the disease, and to attempt to determine the **origin** of the sickle-cell gene in Arabs. Our results suggest that beta-globin gene haplotypes influence the clinical presentation of sickle-cell disease, and that there are at least two major foci for the origin of the sickle-cell gene, one in the eastern part of Saudi Arabia, and the other in the populations of North Africa and the north-western part of the Arabian peninsula

The Benin haplotype was found in patients with severe disease, either as homozygous or in combination with another haplotype. The majority of Syrians and Jordanians had the Benin haplotype, and severe disease. However, one in three Syrians and one in five Jordanians had a milder disease, and the Saudi-Indian haplotype was identified.

All Saudi patients from south-western and north-western areas, where the disease is generally severe, had the Benin haplotype in the homozygous or heterozygous state. Of the Saudi patients from the eastern area, where a mild form of SCD exists, only 9% had the Benin haplotype. The remainder had the Saudi-Indian haplotype, either in its homozygous or heterozygous state

Restriction endonuclease restriction sites have provided a useful insight into the normal polymorphic variations in the DNA surrounding various gene loci, where a combination of two or more polymorphic sites has led to the identification of specific haplotype patterns [13,14]. This has been of significance in the study of the regions surrounding the b-globin gene (i.e. the b-globin gene cluster), where several polymorphic sites have been identified, and population differences have been found on analysis of the haplotype pattern [9]. An interesting observation is that the sickle-cell mutation has occurred on chromosomes carrying different polymorphic sites and different b-globin gene haplotypes, and this seems to play a role in the clinical expression of SCD [9].

We compared the haplotype pattern of SCD patients from different Arabic-speaking countries. Benin haplotype was the major haplotype in all countries with a severe presentation of SCD and it was present in both the homozygous and heterozygous state. This was true for those SCD patients from south-western and north-western areas of Saudi Arabia, and for those from Egypt, Jordan and Syrian Arab Republic. On the other hand, patients from the eastern part of Saudi Arabia, who present with a significantly milder clinical picture, carried the Saudi-Indian b-globin gene haplotype either in its homozygous or heterozygous state."

Source: East Mediterr Health J. 1999 Nov;5(6):1154-8

From the above, one gets the impression that the Benin haplotype is dominant over the other haplotypes wherein it appears in a heterozygous case. Not sure how that specifically goes on a case by case basis, wherein the Benin haplotype would appear with the other African variants, but at the least, this study demonstrates that it is the more likely dominant haplotype over that of the Asian haplotypes; the present author takes this to be the case [with the understanding that from the African side, only the Benin haplotype has notably penetrated the regions under discussion, while the others mentioned are of the Asian type], because it has always been identified with the severe cases of SCD in not only the homozygous cases, but also in the heterozygous cases...that is to say, what are the chances of heterozygous cases, especially in areas like eastern Saudi where the Asian haplotype is the prevalent HbS haplotype, being inclusive of Benin-Asian haplotype pairings? Quite likely—albeit to varying degrees, notwithstanding the discernable distribution patterns of these haplotypes as mentioned in the study. On the other hand, as noted, and to reiterate:

one in three Syrians and one in five Jordanians had a milder disease, and the Saudi-Indian haplotype was identified.

...this even when it occurred in either homozygous or heterozygous cases, not involving the Benin haplotype [which consistently appeared in severe cases of SCD, regardless of whether it was a heterozygous or homozygous case].

Another noteworthy case, is that SCD appears in the remains of predynastic subjects, and the only HbS haplotype noted in Egypt today is the Benin haplotype. As we will shortly see, the Benin haploype also accounts for all the known HbS cases outside of Africa, save for the Asian haplotype, largely confined to eastern Saudi Arabia and the Indian sub-continent.

Use of the amplification refractory mutation system (ARMS) in the study of HbS in predynastic Egyptian remains.

Marin A, Cerutti N, Massa ER.

1999 May-Jun

Dipartimento di Biologia Animale e dell'Uomo, Università degli Studi di Torino.

We conducted a molecular investigation of the presence of sicklemia in six predynastic Egyptian mummies (about 3200 BC) from the Anthropological and Ethnographic Museum of Turin. Previous studies of these remains showed the presence of severe anemia, while histological preparations of mummified tissues revealed hemolytic disorders. DNA was extracted from dental samples with a silica-gel method specific for ancient DNA. A modification of the polymerase chain reaction (PCR), called amplification refractory mutation system (ARMS) was then applied. ARMS is based on specific priming of the PCR and it permits diagnosis of single nucleotide mutations. In this method, amplification can occur only in the presence of the specific mutation being studied. The amplified DNA was analyzed by electrophoresis. In samples of three individuals, there was a band at the level of the HbS mutated fragment, indicating that they were affected by sicklemia. On the basis of our results, we discuss the possible uses of new molecular investigation systems in paleopathological diagnoses of genetic diseases and viral, bacterial and fungal infections.

PMID: 11148985 [PubMed - indexed for MEDLINE]

The interesting thing about the study, is that it too talks of severe cases of SCD, which as we've just seen, is the case with the Benin haplotype as well, and it takes us to a period that precedes any known documentation of slave trade involving sub-Saharan Africans.

Let's take a look at the distribution pattern and type of HbS haplotypes present in global samples:

The Benin haplotype accounts for HbS associated chromosomes in Sicily,4 Northern Greece,10 Southern Turkey,11 and South West Saudi Arabia,6,7 suggesting that these genes had their origin in West Africa. The Asian haplotype is rarely encountered outside its geographic origin because there have been few large population movements and Indian emigrants have been predominantly from non HbS containing populations. However, it is of interest that the Asian haplotype was first described among descendants of Indian indentured laborers in Jamaica.12 — Graham R. Serjeant, MD, FRCP, MRC Laboratories (Jamaica), University of the West Indies, Kingston.

The African type in the above regions appear to be exclusively the Benin haplotype, in contrast to...

From these original foci of the HbS mutation, the gene spread along trading routes to North Africa and the Mediterranean, was transported in large populations to North and South America and the Caribbean during the slave trade, and latterly has spread to Northern Europe by immigration from the Caribbean, directly from Africa to the United Kingdom, France, Belgium, and Holland, and from Turkey to Germany. The relative prevalence of these haplotypes in the Americas reflects the different origins of their African peoples, approximately 70% of HbS associated chromosomes having the Benin haplotype, 10% Senegal and 10% Bantu. Haplotype frequencies in Jamaica are similar to the USA but the Bantu haplotype accounts for the majority of HbS associated chromosomes in Brazil.9 — Graham R. Serjeant, MD, FRCP, MRC Laboratories (Jamaica), University of the West Indies, Kingstom.

It is of no coincidence that the other African haplotypes outside of the Benin haplotype, have found their way into the Americas, regions that have been affected by the historic slave trade, and have large size settler populations of recent common ancestry from sub-Saharan Africa.

What all these show, when taken together, is that blanket attribution of African HbS variants solely to historic slave trade events is a blatant distortion of reality.

Ps - A good map showing the geographical range of HbS haplotype variants:


NileValleys said...

Another excellent write-up. I am surprised to learn that traces of the sickle cell condition appear in ancient Egyptian mummies. That timeline is way before the historic African slave trade, and calls into question assorted claims (variously phrased) of "latecoming" negroes to the Nile Valey and other parts north and west. You mention sickle cell elements appearing in Italy but I have seen Porugal also mentioned in some general accounts. Exactly what is the pattern or breakdown of such appearances in Portugese or Italian populations?

Mystery Solver said...

Not sure if sickle cell disease has any coherent population-structuring in southern European regions [and Southwest Asia], other than to say that it is obviously prevalent, given the repeated detections of the disease in multiple studies undertaken. The fact that the Benin haplotype appears to be the exclusive type found in these regions, belies any attempt to associate its existence to the historic slave trade; in regions that have been impacted by the slave trade -- such as the U.S. and Caribbean countries like Jamaica, as Dr. Graham R. Serjeant noted, have representations for not only the Benin haplotype, but also the Senegalese and Bantu haplotypes!