Prevalence of mutations leading to complete C6 deficiency (C6Q0) in the Western Cape, South Africa and detection of novel mutations leading to C6Q0 in an Irish family.

Complement component C6 is one of five terminal complement components incorporated into the membrane attack complex. Complete deficiency of C6 (C6Q0) leads to an increased susceptibility to Neisseria meningitidis infections, and affected individuals typically present with recurrent meningococcal disease. There is a relatively high prevalence of C6Q0 in the Western Cape, South Africa and three frameshift mutations have previously been described to be responsible for C6Q0 in this area-879delG, 1195delC, and 1936delG (current nomenclature). We have now genotyped a further nine genetically independent individuals with C6Q0, confirming previous reports that the most common defect in the Western Cape is 879delG. Moreover, we report the first identification of the 878delA mutation within the Western Cape, which has previously only been reported in individuals of African descent living in the United States or Europe. We also investigated the genotype of an Irish C6Q0 individual and her sibling, and report two previously undescribed mutations. One mutation alters a tyrosine codon to a stop codon within exon 10. The second mutation is within the 5' donor splice site of intron 3, and would, in all probability, disrupt splicing. These two mutations were shown to segregate independently. We also discuss the nomenclature for reporting C6 and C7 gene mutations, as the current nomenclature does not follow the recognised guidelines.

Autosomal recessive hypercholesterolaemia: discrimination of ARH protein and LDLR function in the homozygous FH phenotype.

BACKGROUND: Phenocopies of homozygous familial hypercholesterolemia (hoFH) having autosomal recessive inheritance, were recently found to arise from defects in the LDL receptor (LDLR) adapter protein, called ARH, which facilitates the clearance of circulating LDL. Discrimination between the two causes of the phenotype at a clinical level may not be possible when parents display moderate hypercholesterolaemia. An effective strategy is thus required to identify the appropriate mechanism for the disorder. METHODS: Fibroblast LDL uptake studies were coupled with Western blotting for ARH protein in cell extracts, to identify the defective gene before DNA studies were initiated. Two subjects with the hoFH phenotype, but with indeterminate dyslipidaemia in their parents, were fully worked up. RESULTS: Defective LDL metabolism was established in both patients by functional and protein studies and further confirmed by detecting deleterious mutations, in the LDLR and ARH genes. The ARH patient is the first subject of Negroid identity to be described and records a specific mutation in this racial grouping. CONCLUSION: This study highlights the occasional complexity and uncertainty of a clinical diagnosis of hoFH and presents Western blotting of leucocyte extracts for ARH protein, as a rapid strategy for the detection of ARH before sequencing the gene for mutation(s). This strategy may be particularly useful in populations where founder mutations for ARH and LDLR defects are rare or co-exist.

X-linked hyper IgM (HIGM1) in an African kindred: the first report from South Africa.

BACKGROUND: The objective of this study was to describe the clinical and molecular features of the first South African family with X-linked hyper-IgM syndrome (HIGM1). METHODS: Diagnoses were based on immunoglobulin results and the absence of CD40 ligand (CD40L) expression on activated T-cells. Complete molecular characterisation involved CD40L cDNA sequencing, and genomic DNA analysis by polymerase chain reaction amplification, restriction enzyme digestion and sequencing. A PCR-based diagnostic assay was established for carrier detection and prenatal diagnosis in this family. RESULTS: There were originally six children, three males and three females. The eldest boy died after being diagnosed with hypogammaglobulinaemia, before HIGM1 was considered. This disorder was diagnosed in the second eldest boy at the age of 5 years, after failing to detect CD40L expression on his activated T-cells. A deficiency of CD40L was also confirmed in the youngest male at the age of 5 years. Both younger brothers have since died of infections relating to HIGM1. Molecular investigation showed that exon 3 was deleted from the CD40L mRNA of the affected males. Genomic DNA analysis identified a 1.5 kilobase deletion, spanning exon 3 and including extended flanking intronic sequence. Carrier status in the mother was confirmed by RT-PCR of her CD40L mRNA. Genetic analysis of the three female children was deferred because they were below the legal consenting age of 18 years. A PCR-based assay for genomic DNA was established for easy identification of female carriers and affected males in the future. CONCLUSIONS: This study confirmed the diagnosis of HIGM1 in the first South African family to be investigated and identified a novel mutation in the CD40L gene.

The AGT gene in Africa: a distinctive minor allele haplotype, a polymorphism (V326I), and a novel PH1 mutation (A112D) in Black Africans.

We describe a novel missense mutation (A112D) and polymorphism (V326I) in the human AGT gene in two black African patients with primary hyperoxaluria type 1, an autosomal recessive disease resulting from a deficiency of the liver peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT; EC 2.6.1.44). V326I was found in DNA from normal control Blacks with an allele frequency of 3%. Expression studies confirmed that A112D reduced AGT enzyme activity by 95% while V326I had no effect. Both A112D and V326I were homozygous in both patients and lie on a variant of the minor allele of the AGT gene. This variant haplotype, Mi(A), includes an intron 1 duplication and intron 4 VNTR (38 repeat) but lacks the P11L and I340M normally associated with the minor allele in Caucasians. Among the South African Blacks tested, the Mi(A) haplotype had an allele frequency of 12% compared to 3 % for the Caucasian-type minor allele haplotype.