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Founders Effect amongst the South African Population

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Founders Effect amongst the South African Population

The Afrikaner community are a geographically isolated group and it is assumed that founder effect has given rise to the high prevalence of some 20 inherited disorders in this community.

Project Aim:

To list all hereditary diseases that affect South Africans, and to attempt to trace the "Founder" individual / individuals who introduced the disease to South Africa.

Disclaimer:

All the information on this page is readily available on the internet, In some cases the "Founders" have been traced by searching for the published dates and places given in the various articles... because these "Founders" are not verified they are listed in brackets... further investigation is welcomed.

FOUNDER EVENTS Imagine you have a jar containing three different colors of marbles: red, yellow and green. If you pick just two or three marbles out of the jar, it's possible you might pick all yellow and red just by chance. If the different colors of marbles were different genes and the three marbles you picked were a new population, the new population would have only red and yellow genes but no green ones -- and that's very similar to the way founder events affect genetic variation. When a small group separates from a larger population and strikes out on its own, that small group might be carrying genes that are rare in the original population. These rare genes will now be common among the new group's descendants. Other genes present in the original population, however, may be absent from the new group altogether. Huntington's Disease, for example, is more common among the Afrikaner or Dutch-descent population of South Africa than in most other populations, because a gene for Huntington's happened to be unusually common among the small group of original Dutch colonists.

The case of Huntington's disease in South Africa provides a striking example of the founder effect.

Only 766 founding fathers were registered between 1691 and 1796 and they are the ancestors of nearly all present-day Afrikaners, according to the genealogical studies.

The Afrikaner population of South Africa is mainly descended from one shipload of immigrants which landed in 1652. The early colonists included individuals with a number of rare genes. The ship of 1652 contained a Dutch man carrying the gene for Huntington's disease, an autosomal dominant disease which does not appear until the sufferer is over 40 years old and leads to certain death within five to 10 years. Most cases of the disease in the modern Afrikaner population can be traced back to that individual.

Curiously, the founder Willem Schalk van der Merwe and his wife Elsje Cloete have been credited with introducing at least four diseases to the Afrikaner: Huntington’s chorea (Haydenetal, 1980), pseudoxanthoma elasticum (Torrington & Viljoen,1991), lipoid proteinosis (Heyl,1970) and schizophrenia (Karayiorgou et al, 2004).

Several diseases with an unusually high frequency in Afrikaners have been suggested to be the result of such founder effects:

  • Variegate porphyria (Dean,1963)
  • Beukes familial hip dysplasia (Cilliers & Beighton,1990)
  • Familial hyper cholestrol emia type I (Jenkins et al. 1980)
  • Huntington’s chorea (Haydenetal. 1980)
  • Fanconi anaemia (Rosendorffetal. 1987)
  • Pseudoxanthoma elasticum (Torrington & Viljoen 1991)
  • Progressive familial heart block type I (Torrington et al. 1986)
  • Lipoid proteinosis (Heyl,1970)
  • Sclerosteosis (Beighton et al. 1977)
  • Cystic fibrosis
  • Gaucher's disease
  • Autosomal recessive polycystic kidney disease
  • Familial colonic polyposis

Variegate porphyria

Variegate porphyria (also known as "Mixed hepatic porphyria", "Mixed porphyria", "South African genetic porphyria", and "South African porphyria") is an autosomal dominant porphyria that can have acute (severe but usually not long-lasting) symptoms along with symptoms that affect the skin. The disorder results from low levels of the enzyme responsible for the seventh step in heme production. Heme is a vital molecule for all of the body's organs. It is a component of hemoglobin, the molecule that carries oxygen in the blood.

Carriers develop a severe and sometimes lethal reaction to barbiturate anaesthetics, but the condition was relatively benign and selectively neutral until the advent of modern medicine. All traceable Afrikaner carriers today prove to be descendants of one couple, Gerritt Jansz and Ariaantje Jacobs, who emigrated from Holland in 1685 and 1688, respectively (Dean, 1972).

The disease most commonly linked with Afrikaaners is porphyria variegate, which affects approximately 1 out of every 200 members of the South African group. In the U.S. porphyria is limited to 1 in 25,000 people (Cartwright et al. 1978: 669).

The Genetic Basis of Common Diseases Oxford University Press, 12 Oct 2002

Huntington's disease

Huntington's chorea is prevalent among the Afrikaner population of South Africa. The origin of the gene for the disorder in this population group has been traced over 14 generations from the present time to the days of the first free burghers at the Cape of Good Hope. Over 200 affected individuals in more than 50 supposedly unrelated families have been found to be ancestrally related through a common progenitor in the 17th century. The individual carriers of the genes for Huntington's disease will have lower fitness than average, and selection will therefore act to reduce the frequency of the gene to zero. Thus its present high frequency suggests that the founder population had an even higher frequency, because it will have probably been decreased by selection since then. Any particular founder sample would not be expected to have a higher than average frequency of the Huntington's disease gene, but if enough colonizing groups set out, some of them are bound to have peculiar, or even very peculiar, gene frequencies.

Pseudoxanthoma Elasticum (PXE)

PXE is a rare heritable disorder of connective tissue with major clinical manifestations involving the cardiovascular system, eyes and skin. In South Africa, more than 100 patients in 58 families have been studied and it has become apparent that a clinically distinct form of the disorder has segregated in the Afrikaner population. The latter have profound eye manifestations with legal blindness often ensuing after the third decade of life.

The Afrikaner group are individuals derived from mainly Dutch, French Huguenot and German stock as a consequence of North European settlement of the Cape. The carrier status of PXE within the Afrikaner population in the Cape Province has been calculated as 1/76, as compared with 1/195 for all Caucasians and 1/289 for Blacks.

Parkinson's Disease

Fanconi Anemia

Sclerosteosis

Sclerosteosis is an autosomal recessive sclerosing bone dysplasia characterized by progressive skeletal overgrowth. The majority of affected individuals have been reported in the Afrikaner population of South Africa, where a high incidence of the disorder occurs as a result of a founder effect. The condition has been traced back 12 generations to three possible carriers who brought the responsible gene into South Africa.

Bone Dysplasia Sclerosteosis Results from Loss of the SOST Gene Product

Lipoid Proteinosis

Lipoid proteinosis is a rare disorder, although the literature contains more than 250 published cases. It occurs worldwide, but is more common in certain areas such as the Northern Cape province of South Africa, including Namaqualand. Here, it occurs predominantly in a population of mixed Khoisan and European origin in which a founder effect has long been suspected.

The original carrier of this condition has been found to be one Jacob Cloete, a German immigrant to the Cape in 1652. His great-grandson Gerrit Cloete migrated to Namaqualand in 1742. Because the area is somewhat isolated, consanguineous marriages were relatively common, leading to a relatively high proportion of homozygous affected individuals

Nasiri, Soheila; Sarrafi-rad, Nima; Kavand, Sima; & Saeedi, Marjan. (2008). Lipoid proteinosis: Report of three siblings. Dermatology Online Journal, 14(11)

Genealogical study of lipoid proteinosis in South Africa.

Autosomal recessive polycystic kidney disease (ARPKD)

Polycystic kidney disease was investigated in 28 families in which at least one member attended the paediatric nephrology clinic at a Johannesburg or Pretoria hospital. Twenty-five (89.3%) of the families had autosomal recessive polycystic kidney disease (ARPKD), and of these a significantly larger number than would have been expected (92%) were Afrikaans-speaking. There was a consanguineous marriage in the ancestry of 2 patients but 6 of the families (26%) had ancestors with one surname in common and 7 of the families (28%) had their origin in the western Transvaal. A point prevalence for ARPKD of 1:26,000 was estimated for the Afrikaans population (based on the age cohort 0 - 19 years) and the carrier rate for the gene was 1:83; on the basis of the live-birth rate for ARPKD of 1:11,000 the carrier rate is 1:53. These findings suggest that ARPKD may be unusually frequent in the Afrikaans-speaking population. A founder effect is probably responsible.

Progressive familial heart block

Two types of familial heart block have been recognized in certain Afrikaans-speaking families distributed over South Africa. These autosomal dominant conditions are progressive, and in the absence of treatment to regulate heart rate the defect of cardiac conduction causes syncope and sudden death. Type I progressive heart block was delineated in a family in which numerous cardiac deaths (22 in three generations) had occurred. Electrocardiographic investigations of 55 further family members demonstrated conduction defects in 31. Type II progressive heart block was recognized in another family with a strong history of sudden deaths, and electrocardiographic studies have revealed 10 affected individuals. These family studies are not yet complete, and the magnitude of the problem has not been determined. However, it is clear that both types of conduction abnormality have serious medical implications, have been spread widely by the founders, and are at present to be found in numerous predominantly Afrikaans-speaking families. The type I founder family can be traced back to the marriage in 1735 between a male immigrant from Lisbon, Portugal, who arrived at the Cape in 1696, and a woman of French descent. They settled in the Eastern Cape, and a large number of descendants from three branches of the family through at least six generations still live in the area. From the fourth generation onwards the family spread widely over South Africa and beyond. The founders of the type II family are the offspring of a Dutch immigrant from Amersfoort, who arrived at the Cape in 1713 and married a local woman in 1720. This couple had 4 children and the condition can be traced to a branch of the family founded by a son who settled in the Eastern Cape, where many descendants still live.

Cystic Fibrosis

Cystic Fibrosis, (CF) is an inherited genetic disease that affects a number of organs in the body, primarily the lungs and pancreas) by clogging them with thick, sticky mucus. Repeated infections and blockages can cause irreversible lung damage and death. Mucus blocks the tiny ducts of the pancreas which supply enzymes required for digestion, and consequently food is not properly digested and nutritional value is lost in the process.

An incidence of the genetic recessive disease cystic fibrosis (mucoviscidosis) far in excess of that reported recently from other countries, has been encountered in the South West African Afrikaner. This has probably resulted from the immigration of a segment of the South African Afrikaner population rich in the gene, into South West Africa, where, for religious reasons and reasons of geographical isolation, the gene has persisted and, perhaps, increased in frequency. Malaria, which killed many of the early settlers, might have selectively spared carriers of the gene, thus enriching its occurrence in the population

CF is one of the most common inherited disorders of Caucasians (whites). In South Africa 1 in 27 individuals in the White population, 1 in 50 in the Coloured population and at least 1 in 90 in the Black population carriers a CF mutation. The total is increasing as more children are correctly diagnosed, treated earlier and living much longer.

Osteodental Dysplasia

In South Africa's Cape Colored population, the high frequency of an autosomal dominant bone disease (osteodental dysplasia) that causes complete loss of teeth by age 20 stems from one polygamous Chinese immigrant named Arnold who, aided by seven wives, transmitted the syndrome to at least 70 of 356 traceable descendants (Jackson, 1951).

The Genetic Basis of Common Diseases Oxford University Press, 12 Oct 2002

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