Phenylketonuria in South Africa. A report on the status quo

During the 1980s a pilot newborn screening programme for the early detection (and treatment) of amino acidopathies, especially phenylketonuria (PKU), was conducted by the Department of National Health and Population Development. The motivation for this pilot programme was the high priority accorded PKU screening in Europe and North America and the presumed similarly high incidence of this condition among South Africans of European origin. From a cohort of 59,600 newborns screened in the Pretoria area over a period of 8 consecutive years (1979-1986), only 1 case of PKU (and 1 of tyrosinaemia) was found. Statistically this result is compatible (Poisson distribution, 95% confidence interval) with a 'true' incidence of not more than 3/59,600 (or about 1/20,000) newborns. It is concluded from this result and other relevant information that newborn screening for PKU and other amino acidopathies is not cost-effective and justifiable, especially against the background of prevailing demographic conditions and more pressing health priorities in South Africa. This particular screening programme was discontinued in 1986. The results and conclusions are presented here for the record.

The prevention of neural tube defects by folic acid supplementation

Neural tube defects, in particular spina bifida and anencephaly, are serious and relatively common congenital abnormalities worldwide. They also occur in South Africa and affect all population groups to varying degrees. The overall incidence in South Africa is approximately 1-2 per 1000 newborns. Higher incidences, up to 6 per 1000 newborns have been recorded in certain parts, especially in some rural areas of the country. In total as many as 1500 newborns could be affected by a neural tube defect each year. The precise aetiology of neural tube defects is still unknown.

Family history as a risk factor for coronary heart disease in South African families

This article presents data on, and applies a procedure for the statistical quantification of, family history as a risk factor for coronary heart disease (CHD) in three sub-samples (groups) of families: I--a healthy control group; II--families with familial hypercholesterolaemia (FH); and III--families identified by an index case with CHD. With regard to the average family history of CHD (calculated as an index for each family, and as a mean index for each group), group II differs significantly from group I and marginally significantly from group III; family groups I and III do not differ from each other statistically. By means of significance tests developed for this purpose, the groups of families are shown to be significantly heterogeneous, by being composed of families highly resistant against and susceptible to CHD. This is illustrated for example in group II, where some FH families can be shown to be highly resistant to CHD, compared with other FH families with a very strong history of CHD. The exact number and proportion of such families at different levels of significance is calculated and the actual families with the highest and lowest calculated family indices, respectively, are then identified (illustrated by examples). The practical significance of the statistical procedure of quantifying and applying family history as a risk factor for CHD, is discussed in terms of epidemiological and preventive health considerations.

Preventing cystic fibrosis in the RSA

Prenatal diagnostic testing for cystic fibrosis (CF) in South Africa has been available by microvillar enzyme (MVE) assay since 1984 and by DNA investigation since 1987. The advantages and practical uses of these two procedures are reviewed. Over the period 1984-1989, 59 MVE assays and 13 DNA investigations (1 woman had both done; total number of pregnancies tested 71) were performed for the prenatal diagnosis of CF in high-risk families. Of the 71 pregnancies tested (65 white woman, 4 mixed race and 2 Indian), 18 fetuses were found to be affected: of these, 15 couples chose to have the pregnancies terminated. In 2 fetuses tested the MVE assays were 'equivocal' and the babies were born affected. By using population genetic and demographic data it is estimated that the present rate of prenatal diagnosis and prevention satisfies about one-quarter of the projected and practically achievable annual need. It is also tentatively shown that only about 60% of the projected number of high-risk families are at present on record. It is concluded that more systematic efforts should be directed at carefully guided information and awareness campaigns, in order to draw more CF families into the mainstream of voluntary genetic services. The identification and cloning of the CF gene (in 1989) has made it possible to extend considerably the present strategy of prevention and to include lower-risk and extended CF families in due time.

Duchenne muscular dystrophy--a molecular service

In 1987 a carrier detection and prenatal diagnostic service for Duchenne muscular dystrophy using molecular technology was instituted at the Department of Human Genetics, University of Cape Town, to serve affected families in southern Africa. DNA samples from 100 affected male subjects and 350 of their relatives from a total of 110 families have been banked. To date restriction fragment length polymorphism (RFLP) analysis and deletion screening has been performed on the DNA of 60 male patients and 116 female relatives at risk of being carriers of the faulty gene. The DNA probes used were pERT 87-1 (MspI polymorphism) pERT 87-15, pXJ1.1, pXJ2.3 (TaqI, polymorphism), pXJ1.2 (BclI polymorphism), P20 (MspI and EcoRV polymorphism) and the cDNA probes. DNA deletions have been detected in 30 of the 60 affected boys and the carrier risks of 49 women have been determined by RFLP analyses. In those families where the risks were uncertain because the affected males had died, prenatal exclusion testing was offered to potential carriers. Two pregnancies were terminated when male fetuses were shown to be affected, since they had the same deletion as that observed in the proband.

Nonrandom association of genetic markers in a sample of South African blacks

According to classical genetic theory, allelic genes at one locus are expected to segregate and be manifested independently of allelic genes at another locus. At the population level any significant deviation from this general hypothesis resulting from specific biologic and genetic effects can be recognized in the form of nonrandom associations between genetic markers. The present data, consisting of 24 genetic polymorphisms determined from a sample of 998 unselected and unrelated South African blacks, offers an opportunity to test whether or not any such nonrandom associations exist between the genetic markers. After appropriate statistical calculations on the population data, we found that 13 pairs of genetic polymorphisms demonstrate a nonrandom association (statistically significant). Because the results cannot be explained in terms of known biologic mechanisms, we conclude that the associations observed could be due to random statistical effects (repeated application of the chi-square test) and/or to real (as yet unknown) biologic phenomena in the population studied. This tentative conclusion can serve as a guideline for more specific investigations.

On the prevention of rhesus immunisation in the RSA

The utilisation rate of anti-Rh antiserum in South African population groups for the years 1983-1985 was investigated. The number of pregnancies at risk of fetomaternal sensitisation because of Rh incompatibility and hence the number of women requiring prophylactic treatment with anti-Rh anti-serum shortly after delivery can be calculated from the known frequency of Rh-negative subjects in each population group. By relating the number of doses of anti-Rh antiserum actually distributed and used to the number of women requiring this prophylactic treatment, the crude utilisation rate of anti-Rh antiserum is calculated as 41-44% for all population groups combined. The rate for blacks is calculated at 14-20%, for whites 89-94%, for Indians 59-64% and for Coloureds 45-51% (the range of variation covers the years 1983-1985). These figures are discussed in terms of the approximations made for the calculations (hence 'crude utilisation rate'), the experience in other countries (specifically the USA) and the need for improving this facet of primary health care.

Linkage between the variegate porphyria (VP) and the alpha-1-antitrypsin (PI) genes on human chromosome 14

From family studies close linkage between the gene locus for variegate porphyria (VP) and the alpha-1-antitrypsin (PI) gene became evident. The maximal lod score from male meioses was 4.33 at theta = 0.04 and from both sexes combined 3.56 at theta = 0.12. Three pedigrees were triple informative regarding loci VP, PI, and IGHC (immunoglobulin heavy chain cluster, Gm polymorphism). In two of the respective meioses recombinations were observed, and in both cases the co-segregating VP and PI alleles were separated from the Gm haplotypes. These findings argue in favour of gene order either VP:PI:IGHC or PI:VP:IGHC.

Neonatal screening for congenital hypothyroidism. A decade's review, including South Africa

Since its introduction in 1974, neonatal screening for congenital hypothyroidism has been very extensively conducted world-wide. It is the commonest disorder found in all neonatal screening programmes, occurring in 1:3,000-8,000 births. Laboratory testing for thyroid function within the first 5 days after birth identifies this disorder weeks to months before clinical symptoms become evident and the clinical diagnosis is made. A screening programme in Pretoria (February 1981- October 1986) has identified 11 hypothyroid neonates in some 45,577 infants tested. Internationally, there is now a 10-year experience (and a follow-up for a number of programmes) of neonatal hypothyroidism diagnosed by laboratory screening tests and treated within weeks of birth. There is conclusive evidence that the physical and mental development of these children is within, or approaches to a great degree, normal limits.

Linkage relationship between the genes for adenosine deaminase and S-adenosyl-homocysteine hydrolase on human chromosome 20

The genes for adenosine deaminase (ADA) and S-adenosyl homocysteine hydrolase (AHCY or SAHH) are known to be syntenic and within measurable distance from each other, on chromosome 20 in man. In the present study an informative family is described in which the recombination fraction (theta) between the respective genes is estimated to be about 0.18. Together with the published finding of theta = 0.15 (Eiberg and Mohr 1985) in informative Danish families, the recombination fraction for the pooled data is calculated to be theta = 0.14 (in men), theta = 0.08 (in women) and theta = 0.13 (both sexes taken together).

Alpha-1-antitrypsin genetic polymorphism in South Africa

The alpha 1-antitrypsin (AAT) or protease inhibitor (Pi) genetic polymorphism was studied in 144 white, 100 coloured, 104 Indian and 127 black (Northern Sotho) healthy individuals (controls), in the Pretoria area. Their Pi phenotype and gene frequency distributions are compared with world-wide data on other population groups. The severely deficient Pi phenotypes S, Z and SZ jointly attain frequencies of 0.3-0.4% in coloureds and whites; in blacks and Indians the corresponding frequencies are very much lower. The implication for preventive medicine and public health is that in South Africa the sequelae of Pi deficiencies such as cirrhosis of the liver and/or emphysema of the lung are of practical importance in whites and coloureds and much less so in blacks and Indians. In 176 white breast cancer patients studied, the Pi phenotype and gene frequency distributions were found to be similar to those of healthy controls (not statistically significant). Cohorts of other patients were also phenotyped because of their low alpha 1-globulin concentrations in routine serum protein electrophoresis and/or their specific disease condition (cirrhosis of the liver or emphysema of the lung) known to be associated with AAT deficiency. These results are discussed in terms of their significance for family follow-up, genetic counselling and a preventive service. The need to avoid atmospheric pollution, especially cigarette smoke, is emphasised as a major and cost-effective preventive measure.

The plasminogen polymorphism in South African Negro populations: genetics and anthropogenetics

Genetic polymorphism of human plasminogen (PLG) was investigated in 1252 unrelated individuals from eight South African Bantu-speaking Negro tribes. PLG phenotypes were determined by isoelectric focusing (pH 3.5-9.5 and 5-8 gradients) of neuraminidase-treated samples and subsequent detection by caseinolytic overlay or immunoblotting with specific antibody. No significant difference in the distribution of PLG alleles among the eight ethnic groups was observed. The combined allele frequencies of the common alleles in South African Negroes were 0.6977 for PLG*A, 0.2736 for PLG*B. In addition, six rare alleles were seen: PLG*A3, *A1, *M2, *B1, *B2, *B3. The rare variant PLG*B2 was proven to segregate by autosomal Mendelian inheritance in a family. The combined frequency for the rare alleles was 0.0287. The distribution of phenotypes in the total population sample was found to be in Hardy-Weinberg equilibrium. A striking difference in PLG allele distribution between Negroes from South Africa and published Negroid frequencies from North America could be observed. This difference was also seen in comparison with Mongoloid populations; in contrast, PLG frequencies for South African Negroes were similar or almost identical to known Caucasoid distributions.

On the genetic interrelationships of South African Negroes

This study addresses the comparative genetic interrelationships between South African Negro groups. For this the genetic distances between seven ethnically defined Negro subsamples (total of 998 individuals) based on 24 genetic loci/polymorphisms are calculated by applying standard distance formulae. These computations offer an opportunity to evaluate the different polymorphisms in terms of their effects on the genetic distances. The genetic interrelationships thus computed are illustrated by way of dendrograms and are discussed in terms of their comparative significance. It follows from the findings that the Ndebele, Northern Sotho (Pedi), and Tswana form a closely related subcluster and that the Zulu and Swazi as well as the Venda and Shangana-Tsonga form two additional, more distant, subclusters. These results are discussed and tentatively interpreted against the background of the reported Khoisan admixture of the populations concerned as well as their ethnological history. The data are also compared to those derived from metric and dermatoglyphic studies. It is concluded that whereas there is some agreement between these categories of variation (genetic, metric, and dermatoglyphic) as far as the comparative evaluation of South African Negro groups is concerned, there also are discrepancies. These conclusions need to be explained in terms of evolutionary mechanisms (such as historic origins, hybridization, natural selection, and genetic drift) in order to obtain a more consistent and comprehensive comparative picture of the physical anthropology of southern African populations.

Maternal serum alpha-fetoprotein screening in Natal. Results of over 12 000 tests

Mass screening of maternal serum alpha-fetoprotein levels for the prenatal detection of fetuses with neural tube defects (NTDs) was introduced in Natal during 1979. From then until 31 October 1982, 12318 pregnancies were monitored and 16 fetuses with NTDs identified. A further 7 abnormal fetuses, 3 with defects other than of the neural tube, were encountered. It is shown that the screening process is reliable and that it does not generate much additional work for ultrasonographers or other laboratories. Few problems related to the management of the screening programme were met; they are discussed with particular reference to the role of ultrasonography.

Screening for inborn errors of metabolism among mentally retarded patients. Outcome of a surgery at the Witrand Care and Rehabilitation Centre

The prevalence of different types of inborn errors of metabolism among the mentally retarded patients at the Witrand Care and Rehabilitation Centre, were determined by means of a biochemical screening survey. These results are compared with those of other surveys in South Africa and abroad. One important result points to substantial differences in the recorded incidences of metabolic defects between surveys. This observation could partially be due to significant differences between the different studies in terms of methodology employed and sampling procedures. The questions raised in this regard are documented and discussed.

Anthropogenetic relationship between Negro population groups in South Africa based on genetic distances

The present study aims to establish the anthropogenetic relationship between Negro population groups in South Africa. The method applied for this purpose is based on the calculation of genetic distances for which several formulae have been developed in the recent scientific literature.

Age-dependency of somatic selection in South African Negro G-6-PD heterozygotes

The distribution of red cell G-6-PD phenotypes was analysed in terms of their manifestation in South African Negro females of different age classes. It was found that with increasing chronological age, over the age range from the teens to adulthood, the relative proportion of females expressing a "non BB" phenotype gradually decreases and the relative proportion of females expressing a "B" phenotype increases concomitantly. It is suggested that this phenomenon is attributable to age-dependent somatic selection against the deficient A- cell line in appropriate heterozygotes. This process can give rise to the misinterpretation of recording a growing excess of "homozygotes" and a corresponding increasing deficit of "heterozygotes" with ongoing chronological ageing. It is speculated that the postulated process of age-dependent somatic selection may be in response to malaria as the underlying selective agent. Considerations in this respect are discussed in greater detail.

South African Negroes: isoenzyme polymorphisms (GPT, PGM1, PGM2, AcP, AK, and ADA) and tentative genetic distances

Data on the genetic polymorphisms of the isoenzyme markers GPT, PGM1, PGM2, AcP, AK and ADA are presented for 7 ethnically defined Negro sub-samples (total n = 847-973, depending on the genetic marker concerned) from South Africa. The results are discussed in terms of previous findings, of the anthropogenetic interrelatedness of the populations concerned and of the factors assumed to have an influence on the distribution of the particular genetic markers. The entire data is subjected to the computation of "genetic distances" using several of the formulae available for this purpose. Some tentative conclusions on these results as well as on the formulae applied are drawn.

Erythrocyte G-6-PD and 6-PGD genetic polymorphisms in South African Negroes, with a note on G-6-PD and the malaria hypothesis

Sample of 981 and 998 South African Negroes belonging to seven different ethnic groups were screened for G-6-PD and 6-PGD phenotypes, respectively. The results are discussed in terms of the interethnic variability and the possible adaptive values of these genetic polymorphisms. Particular attention is paid to the geographic co-distribution and interrelationship of G-6-PD deficiency and the occurrence of malaria in South Africa.

A population genetic study on the Ag polymorphism in South African Indians and Negroids

Two population samples from South Africa, i.e. 555 Bantu-speaking Negroids and 75 Indians, were tested for selected immunogenetic specificities comprising the Ag polymorphism. Within the Negroid sample a new Ag phenotype (i.e. the 49th) was discovered. Interpopulational comparisons involving the present two, as well as three previously studied samples, reveal characteristic differences in regard to the frequency distribution of Ag phenotypes, to the Ag allele frequencies and to the occurrence of Ag haplotypes. These results are discussed. All the data could be interpreted satisfactorily in terms of the genetic model regarding the Ag system as postulated by Bütler and his co-workers.

Genetic markers and leprosy in South African negroes: Part II. Erythrocyte enzyme polymorphisms

The phenotype frequencies of the erythrocyte enzyme polymorphisms acid phosphatase (aP), phosphoglucomutase loci 1 and 2 (PGM1 and PGM2), adenylate kinase (AK), adenosine desaminase (ADA), esterase D (EsD) and 6-phosphogluconate dehydrogenase (6-PGD) were determined on a sample of 234-248 South African Negroes with leprosy. These results were compared with data of 841--997 healthy Negro controls of similar geographical and ethnic origin, in order to determine whether or not any association exists between specific phenotypes and the manifestation of leprosy. A part of the data included in the present study were compared with the data of a similar comparative analysis on Mozambican Negroes. With regard to the polymorphisms aP, PGM1 and PGM2, the results derived from South Africa and Mozambique exhibit reverse patterns of deviations from the null hypothesis. From this it does not appear justified to postulate an association between these genetic markers and the occurrence of leprosy. For the enzyme polymorphisms ADA, AK and EsD (data are confined to South African Negroes only) the distribution of phenotypes between patients and controls was very similar. The differences were not statistically significant. However, observations on the 6-PGD polymorphism (data are confined to South African Negroes only) showed an excess of phenotype PGD A among leprosy patients as compared with controls. The difference was statistically highly significant. Further studies based on additional samples are required to substantiate whether or not the statistical outcome reflects a true association between this phenotype and leprosy.

On the population genetics of beta2-glycoprotein I

Beta2-glycoprotein I typings on 152 healthy Germans and 150 patients with atopic diseases did not show any differences in the serum protein concentrations or in the phenotype and gene frequencies. Compared to these German samples, Philippinos (n = 88) as well as healthy Negroes from South Africa (n = 192) revealed statistically significant lower concentrations of this serum protein. They differ also from the Germans with regard to phenotype and gene frequencies. A most striking result was found in the comparison of healthy and leprous Negroes (n = 250) from South Africa. In these, quite different and statistically significant beta 2-Glycoprotein I concentrations, respectively, phenotype and gene frequencies were seen, which may be due to this disease. The possible reasons for these observations as well as for the observed population differences are discussed.

Genetic markers and leprosy in South African negroes. Part I. Serum protein polymorphisms

The phenotype frequencies of the serum protein polymorphisms Hp, Gc, Tf, Gm and Inv were determined on a sample of 250 South African Negroes with leprosy. These results were compared with data derived from 918-977 (depending on the polymorphism tested) healthy Negro controls of similar geographical and ethnic origin, in order to determine whether or not any association existed between specific phenotypes and the occurrence of leprosy. The data derived from the present study were also compared with those of similar comparative analyses on African and non-African populations. Because of the contradictory results between samples with regard to the polymorphisms Hp, Gc and Inv, an association of any of these phenotypes with leprosy appears to be highly improbable. With regard to the polymorphisms Tf and Gm, however, such associations cannot be ruled out. The questions arising from the results are discussed.

Preferential X inactivation in human placenta membranes: is the paternal X inactive in early embryonic development of female mammals?

In placenta membranes of newborn girls carrying electrophoretically distinguishable G6PD alleles, the maternally derived isozyme is expressed preferentially. This phenomenon cannot be explained by allelic differences in enzyme activity or by somatic selection directed against cells with particular G6PD phenotypes. Instead, it may be that in this tissue X inactivation is nonrandom. Preferential expression of the maternal X chromosome, as has been shown in marsupials and in extraembryonic membranes of rodents and now in man, may reflect the state of activity of the X chromosomes in the early stages of female embryonic development.

Distribution of G6PD phenotypes in red blood cells of Southern African Negroids: evidence for somatic selection

In a recent population study, we observed a striking deficit of G6PD heterozygotes among Southern African Negroid females. This finding was interpreted tentatively as evidence for a small number of hematopoetic stem cells in man. In a follow-up study we examined peripheral blood and cord blood in 547 mothers and in their newborn offspring. In mothers and sons, the frequencies of the G6PD alleles are apparently quite different. When the allele frequencies determined in sons are used for calculation of the expected phenotype frequencies in mothers and daughters, there is a large deficit of maternal G6PD AB phenotypes, and an equivalent surplus of G6PD homozygotes. However, no relevant heterozygote deficit is observed in newborn daughters. This discrepancy may be explained by the assumption that in peripheral blood of heterozygotes carrying the GdA- allele, G6PD-deficient cells progressively become eliminated during development from birth to adulthood. In other words, the large heterozygote deficit observed in adult females may be due to somatic selection rather than to a small pool of hematopoetic cells at the time of X differentiation.

The nomenclature of properdin factor B allotypes

In a comparative study the presently known eleven allotypes of properdin factor B (Bf) were examined. Bf polymorphism consists of the two common alleles F and S, the two less common alleles F 1 and S 1 and seven further rare alleles. A variant designation has been proposed according to their relative electrophoretic mobility in comparison to the migration difference between the S and F 1 band. There rare variant alleles were redesignated: F 1.55, SO.45 and SO.7, which previously had been described as F 1.6, S 0.8 and S 1, respectively. Conversion studies did neither reveal variant mobility in the Bb nor in the Ba fragment of factor B in three of the rare alleles. This finding confirms the earlier report on one of the variants, possibly suggesting the existence of a so far unknown third clearing fragment.

Serum protein polymorphisms Hp, Tf, Gc, Gm, Inv and Pt in Bantu speaking South African Negroids

Frequencies for the genetic polymorphisms of the serum proteins Hp, Tf, Gc, Gm, Inv and Pt are presented for 7 ethnically defined Negroid sub-samples (total n = 918--977, depending on genetic marker system) from South Africa. The results are discussed in terms of previous findings and of factors assumed to have an influence on the distribution of the particular genetic markers. The present report on the Pt ("post transferrin" approximately C3) polymorphism represents the first report on the genetic variation of the corresponding beta1A-globulin fraction within African Negroids (PtA = 0.009, PtB = 0.966, PtC = 0.025).

Glyoxalase I polymorphism in South African Bantu-speaking negroids

The newly described genetic polymorphism of glyoxalase I (GLO) is studied in seven ethnically defined Negroid samples from South Africa (total: n = 843). The allele frequencies between the different Negroid samples studied vary only marginally. However, the allele frequency of GLO1 for the South African Negroid samples combined (i.e., p = 0.259), is highly significantly lower than that for Caucasoid samples.

No evidence for linkage disequilibrium between Bf and GLO in African negroids

A sample of South African Negroids (n = 791) was scored for each individual's Bf and GLO phenotype. (The genes for the Bf and GLO olymorphisms are included in a known cluster of linked genes on chromosome 6.) Following a x2-test the respective two series of alleles were found to be disturbed at random, i.e., there was no evidence for a linkage disequilibrium. This result is discussed in terms of the linkage relationships and map distances of the genetic markers involved.

Tentative evidence for 3--4 haematopoetic stem cells in man

Blood specimens from a random sample of 981 South African Negroid females were typed electrophoretically inter alia for their G-6-PD phenotypes, The allele frequency for GdB and GdnonB was found to be 0.8126 and 0.1874 respectively. Calculating the number of individuals expected for each phenotypic class, a highly significant deviation from the Hardy-Weinberg equilibrium became manifest, i.e. there was a deficit of 24.6% of heterozygotes and an excess of 12.3% of each of the two classes of homozygotes. Several possible reasons for this discrepancy e.g. the effects of pooling sub-samples, selection and misclassifications due to insufficient staining were examined and were found not be likely explanations for the observed phenomenon. Instead, the result is interpreted as due to only 3--4 stem cells which give rise to the haematopoetic system in man.

Polymorphism of properdin factor B in South African Negroid, Indian and Colored populations

Properdin factor B phenotypes were determined in 7 Bantu speaking Negroid populations, 1 Indian, and 1 Colored population of South Africa in a total of 1258 individuals. In the Negroid populations allele frequencies were: BfF 0.655, BfS 0.282, BfRARE 0.063, in the Indian population: BfF 0.322, BfS 0.645, BfRARE 0.033, and in the Colored population: BfF 0.513, BfS 0.435, BfRARE 0.052. In addition, 2 so far unknown F alleles and possibly 1 new S allele were discovered.