Browsing by Author "Loubser, Odell"
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- Item'n Ondersoek na die hiperlipidemie-veroorsakende mutasies en hul geassosieerde haplotipes in Suid-Afrikaanse hiperlipidemiese kleurlinge(Stellenbosch : Stellenbosch University, 1994) Loubser, Odell; Odendaal, H. J.; Kotze, Maritha J.; Stellenbosch University. Faculty of Medicine and Health Sciences. Department of Biomedical Sciences.ENGLISH ABSTRACT: Coronary heart disease (CHD), common among South African whites, is increasingly manifesting itself in the coloured population of South Africa. More than 70% of coloureds participating in the CRISIC I study, a study undertaken to investigate CHD risk factors in the coloureds of the Cape Peninsula, had serum cholesterol levels imparting CHD risk. Increased levels of plasma low-density lipoprotein cholesterol (LDL-C) are the hallmark of the disease familial hypercholesterolemia (FH). The underlying molecular defect of FH consists of mutations in the low-density lipoprotein (LDL) receptor gene. The coloured study population was screened for the presence of the three founder related point mutations in the LDL receptor gene that account for more than 90% of FH cases in the Afrikaner population of South Africa. The prevalence of the FH Afrikaner mutations was determined in 31 FH heterozygotes and 39 hyperlipidemic coloureds. The 39 hyperlipidemic coloureds did not meet the criteria for heterozygous FH. The FH Afrikaner-1 (Asp₂₀₆→Glu , FH-1) en FH-Afrikaner-2 (Val₄₀₈→Met FH-2) mutations had the same prevalence, 12.9% (4 individuals), in the FH group. The FH Afrikaner-3 (Asp₁₅₄→Asn, FH-3) mutation was absent in the FH group. The three FH Afrikaner mutations were not detected in the hyperlipidemic group of 39 individuals. Haplotype analysis in the families with the FH-1 mutation showed that the chromosomal background of the mutation was compatible with that described in the Afrikaner patients. In the FH-2 heterozygous patients the mutation was found on a haplotype that differed from the Afrikaner haplotype at polymorphic sites at both sides of the defect in exon 9. These results indicate that identical LDL-receptor gene mutations originated in two different South African population groups due to independent events at a potential CpG dinucleotide "hotspot". The clinical effects of the FH-1 and FH-2 mutations were the same as described in the Afrikaners. Heteroduplex analysis detected a 3 bp deletion that causes deletion of amino acid Gly₁₉₇ (FH Lithuania) in one of 23 FH heterozygous coloured patients (4.3 %) and in one indiviaual of 90 hyperlipidemic coloureds (1.1%) already screened for the FH Afrikaner mutations , by means of heteroduplex analysis. A hyperlipidemic group of 66 coloureds, screened for the first time for mutations in exon 4 of the LDL-receptor gene by means of heteroduplex analysis, showed one individual (1.5%) with the FH Lithuania mutation. In all cases, the haplotype of the mutant allele was compatible with that described in Ashkenazi Jews. The heteroduplex screening method was sensitive enough to detect the FH-1 mutation, as 5 individuals of the group of 66 hyperlipidemic coloureds were found to have the FH-1 mutation. A total of 22 FH heterozygous coloured patients, negative for the above mentioned mutations, were screened for new mutations in exon 4 of the LDL-receptor gene by means of the single-strand-conformation-polymorphism (SSCP) technique and the heteroduplex method. A new mutation, hitherto not described, was detected in one patient by means of the SSCP technique. A point mutation (A to C) at nucleotide 671 in exon 4 of the LDL receptor cDNA results in an amino acid change from aspartic acid to alanine at residue 203 in the cysteine rich ligand binding domain of the LDL receptor. The mutation gives rise to an additional Hae Ill restriction site in the DNA of affected subjects. The accurate diagnosis of this so-called FH Tygerberg mutation in subjects with FH is possible now by means of the amplification of genomic DNA (using the polymerase chain reaction) and restriction enzyme analysis. The allele frequencies of four restriction-fragment-length-polymorphisms (RFLP's) in the LDL-receptor gene were determined for 17 FH heterozygous patients and 21 control individuals. Statistical analysis indicated a significant rise in the frequency of the least frequent allele of the Ava 11 polymorphism (x2= 7.14, P<0.01). This indicates population association of the allele with a gene that causes FH. The heterozygosities and PIC values of each RFLP site were determined, and these values disclosed that the Sma I site would be the most informative in genetic linkage studies, followed by the Ava II and Nco I sites. These three sites were all moderately polymorphic. The Stu I site was the least polymorphic and therefore would be the least informative of the four RFLP sites in genetic studies. A 478 bp deletion in exon 18 of the LDL-receptor gene was detected in 2 FH heterozygous coloured patients. One of the two patients was heterozygous for the FH-1 mutation. This deletion is most likely a polymorphism as it was detected in the 3'-nontranslated region of the LDL-receptor gene and therefore it may be used in genetic marker analysis. This study suggests that the moderately informative RFLP sites (Sma I, Ava II and Nco I) must be combined with the TA dinucleotide repeat locus, also moderately informative, to indirectly diagnose FH in families where the direct molecular diagnosis of FH, by means of the detection of the FH associated mutations, is not possible. A better molecular diagnostic service is now possible for the coloureds with a high risk for CHD by combining the screening for FH associated mutations with RFLP and dinucleotide repeat polymorphism studies.