Utilisation of predictive, prenatal and diagnostic testing for Huntington's disease in Johannesburg

CONTEXT

Huntington's disease (HD) is a dominantly inherited condition in which the gene defect is known. As such individuals in at-risk families can be tested before symptoms occur, prenatally, or after symptoms appear to confirm the diagnosis.

OBJECTIVES

To investigate the utilisation and sequelae of the predictive, prenatal and diagnostic services offered to families with suspected Huntington's disease.

DESIGN

A retrospective design was used. The 1975-1997 records of the Department of Human Genetics for all families with a history of HD presenting for genetic counselling and DNA analysis were studied.

SETTING

Department of Human Genetics, South African Institute for Medical Research and University of the Witwatersrand, Johannesburg.

SUBJECTS

There were 30 at-risk (50% risk) subjects for predictive testing, 7 women (10 pregnancies) for prenatal testing, and 52 subjects for diagnostic testing.

OUTCOME MEASURES

These were provided by the results from molecular studies and by the action taken by subjects after a predictive or prenatal result was given.

RESULTS

Altogether 15 (50%) subjects for predictive testing had a positive result, but none had serious psychiatric sequelae. Two women were found to be carrying an affected fetus and both requested pregnancy termination. Of 52 diagnostic tests, 33 (63%) were positive.

CONCLUSION

The service was used appropriately, and there were no traumatic incidents following positive results. There was no genotypic or sex bias in subjects presenting for testing. Black and white patients were equally likely to be positive for HD on diagnostic testing. The families appreciated the service and found it useful in the detection and prevention of HD.

High polymorphism at the human melanocortin 1 receptor locus

Variation in human skin/hair pigmentation is due to varied amounts of eumelanin (brown/black melanins) and phaeomelanin (red/yellow melanins) produced by the melanocytes. The melanocortin 1 receptor (MC1R) is a regulator of eu- and phaeomelanin production in the melanocytes, and MC1R mutations causing coat color changes are known in many mammals. We have sequenced the MC1R gene in 121 individuals sampled from world populations with an emphasis on Asian populations. We found variation at five nonsynonymous sites (resulting in the variants Arg67Gln, Asp84Glu, Val92Met, Arg151Cys, and Arg163Gln), but at only one synonymous site (A942G). Interestingly, the human consensus protein sequence is observed in all 25 African individuals studied, but at lower frequencies in the other populations examined, especially in East and Southeast Asians. The Arg163Gln variant is absent in the Africans studied, almost absent in Europeans, and at a low frequency (7%) in Indians, but is at an exceptionally high frequency (70%) in East and Southeast Asians. The MC1R gene in common and pygmy chimpanzees, gorilla, orangutan, and baboon was sequenced to study the evolution of MC1R. The ancestral human MC1R sequence is identical to the human consensus protein sequence, while MC1R varies considerably among higher primates. A comparison of the rates of substitution in genes in the melanocortin receptor family indicates that MC1R has evolved the fastest. In addition, the nucleotide diversity at the MC1R locus is shown to be several times higher than the average nucleotide diversity in human populations, possibly due to diversifying selection.

The molecular basis of transferase galactosaemia in South African negroids

Transferase galactosaemia is an autosomal recessively inherited disorder caused by a deficiency of galactose-1-phosphate uridyltransferase (GALT). Manifestations include jaundice, vomiting, cataracts, mental retardation, speech abnormalities and poor growth. The GALT gene has been mapped and sequenced. The S135L mutation accounts for approximately 48% of galactosaemia alleles in African Americans and has been found to account for about 91% of galactosaemia alleles in negroid South African patients which suggested that the mutation had an African origin. We have calculated the S135L allele frequency (+/- 1SE) in a sample of healthy unrelated negroid South Africans to be 0.0067 (+/- 0.0024). The S135L mutation was also detected in negroid populations from other regions of Africa confirming its African origin.

Brachydactyly type B with its distinct facies and 'Cooks syndrome' are the same entity

A sibling pair with brachydactyly type B born to a normal non-consanguineous couple are described and the severity of their condition discussed. It is proposed that a subgroup of individuals with brachydactyly type B principally involving the nails and distal phalanges, and also having distinct facies, might be identical to individuals having 'Cooks syndrome'.

Cystic fibrosis carrier frequencies in populations of African origin

Cystic fibrosis (CF) is a common autosomal recessive disorder in populations of European descent. However, very little is known about CF in populations of African origin among whom it has been believed to be extremely rare. The aim of this study was to determine if this is the case or whether it is under-reported. A CFTR mutation, 3120+1G-->A, which was first reported in three African-American CF patients, has been shown to account for 9-14% of African-American CF chromosomes. It has also been found in 4/6 CF chromosomes in South African blacks and one CF chromosome of Cameroonian origin. In order to determine the carrier frequency of the 3120+1G-->A mutation in Africa, 1360 unrelated, healthy subjects were screened. Nine carriers were identified. In addition, two out of five black CF patients with positive sweat tests were found to be heterozygous for the 3120+1G-->A mutation and two out of another four black patients with symptoms suggestive of CF, but unconfirmed by sweat tests, were heterozygous for the D1270N mutation. A further three CFTR mutations, A559T, S1255X, and 444delA, which had been found in African-American CF patients, were not identified in the patients or in over 373 healthy subjects tested. The 3120+1G-->A mutation has a carrier frequency of 1 in 91 (8/728) in South African blacks with a 95% confidence interval of 1 in 46 to 1 in 197. Since this mutation accounts for between 15% and 65% of CF chromosomes in South African blacks, a corrected CF carrier frequency would be between 1 in 14 and 1 in 59. Hence, the incidence of CF would be predicted to be between 1 in 784 and 1 in 13924 births in this population. There are several possible reasons why these people are not being detected. Some of these are misdiagnosis as chronic pulmonary infection, malnutrition, tuberculosis, infantile diarrhoea, failure to thrive, or a high infant mortality rate.

Mapping tissue angiotensin-converting enzyme and angiotensin AT1, AT2 and AT4 receptors

BACKGROUND

The renin-angiotensin system (RAS) functions as both a circulating endocrine system and a tissue paracrine/autocrine system. As a circulating peptide, angiotensin II (Ang II) plays a prominent role in blood-pressure control and body fluid and electrolyte balance by acting on the AT1 receptor in the brain and peripheral tissues. As a paracrine/autocrine peptide, locally formed Ang II also plays additional roles in tissues involving the regulation of regional haemodynamics, cell growth and remodelling, and neurotransmitter release. Evidence is emerging that Ang II is not the only active peptide of the RAS, and other Ang II fragments may also have important biological activities.

OBJECTIVES

To provide a morphological basis for understanding novel actions of angiotensin-converting enzyme (ACE), Ang II and related peptides in tissues, this article will review the localization of ACE and AT1, AT2 and AT4 receptors in the central nervous system, blood vessels and kidney.

RESULTS AND CONCLUSION

Autoradiographic mapping of the major components of the RAS has proved a valuable strategy to reveal, or suggest, cellular sites of novel actions for Ang II and related peptides in tissues. First, colocalization of ACE and AT1 receptors in the substantia nigra, the caudate nucleus and putamen of human and rat brain, which contain the dopamine-synthesizing neurons, suggests that the central RAS may be important in modulating central dopamine release. Secondly, the distribution of AT4 receptors with a striking association with cholinergic neurons, motor and sensory nuclei in the brain reveals that Ang IV may modulate central motor and sensory activities and memory. Thirdly, the occurrence of high levels of ACE and AT1 and/or AT2 receptors in the adventitia of blood vessels suggests important paracrine roles of the vascular RAS. Finally, the identification of abundant AT1 receptor and elucidation of its roles in the renomedullary interstitial cells of the kidney may provide a new impetus to study further the role of Ang II in the regulation of renal medullary function and blood pressure. Overall, circulating and locally produced Ang II and related peptides may exert a remarkable range of actions in the brain, kidney and cardiovascular system through multiple angiotensin receptors.

Failure to exclude a possible schizophrenia susceptibility locus on chromosome 13q14.1-q32 in southern African Bantu-speaking families

Several recent reports have provided evidence suggesting linkage of markers on chromosome 13q14.1-q32 to schizophrenia in families from England, Wales, Japan and the USA, but not in Chinese families. We tested for linkage between markers in this region and schizophrenia in a sample of 16 families multiply affected with schizophrenia drawn from the Bantu-speaking black population of South Africa. Twelve markers spanning 76 cM of chromosome 13q were examined in these analyses, including 10 markers covering the most positive region in the studies of the English, Welsh and Chinese families, and two additional markers yielding the largest positive LOD scores in the American study. The map of markers used was D13S126-14.6cM-D13S119-12.2cM-D13S144-10.+ ++2cM-D13S160-7.9cM-D13S121-6.3cM -D13S71-1.6cM-D13S122- 4.9cM-D13S128-8.9cM-D13S770-1.4cM-D13S7 79-2.2cM-D13S64-7.4cM-D13S173. Parametric two-point analysis yields strongly negative LOD scores across the region D13S71-D13S64 under all models, and D13S71-D13S173 under a recessive model, when analysing either the whole sample or affected individuals only. ALOD maxima are 0.0 when allowing for heterogeneity for all markers in this subset. Under recessive modelling, the ALOD maximum is 0.717, theta = 0.0, alpha = 0.45, for D13S126 when analysing all samples. Affected-only analysis of this marker yields a maximum LOD score of 0.645, theta = 0.1, and an ALOD maximum of 0.697, theta = 0.0, alpha = 0.55. Non-parametric multipoint analysis of these markers provides no support for excess sharing of alleles identical by descent, although D13S119 and D13S770 show some evidence for excess sharing of alleles identical by state.

A global survey of haplotype frequencies and linkage disequilibrium at the DRD2 locus

A four-site haplotype system at the dopamine D2 receptor locus (DRD2) has been studied in a global sample of 28 distinct populations. The haplotype system spans about 25 kb, encompassing the coding region of the gene. The four individual markers include three TaqI restriction site polymorphisms (RSPs) -- TaqI "A", "B", and "D" sites -- and one dinucleotide short tandem repeat polymorphism (STRP). All four of the marker systems are polymorphic in all regions of the world and in most individual populations. The haplotype system shows the highest average heterozygosity in Africa, a slightly lower average heterozygosity in Europe, and the lowest average heterozygosities in East Asia and the Americas. Across all populations, 20 of the 48 possible haplotypes reached a frequency of at least 5% in at least one population sample. However, no single population had more than six haplotypes reaching that frequency. In general, African populations had more haplotypes present in each population and more haplotypes occurring at a frequency of at least 5% in that population. Permutation tests for significance of overall disequilibrium (all sites considered simultaneously) were highly significant (P<0.001) in all 28 populations. Except for three African samples, the pairwise disequilibrium between the outermost RSP markers, TaqI "B" and "A", was highly significant with D' values greater than 0.8; in two of those exceptions the RSP marker was not polymorphic. Except for those same two African populations, the 16-repeat allele at the STRP also showed highly significant disequilibrium with the TaqI "B" site in all populations, with D' values usually greater than 0.7. Only four haplotypes account for more than 70% of all chromosomes in virtually all non-African populations, and two of those haplotypes account for more than 70% of all chromosomes in most East Asian and Amerindian populations. A new measure of the amount of overall disequilibrium shows least disequilibrium in African populations, somewhat more in European populations, and the greatest amount in East Asian and Amerindian populations. This pattern seems best explained by random genetic drift with low levels of recombination, a low mutation rate at the STRP, and essentially no recurrent mutation at the RSP sites, all in conjunction with an "Out of Africa" model for recent human evolution.

A global haplotype analysis of the myotonic dystrophy locus: implications for the evolution of modern humans and for the origin of myotonic dystrophy mutations

Haplotypes consisting of the (CTG)n repeat, as well as several flanking markers at the myotonic dystrophy (DM) locus, were analyzed in normal individuals from 25 human populations (5 African, 2 Middle Eastern, 3 European, 6 East Asian, 3 Pacific/Australo-Melanesian, and 6 Amerindian) and in five nonhuman primate species. Non-African populations have a subset of the haplotype diversity present in Africa, as well as a shared pattern of allelic association. (CTG)18-35 alleles (large normal) were observed only in northeastern African and non-African populations and exhibit strong linkage disequilibrium with three markers flanking the (CTG)n repeat. The pattern of haplotype diversity and linkage disequilibrium observed supports a recent African-origin model of modern human evolution and suggests that the original mutation event that gave rise to DM-causing alleles arose in a population ancestral to non-Africans prior to migration of modern humans out of Africa.

Management of tuberculosis in Wales: 1986-92

OBJECTIVES

To describe the epidemiology of childhood tuberculosis in Wales and to assess the standard of management of patients with tuberculosis.

DESIGN

Retrospective study of data retrieved from case notes and review of radiographs of all identified patients.

SUBJECTS

Forty eight cases of tuberculosis and 10 of tuberculosis chemoprophylaxis in children under 15 years of age, in Wales, between January 1986 and December 1992.

MAIN OUTCOME MEASURES

Management of childhood tuberculosis in Wales compared with the published recommendations of the Joint Tuberculosis Committee of the British Thoracic Society.

RESULTS

Documentation was poor in most of the 48 cases of tuberculosis and only 31 (65%) were formally notified. One third of patients were asymptomatic and were detected by contact tracing; only eight (17%) were culture positive. Only 20% of patients from ethnic minorities had previously been immunised with BCG. Management and chemotherapy varied widely. Few patients were managed jointly by paediatricians and chest doctors. Only 10% completed treatment with a recommended chemotherapy regimen. In 37% of patients treatment was inadequate, and in the remainder either the choice of drugs or the duration of treatment was inappropriate. No patient died, nor had any relapsed by June 1995.

CONCLUSIONS

Very few cases of childhood tuberculosis were managed according to the recommendations of the British Thoracic Society.

Out of Africa and back again: nested cladistic analysis of human Y chromosome variation

We surveyed nine diallelic polymorphic sites on the Y chromosomes of 1,544 individuals from Africa, Asia, Europe, Oceania, and the New World. Phylogenetic analyses of these nine sites resulted in a tree for 10 distinct Y haplotypes with a coalescence time of approximately 150,000 years. The 10 haplotypes were unevenly distributed among human populations: 5 were restricted to a particular continent, 2 were shared between Africa and Europe, 1 was present only in the Old World, and 2 were found in all geographic regions surveyed. The ancestral haplotype was limited to African populations. Random permutation procedures revealed statistically significant patterns of geographical structuring of this paternal genetic variation. The results of a nested cladistic analysis indicated that these geographical associations arose through a combination of processes, including restricted, recurrent gene flow (isolation by distance) and range expansions. We inferred that one of the oldest events in the nested cladistic analysis was a range expansion out of Africa which resulted in the complete replacement of Y chromosomes throughout the Old World, a finding consistent with many versions of the Out of Africa Replacement Model. A second and more recent range expansion brought Asian Y chromosomes back to Africa without replacing the indigenous African male gene pool. Thus, the previously observed high levels of Y chromosomal genetic diversity in Africa may be due in part to bidirectional population movements. Finally, a comparison of our results with those from nested cladistic analyses of human mtDNA and beta-globin data revealed different patterns of inferences for males and females concerning the relative roles of population history (range expansions) and population structure (recurrent gene flow), thereby adding a new sex-specific component to models of human evolution.

Rufous oculocutaneous albinism in southern African Blacks is caused by mutations in the TYRP1 gene

Oculocutaneous albinism (OCA) is the most common autosomal recessive disorder among southern African Blacks. There are three forms that account for almost all OCA types in this region. Tyrosinase-positive OCA (OCA2), which is the most common, affects approximately 1/3,900 newborns and has a carrier frequency of approximately 1/33. It is caused by mutations in the P gene on chromosome 15. Brown OCA (BOCA) and rufous OCA (ROCA) account for the majority of the remaining phenotypes. The prevalence of BOCA is unknown, but for ROCA it is approximately 1/8,500. Linkage analysis performed on nine ROCA families showed that ROCA was linked to an intragenic marker at the TYRP1 locus (maximum LOD score = 3.80 at straight theta=.00). Mutation analysis of 19 unrelated ROCA individuals revealed a nonsense mutation at codon 166 (S166X) in 17 (45%) of 38 ROCA chromosomes, and a second mutation (368delA) was found in an additional 19 (50%) of 38 chromosomes; mutations were not identified in the remaining 2 ROCA chromosomes. In one family, two siblings with a phenotypically unclassified form of albinism were found to be compound heterozygotes for mutations (S166X/368delA) at the TYRP1 locus and were heterozygous for a common 2.7-kb deletion in the P gene. These findings have highlighted the influence of genetic background on phenotype, in which the genotype at one locus can be influenced by the genotype at a second locus, leading to a modified phenotype. ROCA, which in southern African Blacks is caused by mutations in the TYRP1 gene, therefore should be referred to as "OCA3," since this is the third locus that has been shown to cause an OCA phenotype in humans.

Alu insertion polymorphisms and human evolution: evidence for a larger population size in Africa

Alu insertion polymorphisms (polymorphisms consisting of the presence/absence of an Alu element at a particular chromosomal location) offer several advantages over other nuclear DNA polymorphisms for human evolution studies. First, they are typed by rapid, simple, PCR-based assays; second, they are stable polymorphisms-newly inserted Alu elements rarely undergo deletion; third, the presence of an Alu element represents identity by descent-the probability that different Alu elements would independently insert into the exact same chromosomal location is negligible; and fourth, the ancestral state is known with certainty to be the absence of an Alu element. We report here a study of 8 loci in 1500 individuals from 34 worldwide populations. African populations exhibit the most between-population differentiation, and the population tree is rooted in Africa; moreover, the estimated effective time of separation of African versus non-African populations is 137,000 +/- 15,000 years ago, in accordance with other genetic data. However, a principal coordinates analysis indicates that populations from Sahul (Australia and New Guinea) are nearly as close to the hypothetical ancestor as are African populations, suggesting that there was an early expansion of tropical populations of our species. An analysis of heterozygosity versus genetic distance suggests that African populations have had a larger effective population size than non-African populations. Overall, these results support the African origin of modern humans in that an earlier expansion of the ancestors of African populations is indicated.

The founding mitochondrial DNA lineages of Tristan da Cunha Islanders

Genealogical histories show that the inhabitants of Tristan da Cunha are derived from a known number of founders. Using the transmission of mitochondrial DNA (mtDNA) from mother to offspring pairs, we traced the mtDNA types found in 161 extant individuals to five female founders. Although the historical data claimed that two pairs of sisters were among the founding females, mtDNA data showed support for only one pair of sisters. We also studied the fidelity of mtDNA transmission in conjunction with the genealogical data. We did not detect any mutations from 698 base pairs of sequence data from 75 individuals, which together accounted for 108 independent transmissions of mtDNA from mother to offspring. Based on this observation, we estimate that the mtDNA mutation rate is no more than one new mutation every 36 transmissions. These results indicate a high fidelity of maternal mtDNA transmission and support the utility of mtDNA in evolutionary and forensic studies.

Detection of numerous Y chromosome biallelic polymorphisms by denaturing high-performance liquid chromatography

Y chromosome haplotypes are particularly useful in deciphering human evolutionary history because they accentuate the effects of drift, migration, and range expansion. Significant acceleration of Y biallelic marker discovery and subsequent typing involving heteroduplex detection has been achieved by implementing an innovative and cost-efficient method called denaturing high-performance liquid chromatography (DHPLC). The power of the method resides in its sensitivity and ability to rapidly compare amplified sequences in an automated manner. We have determined the allelic states of 22 Y polymorphisms; 19 of which are unreported, in 718 diverse extant chromosomes; established haplotype frequencies; and deduced a phylogeny. All major geographic regions, including Eurasia, are characterized by mutations reflecting episodes of genetic drift and expansion. Most biallelic markers are localized regionally. However, some show wider dispersal and designate older, core haplotypes. One transversion defines a major haplogroup that distinguishes a previously unknown deep, apparently non-African branch. It provides evidence of an ancient bottleneck event. It is now possible to anticipate the inevitable detailed reconstruction of human Y chromosome genealogy based on several tens to even hundreds of these important polymorphisms.

Localization of the gene causing keratolytic winter erythema to chromosome 8p22-p23, and evidence for a founder effect in South African Afrikaans-speakers

Keratolytic winter erythema (KWE), also known as "Oudtshoorn skin disease," or "erythrokeratolysis hiemalis," is an autosomal dominant skin disorder of unknown etiology characterized by a cyclical erythema, hyperkeratosis, and recurrent and intermittent peeling of the palms and soles, particularly during winter. Initially KWE was believed to be unique to South Africa, but recently a large pedigree of German origin has been identified. The disorder occurs with a prevalence of 1/7,000 in the South African Afrikaans-speaking Caucasoid population, and this high frequency has been attributed to founder effect. After a number of candidate regions were excluded from linkage to KWE in both the German family and several South African families, a genomewide analysis was embarked on. Linkage to the microsatellite marker D8S550 on chromosome 8p22-p23 was initially observed, with a maximum LOD score (Z(max)) of 9.2 at a maximum recombination fraction (theta(max)) of .0 in the German family. Linkage was also demonstrated in five of the larger South African families, with Z(max) = 7.4 at theta(max) = .02. When haplotypes were constructed, 11 of 14 South African KWE families had the complete "ancestral" haplotype, and 3 demonstrated conservation of parts of this haplotype, supporting the hypothesis of founder effect. The chromosome segregating with the disease in the German family demonstrated a different haplotype, suggesting that these chromosomes do not have a common origin. Recombination events place the KWE gene in a 6-cM interval between D8S550 and D8S552. If it is assumed that there was a single South African founder, a proposed ancestral recombinant suggests that the gene is most likely in a 1-cM interval between D8S550 and D8S265.

Two newly identified mutations (Thr233Ile and Leu152Met) in partially adenosine deaminase-deficient (ADA-) individuals that result in differing biochemical and metabolic phenotypes

Deficiency of adenosine deaminase (ADA-) results in autosomal recessive immunodeficiency disease of varying severity. Partial ADA- [ADA deficiency in erythrocytes (RBCs) but substantial ADA in non-RBCs] has also been identified, primarily by population screening of healthy adults in Africa and newborns in New York State. Normal immune function and/or minimal elevations of toxic metabolites in childhood suggested that partial ADA deficiency was benign and therefore that six mutations identified in partially ADA-deficient newborns and expressing 8-80% of normal ADA in non-RBCs were not pathogenic. However, the lowest activity mutation (Arg211Cys) has now been reported in patients with adult-onset immunodeficiency. We have now molecularly and biochemically studied two additional individuals whom we found to represent opposite ends of the spectrum of partial ADA deficiency as to biochemical abnormalities and age of ascertainment. Homozygosity for a newly identified Leu152Met mutation expressing considerably less activity than the pathogenic Arg211Cys mutation was found in a currently healthy 10-year-old Afghanistani child (ascertained at birth). He had the highest accumulation of the metabolite dATP among 13 partially ADA-deficient patients studied, but considerably lower than in those with immunodeficiency. Homozygosity for a newly identified Thr233Ile mutation expressing somewhat greater ADA activity than Arg211Cys was found in a healthy young adult Kung individual, associated with very low metabolite concentrations. Biochemical findings and a family history suggestive of immunodeficiency in prior offspring support the idea that the Leu152Met mutation could result in disease in homozygous individuals challenged by severe environmental insult or in heterozygosity with a null mutation. The pathogenicity of the Thr233Ile mutation, as well as a previously described Ala215Thr mutation with relatively lower activity is less likely but will only be determined by long-term observation of individuals carrying these mutations. Although, in contrast to other partial mutations, neither of these two mutations are at CpG hot spots, the frequency of CpG mutations remains high for partial mutations but is also similarly high in ADA- immunodeficient patients (5/8 vs 12/21).

A linkage study of the N-methyl-D-aspartate receptor subunit gene loci and schizophrenia in southern African Bantu-speaking families

Both direct and indirect evidence implicate excitatory amino acid neurotransmission in the aetiology of schizophrenia. The data are particularly suggestive for N-methyl-D-aspartate (NMDA) neurotransmission. Four of the six genes coding for subunits of the neural NMDA receptor have been mapped. We have studied segregation and allele sharing of markers in these four regions in a sample of southern African Bantu-speaking families multiply affected with DSM-III-R schizophrenia. This population was chosen because anthropological and linguistic data suggest that it has diverged from a small initial population within the past 1000 years, making shared genetic aetiology more likely. We find positive LOD score maxima of 0.876 at a marker D9S1838 on chromosome 9q34.3 near the NMDAR1 central subunit gene, 0.758 at marker D17S784 on chromosome 17q25 near the NMDAR2C potentiating subunit gene, and 0.453 at marker D12S77 near the NMDAR2B gene on chromosome 12p12 when analysing affected samples only. Only the region of NMDAR2A, on chromosome 16p13, can be excluded in this population. There is evidence of increased allele sharing on chromosomes 9p34.3 and 17q25 using APM. Multipoint allele-sharing analysis using GENEHUNTER does not reject possible effects on chromosome 9q34.3, but does not support any involvement of chromosome 17q25. We propose that the NMDA receptor may be involved in the genetic predisposition to schizophrenia in this population through covariation in several of the subunits, which is consistent with the genetic models of the inheritance of the disease.

Feasibility of prenatal diagnosis and carrier detection in South African haemophilia A patients

The feasibility of DNA diagnosis for haemophilia A was tested in South African patients and families by screening for the common inversion mutation in the factor VIII gene and for the intragenic microsatellite markers in introns 13 and 22. The allele frequencies at the two microsatellite loci were significantly different, with informativity being higher in the Negroid (100%) than the Caucasoid group (67%). In severely affected haemophiliacs the inversion was found in 43% (6/14) of Negroids but in only 32% (13/41) of Caucasoids. Presence of a second common unidentified mutation may account for the low frequency in the latter. Haplotype analysis shows a disproportionately high frequency of an (AC)20 intron 13-(AC)26 intron 22 inversion negative Caucasoid haemophilia chromosome, supporting a founder effect.

Oculocutaneous albinism (OCA2) in sub-Saharan Africa: distribution of the common 2.7-kb P gene deletion mutation

Oculocutaneous albinism (OCA2) is the most common autosomal recessive disorder in the South African Negroid population, occurring with a prevalence of 1/3900 individuals. The OCA2 locus, P, has been mapped to chromosome 15q11-q13 and a 2.7-kb interstitial deletion has been found to be the common mutation in Africa. This study reports the detection of the deletion allele in OCA2-affected individuals from the southern African, Zambian and Central African Republic (CAR) Negroid populations (0.77, 131/170 OCA2 chromosomes; 0.79, 11/14; 0.33, 4/12, respectively). Normally pigmented individuals from different African countries were also tested. The deletion mutation was found at a frequency of 0.013 (10/780) in the normally pigmented southern African Negroid population and at a lower frequency in individuals from central Africa (0.002; 2/834), including individuals from Zambia, Cameroon, Zaire and the CAR. The study confirms the African origin of this deletion allele. Haplotype analysis suggests that the deletion mutation probably occurred only once and that it arose before the divergence of these African populations, which is estimated to be about 2000-3000 years ago. The unusually high frequency of OCA2 mutations, in particular the 2.7-kb deletion, suggests some selective agent or genetic drift.