CBS Mutations Research Articles

Development of low-cost in-house tetra-ARMS-PCR assay for the screening of five CBS mutations found in Pakistani homocystinuria patients

Background Classical homocystinuria is an inborn amino acid metabolism disorder resulting from mutations in the Cystathionine-β-Synthase (CBS) gene. These mutations lead to elevated homocysteine and methionine levels and reduced cysteine levels in the blood. Typically, diagnosis occurs after patients display symptoms, and various lab methods confirm it. DNA sequencing is the best option for early detection of genetic variants in asymptomatic suspected individuals. Unfortunately, its high cost can hinder its use, especially in low-income countries like Pakistan. Objective Aim of this study was to devise a robust low-cost diagnostic/screening assay based on Tetra-ARMS-PCR for five prevalent genetic variants found in Pakistani classical homocystinuria patients. Materials and methods In the current study, T-ARMS-PCR assays were developed for five mutations (c.975G > C, c.770C > T, c.752T > C, c.1039 + 1G > T, c.451 + 1GG > TA), which were characterized previously in classical homocystinuria patients. These low-cost T-ARMS-PCR assays were then used to screen the affected individuals and their family members to identify their genotypes for pathogenic variations in the asymptomatic patients and carriers in their respective families. Results The outcomes were entirely consistent with those obtained from Sanger DNA sequencing, confirming the sensitivity, specificity, and reliability of the T-ARMS-PCR assay for detecting CBS mutations. Conclusion T-ARMS-PCR has wide applications for low-income countries for the screening and early diagnosis of asymptomatic patients and carriers in the homocystinuria affected families as well as other inherited diseases.

Clinical exome sequencing uncovers a high frequency of Mendelian disorders in infants with stroke: A retrospective analysis.

Stroke causes significant disability and is a common cause of death worldwide. Previous studies have estimated that 1%-5% of stroke is attributable to monogenic etiologies. We set out to assess the utility of clinical exome sequencing (ES) in the evaluation of stroke. We retrospectively analyzed 124 individuals who received ES at the Baylor Genetics reference lab between 2012 and 2021 who had stroke as a major part of their reported phenotype. Ages ranged from 10 days to 69 years. 8.9% of the cohort received a diagnosis, including 25% of infants less than 1year old; an additional 10.5% of the cohort received a probable diagnosis. We identified several syndromes that predispose to stroke such as COL4A1-related brain small vessel disease, homocystinuria caused by CBS mutation, POLG-related disorders, TTC19-linked mitochondrial disease, and RNASEH2A associated Aicardi-Goutieres syndrome. We also observed pathogenic variants in NSD1, PKHD1, HRAS, and ATP13A2, which are genes rarely associated with stroke. Although stroke is a complex phenotype with varying pathologies and risk factors, these results show that use of exome sequencing can be highly relevant in stroke, especially for those presenting <1year of age.

Suspecting classical homocystinuria in an adolescent born before the newborn screening program

Introduction: Classical homocystinuria (HCU) is an autosomal recessive disorder caused by a deficiency in the cystathionine beta-synthase enzyme and associated with a high probability of vascular complications. Herein is presented the case of an adolescent diagnosed with HCU during cerebral venous sinus thrombosis (CVST) study. Case Report: A 14-year-old girl presented with thrombophilia screening tests suggestive of HCU during CVST study. After referral to an Inherited Metabolic Diseases Unit, she started supplementation with pyridoxine, folic acid, vitamin B12, betaine anhydrous, and cysteine and was advised to restrict natural proteins and methionine from diet. Genetic analysis revealed a homozygous CBS mutation (c.572C>T (p.T191M) with c.699C>T (p.Y233Y) polymorphism. Discussion: In adolescents born before 2004 (year of implementation of the Portuguese newborn screening program), HCU should be considered when studying hypercoagulability syndromes, as it is a treatable condition and treatment can prevent major morbidity and mortality causes.

Screening for inherited thrombophilia and genome sequencing

To screening coagulation and anticoagulation genes and mutations in Chinese patients with venous thromboembolism suspected of inhered thrombophilia. Patients with venous thromboembolism caused by inherited thrombophilia were collected from the second Affiliated Hospital, Zhejiang University School of Medicine during June 2015 and December 2017. Gene sequencing was conducted to screen the anticoagulant-related genes PROC, PROS1, SERPINC1 and coagulation-related genes F5, F2, F8, CBS, MTHFR. The thousands of human genome database, ESP6500 database, Genoma database, HGMD mutation database were used to compare the mutation sites, and the pathogenic mutation sites were predicted according to SIFT, Polyphen, MutationTaster and CADD databases. Among 25 patients screened, eight patients with PROC mutations, two patients with PROS1 mutations, three patients with SERPINC1 mutations; one patient with F5 mutation, one patient with F2 mutation, one patient with F8 mutation, two patient with CBS mutations, and one patient with MTHFR mutation. The second generation gene sequencing helps to diagnose inherited thrombophilia and guide the decision of anticoagulation duration.

Restoration of skeletal muscle homeostasis by hydrogen sulfide during hyperhomocysteinemia-mediated oxidative/ER stress condition 1.

Elevated homocysteine (Hcy), i.e., hyperhomocysteinemia (HHcy), causes skeletal muscle myopathy. Among many cellular and metabolic alterations caused by HHcy, oxidative and endoplasmic reticulum (ER) stress are considered the major ones; however, the precise molecular mechanism(s) in this process is unclear. Nevertheless, there is no treatment option available to treat HHcy-mediated muscle injury. Hydrogen sulfide (H2S) is increasingly recognized as a potent anti-oxidant, anti-apoptotic/necrotic/pyroptotic, and anti-inflammatory compound and also has been shown to improve angiogenesis during ischemic injury. Patients with CBS mutation produce less H2S, making them vulnerable to Hcy-mediated cellular damage. Many studies have reported bidirectional regulation of ER stress in apoptosis through JNK activation and concomitant attenuation of cell proliferation and protein synthesis via PI3K/AKT axis. Whether H2S mitigates these detrimental effects of HHcy on muscle remains unexplored. In this review, we discuss molecular mechanisms of HHcy-mediated oxidative/ER stress responses, apoptosis, angiogenesis, and atrophic changes in skeletal muscle and how H2S can restore skeletal muscle homeostasis during HHcy condition. This review also highlights the molecular mechanisms on how H2S could be developed as a clinically relevant therapeutic option for chronic conditions that are aggravated by HHcy.

CBS mutations are good predictors for B6-responsiveness: A study based on the analysis of 35 Brazilian Classical Homocystinuria patients.

CBS mutations are good predictors for B6-responsiveness: A study based on the analysis of 35 Brazilian Classical Homocystinuria patients.

Hydrogen sulfide alleviates hyperhomocysteinemia-mediated skeletal muscle atrophy via mitigation of oxidative and endoplasmic reticulum stress injury.

Although hyperhomocysteinemia (HHcy) occurs because of the deficiency in cystathionine-β-synthase (CBS) causing skeletal muscle dysfunction, it is still unclear whether this effect is mediated through oxidative stress, endoplasmic reticulum (ER) stress, or both. Nevertheless, there is no treatment option available to improve HHcy-mediated muscle injury. Hydrogen sulfide (H2S) is an antioxidant compound, and patients with CBS mutation do not produce H2S. In this study, we hypothesized that H2S mitigates HHcy-induced redox imbalance/ER stress during skeletal muscle atrophy via JNK phosphorylation. We used CBS+/- mice to study HHcy-mediated muscle atrophy, and treated them with sodium hydrogen sulfide (NaHS; an H2S donor). Proteins and mRNAs were examined by Western blots and quantitative PCR. Proinflammatory cytokines were also measured. Muscle mass and strength were studied via fatigue susceptibility test. Our data revealed that HHcy was detrimental to skeletal mass, particularly gastrocnemius and quadriceps muscle weight. We noticed that oxidative stress was reversed by NaHS in homocysteine (Hcy)-treated C2C12 cells. Interestingly, ER stress markers (GRP78, ATF6, pIRE1α, and pJNK) were elevated in vivo and in vitro, and NaHS mitigated these effects. Additionally, we observed that JNK phosphorylation was upregulated in C2C12 after Hcy treatment, but NaHS could not reduce this effect. Furthermore, inflammatory cytokines IL-6 and TNF-α were higher in plasma from CBS as compared with wild-type mice. FOXO1-mediated Atrogin-1 and MuRF-1 upregulation were attenuated by NaHS. Functional studies revealed that NaHS administration improved muscle fatigability in CBS+/- mice. In conclusion, our work provides evidence that NaHS is beneficial in mitigating HHcy-mediated skeletal injury incited by oxidative/ER stress responses.

CBS mutations are good predictors for B6-responsiveness: A study based on the analysis of 35 Brazilian Classical Homocystinuria patients.

BackgroundClassical homocystinuria (HCU) is a monogenic disease caused by the deficient activity of cystathionine β‐synthase (CβS). The objective of this study was to identify the CBS mutations in Brazilian patients with HCU.Methods gDNA samples were obtained for 35 patients (30 families) with biochemically confirmed diagnosis of HCU. All exons and exon‐intron boundaries of CBS gene were sequenced. Gene expression analysis by qRT‐PCR was performed in six patients. Novel missense point mutations were expressed in E. coli by site‐directed mutagenesis.ResultsParental consanguinity was reported in 16 families, and pyridoxine responsiveness in five (15%) patients. Among individuals from the same family, all presented the same phenotype. Both pathogenic mutations were identified in 29/30 patients. Twenty‐one different mutations were detected in nine exons and three introns; being six common mutations. Most prevalent were p.Ile278Thr (18.2%), p.Trp323Ter (11.3%), p.Thr191Met (11.3%), and c.828+1G>A (11.3%). Eight novel mutations were found [c.2T>C, c.209+1delG, c.284T>C, c.329A>T, c.444delG, c.864_868delGAG c.989_991delAGG, and c.1223+5G>T]. Enzyme activity in E. coli‐expressed mutations was 1.5% for c.329A>T and 17.5% for c.284T>C. qRT‐PCR analysis revealed reduced gene expression in all evaluated genotypes: [c.209+1delG; c.572C>T]; [c.2T>C; c.828+1G>A]; [c.828+1G>A; c.1126G>A]; [c.833T>C; c.989_991delAGG]; [c.1058C>T; c.146C>T]; and [c.444delG; c.444delG]. The expected phenotype according to the genotype (pyridoxine responsiveness) matched in all cases.ConclusionsMost patients studied were pyridoxine nonresponsive and presented early manifestations, suggesting severe phenotypes. Many private mutations were observed, but the four most prevalent mutations together accounted for over 50% of mutated alleles. A good genotype–phenotype relationship was observed within families and for the four most common mutations.

S-adenosylhomocysteine hydrolase over-expression does not alter S-adenosylmethionine or S-adenosylhomocysteine levels in CBS deficient mice

Elevated plasma total homocysteine (tHcy) is associated with a number of human diseases including coronary artery disease, stroke, osteoporosis and dementia. It is highly correlated with intracellular S-adenosylhomocysteine (SAH). Since SAH is a strong inhibitor of methyl-transfer reactions involving the methyl-donor S-adenosylmethionine (SAM), elevation in SAH could be an explanation for the wide association of tHcy and human disease. Here, we have created a transgenic mouse (Tg-hAHCY) that expresses human S-adenosylhomocysteine hydrolase (AHCY) from a zinc-inducible promoter in the liver and kidney. Protein analysis shows that human AHCY is expressed well in both liver and kidney, but elevated AHCY enzyme activity (131% increase) is only detected in the kidney due to the high levels of endogenous mouse AHCY expression in liver. Tg-hAHCY mice were crossed with mice lacking cystathionine β-synthase activity (Tg-I278T Cbs−/−) to explore the effect to AHCY overexpression in the context of elevated serum tHcy and elevated tissue SAM and SAH. Overexpression of AHCY had no significant effect on the phenotypes of Tg-I278T Cbs−/− mice or any effect on the steady state concentrations of methionine, total homocysteine, SAM, SAH, and SAM/SAH ratio in the liver and kidney. Furthermore, enhanced AHCY activity did not lower serum and tissue tHcy or methionine levels. Our data suggests that enhancing AHCY activity does not alter the distribution of methionine recycling metabolites, even when they are greatly elevated by Cbs mutations.

The Effects of Plasma Homocysteine in PCOS Women: A Review

Homocysteine is an intermediate substance formed during the breakdown of the amino acid methionine and may undergo remethylation to methionine or trans-sulfuration to cystathionine or cysteine. The metabolism occurs via two pathways: remethylation to methionine, which requires folate and vitamin B12; and transsulfuration to cystathionine, which requires pyridoxal-5’-phosphate. The disturbances in the metabolic pathways lead to the accumulation of Hcy, either by insufficient transsulfuration (through CBS mutations or vitamin B6 deficiency) or by a blockage of remethylation. In the latter case, folate or vitamin B12 deficiency may be involved, as well as MTHFR. High levels of Hcy induce sustained injury of arterial endothelial cells, proliferation of arterial smooth muscle cells and enhance activity of key participants in vascular inflammation, atherogenesis, and vulnerability of the established atherosclerotic plaque. Hyperhomocysteinemia has become the topic of interest in recent years. It has been highly associated with increased risk for cardiovascular disorders, such as, atherosclerosis, thromboembolism and dyslipidemia. Women with PCOS show constellation of metabolic syndromes. Obesity, hyperandrogenemia and type 2 diabetes mellitus is the hallmark of PCOS which later becomes the risk factors for cardiovascular disease. Various studies had revealed the presence of increased Hcy level in PCOS women which may or may not be associated with other biochemical parameters. Intense treatment for PCOS can influence homocysteine levels.

A Clinically Relevant Variant of the Human Hydrogen Sulfide-Synthesizing Enzyme Cystathionine β-Synthase: Increased CO Reactivity as a Novel Molecular Mechanism of Pathogenicity?

The human disease classical homocystinuria results from mutations in the gene encoding the pyridoxal 5′-phosphate- (PLP-) dependent cystathionine β-synthase (CBS), a key enzyme in the transsulfuration pathway that controls homocysteine levels, and is a major source of the signaling molecule hydrogen sulfide (H2S). CBS activity, contributing to cellular redox homeostasis, is positively regulated by S-adenosyl-L-methionine (AdoMet) but fully inhibited upon CO or NO• binding to a noncatalytic heme moiety. Despite extensive studies, the molecular basis of several pathogenic CBS mutations is not yet fully understood. Here we found that the ferrous heme of the reportedly mild p.P49L CBS variant has altered spectral properties and markedly increased affinity for CO, making the protein much more prone than wild type (WT) CBS to inactivation at physiological CO levels. The higher CO affinity could result from the slightly higher flexibility in the heme surroundings revealed by solving at 2.80-Å resolution the crystallographic structure of a truncated p.P49L. Additionally, we report that p.P49L displays impaired H2S-generating activity, fully rescued by PLP supplementation along the purification, despite a minor responsiveness to AdoMet. Altogether, the results highlight how increased propensity to CO inactivation of an otherwise WT-like variant may represent a novel pathogenic mechanism in classical homocystinuria.

Characterization of two pathogenic mutations in cystathionine beta-synthase: Different intracellular locations for wild-type and mutant proteins

Cystathionine β-synthase (CBS) is a pyridoxal 5′-phosphate (PLP)-dependent enzyme that catalyzes the condensation of homocysteine with serine to generate cystathionine. Homocystinuria is an autosomal recessive disorder commonly caused by a deficiency of CBS activity. Here, we characterized a novel CBS mutation (c.260C>A (p.T87N)) and a previously reported variant (c.700G>A (p.D234N)) found in Venezuelan homocystinuric patients, one nonresponsive and one responsive to vitamin B6. Both mutant proteins were expressed in vitro in prokaryotic and eukaryotic cells, finding lower soluble expression in HEK-293 cells (19% T87N and 23% D234N) compared to wild-type CBS. Residual activities obtained for the mutant proteins were 3.5% T87N and 43% D234N. Gel exclusion chromatography demonstrated a tendency of the T87N mutant to aggregate while the distribution of the D234N mutant was similar to wild-type enzyme. Using immunofluorescence microscopy, an unexpected difference in intracellular localization was observed between the wild-type and mutant proteins. While the T87N mutant exhibited a punctate appearance, the wild-type protein was homogeneously distributed inside the cell. Interestingly, the D234N protein showed both distributions. This study demonstrates that the pathogenic CBS mutations generate unstable proteins that are unable (T87N) or partially unable (D234N) to assemble into a functional enzyme, implying that these mutations might be responsible for the homocystinuria phenotype.

Endothelial Dysfunction and Paradoxical Resistance to Thrombosis in a Transgenic Mouse Model of Severe Hyperhomocysteinemia.

Hyperhomocysteinemia induced by low folate or high methionine diets produces endothelial dysfunction and accelerated thrombosis in mice, but it is not known if these vascular effects are influenced by homocysteine-independent effects of dietary perturbation. To determine the vascular phenotype of severe hyperhomocysteinemia in the absence of dietary intervention, we utilized a recently developed transgenic mouse model of cystathionine β-synthase (CBS) deficiency. In this model, the early lethality of homozygous Cbs-deficient mice (Cbs-/-) is rescued by a zinc-inducible human CBSI278T (hCBS-I278T) transgene under the control of a metallotheinein promoter. The I278T mutation is a common CBS mutation found in human patients with homocystinuria. We bred Cbs+/− male to Cbs+/− hCBS-I278T female mice. The mothers were given drinking water supplemented with 25 mmol/L zinc sulfate. Pups were weaned at 4–5 weeks of age and did not receive supplemental zinc after weaning. At 20 weeks of age, plasma total homocysteine was significantly higher in Cbs-/- hCBS-I278T mice (350±17 μmol/L) compared with Cbs+/+ hCBS-I278T (8.0±0.6 μmol/L) or wild-type Cbs+/+ (7.2±0.8 μmol/L) littermates (P<0.001). Maximal dilatation of pial arterioles to the endotheliumdependent dilator, acetylcholine, was decreased significantly in Cbs-/- CBS-I278T mice (10±2 %) compared with Cbs+/+ hCBS-I278T (25±2 %) mice (P<0.001). Dilatation responses to the endothelium-independent vasodilator, nitroprusside, were similar in both groups. Surprisingly, the time to stable occlusion after photochemical injury (rose bengal and green laser) of the carotid artery was significantly delayed in Cbs-/- hCBSI278T mice (71±10 minutes) compared with Cbs+/+ hCBS-I278T (30±10 minutes) or Cbs+/+ mice (33±8 minutes) (P<0.05). The tail bleeding time was comparable in all 3 groups. We conclude that severe hyperhomocysteinemia due to genetic deficiency of Cbs produces endothelial dysfunction but not accelerated thrombosis in the absence of dietary intervention in mice.

Characterization of cystathionine β-synthase gene mutations in homocystinuric Venezuelan patients: identification of one novel mutation in exon 6

This study describes for the first time the cystathionine β-synthase (CBS) gene mutations in Venezuelan patients. A total of five disease-causing mutations were identified in 9 out of 10 independent chromosomes. Four of the mutations have been previously described (G85R, T191M, D234N, and D444N) and a novel mutation was found (Q243X). Two common polymorphisms (699C/T and 1080C/T) were found in the CBS gene. Mutation analysis was performed using a combined screening approach for CBS mutations: restriction analysis, single-strand conformational polymorphism (SSCP) scanning, and sequencing. All the mutations were detected in homozygous state, except for Q243X, detected in three heterozygous siblings. Each one of the patients studied presented a different mutation. All mutations and polymorphisms detected involved hypermutable CpG sites, except for the novel mutation Q243X. The most common mutations I278T and G307S were not found in any of the patients. The CBS mutations present in each country differ from each other depending on the demographic profile; therefore, specific mutations scanning must be performed in each population for diagnosis and prognosis purposes.

Structural insights into mutations of cystathionine β-synthase

Cystathionine β-synthase (CBS) is a unique heme-containing enzyme that catalyses a pyridoxal 5′-phosphate (PLP)-dependent condensation of serine and homocysteine to give cystathionine. Deficiency of CBS leads to homocystinuria, an inherited disease of sulfur amino acid metabolism characterised by increased levels of homocysteine and methionine and decreased levels of cysteine. Presently, more than 100 CBS mutations have been described which lead to homocystinuria with different degrees of severity in the patients. We have recently solved the crystal structure of a truncated form of this enzyme, which enables us to correlate some of these mutations with the structure.

Intermediate and Severe Hyperhomocysteinemia with Thrombosis: A Study of Genetic Determinants

Hyperhomocysteinemia is an independent risk factor for cardiovascular disease. In search of genetic factors causing elevated levels of total homocysteine in plasma (tHcy), we investigated a cohort of consecutively identified, unrelated thrombosis patients (n = 28) having intermediate or severe hyperhomocysteinemia (30 micromol/l<tHcy < or =100 micromol/l, and tHcy > 100 micromol/l, respectively). The methylene-tetrahydrofolate reductase (MTHFR) 677C-->T genotype, and the complete cystathionine beta-synthase (CBS) genotype was determined in all patients. We found that the MTHFR T/T genotype was strongly correlated with intermediate hyperhomocysteinemia, being present in 73.9% of those cases (17 of 23). In three of five patients with severe hyperhomocysteinemia, compound heterozygosity for CBS mutations was detected. Among the mutations, two novel missense mutations: 1265C-->T (S422L) and 1397C-->T (S466L) were detected. The phenotype in those patients was quite mild, thromboembolism apart. This indicates that a search for CBS mutations in patients with severe hyperhomocysteinemia is important to ensure the detection of a possible CBS deficiency, thus enabling treatment. Co-existence of the MTHFR T/T genotype and the common CBS 844ins68 variant was significantly higher among patients (10.7%) as compared to controls (1.2%), indicating that this genotype combination is a thrombotic risk factor (P <0.05). In a few patients, hyperhomocysteinemia could not be explained by this genetic approach, suggesting that other genetic risk factors were implicated.

Four Common Mutations of the Cystathionine β-Synthase Gene Detected by Multiplex PCR and Matrix-assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

A deficiency of cystathionine beta-synthase (CBS) is the most frequent cause of homocystinuria. The effect of therapy is related to the underlying CBS genotype, which makes early diagnosis of this genetic defect important. Our aim was to develop a fast and reliable method based on matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry for the determination of common mutations of the CBS gene. We used MALDI-TOF mass spectrometry to detect four common CBS mutations (G307S, T272M, I278T, and V320A). The method is based on multiplex PCR of exons 7, 8, and 9, followed by single nucleotide extension in the presence of dideoxy NTPs of four primers targeted at the separate mutation sites. The extension products, as well as the 3-hydroxypicolinic acid matrix, were incubated with cation-exchange beads to remove disturbing salt contaminants. The above-mentioned mutations were determined in samples from 12 homocystinuria patients. The MALDI-TOF spectra allowed unambiguous discrimination between primers and extension products (>9 Da) in the mass range between 4500 and 7500 Da. No labeled primers or ddNTPs were required. The genotyping was verified by reference technique. Our results demonstrate fast, simple, and unambiguous multiplex genotyping of four common CBS mutations by MALDI-TOF mass spectrometry.

Detection of a novel deletion in the cystathionine β-synthase (CBS) gene using an improved genomic DNA based method

We elucidated the intron-exon boundaries of the 15 coding exons of the human cystathionine β-synthase (CBS) gene in order to establish an improved method based on PCR and direct sequencing for detection of CBS mutations. Using this method we identified the pathogenic mutations in two Danish siblings with CBS deficiency. Patients were compound heterozygotes: we detected the 833T→C mutation and a novel 22 bp deletion of exon 4 (493–514del) that introduces a frameshift and a stop codon immediately after the deletion. The deletion resulted in no detectable mRNA from this allele, as assessed by sequencing of cDNA. The established method represents an improvement of the existing method based on sequencing of cDNA because it permits the detection of mutations within the entire coding region of the CBS gene from a peripheral blood sample, including splice mutations and mutations resulting in the lack or a reduced amount of transcript.

Heterogeneous Ethnic Distribution of the 844ins68 in the Cystathionine β-Synthase Gene

A novel mutation in the cystathionine β-synthase (CBS) gene (a 68-bp insertion in the coding region of exon 8: 844ins68) was recently described, but its prevalence in different human populations is unknown. We analyzed the prevalence of the 68-bp insertion in the CBS gene in 405 unrelated individuals (810 chromosomes) of European, African, Asian and Amerindian origins. PCR amplification of a DNA fragment containing exon 8 of the CBS gene was employed. In addition, screening for the T833C CBS mutation by BsrI digestion was carried out in all samples bearing the 844ins68 mutation, since both mutations were previously reported to be associated in cis. The insertion was found in heterozygosity in 14 out of 104 whites (13.5%), was absent among Asians, and was found solely in 1 out of 220 Amerindian chromosomes analyzed, whereas a much higher prevalence was observed among blacks (37.7% of heterozygotes and 4% of mutant homozygotes). Furthermore, the T833C CBS mutation was found to cosegregate in cis with 844ins68 in all carriers of the insertion. The finding of the double mutant among blacks and Caucasians suggests that it antedated the racial divergence between Africans and non-Africans, and provides evidence for a partly or completely neutralizing effect conferred by the 68-bp insertion, since it allows the skipping of the T833C mutation. Our study represents the first analysis of the 844ins68 insertion in the CBS gene in different human populations, and reveals an extensive ethnic and geographic variability associated with this mutation.

Simultaneous detection and screening of T 833C and G 919A mutations of the cystathionine β-synthase gene by single-strand conformational polymorphism

Objective: We used single-strand conformational polymorphism (SSCP) to screen for mutations at nucleotides 833 and 919 of the cystathionine β-synthase (CBS) gene in 13 patients with homocystinuria and 11 of their relatives. Methods: Exon 8 of genomic DNA was selectively amplified by PCR using primers derived from intronic sequences of the human CBS gene. SSCP analysis was performed on the amplified products. Genotypes identified by SSCP were confirmed by DNA sequencing and an allele-specific PCR method. Results: SSCP identified 5 patterns corresponding to five genotypes. We confirmed that the different genotypes result from mutations at nucleotides 833 and 919 of the CBS gene, and that these 2 mutations account for approximately 50% of affected alleles in homocystinuria patients. Conclusion: Our recent elucidation of intron-exon borders and intronic sequences of the CBS gene has made possible the use of SSCP to screen for known/unknown mutations in the CBS gene. Because T 833C and G 919A represent the two most common mutations and both are located within exon 8 of the CBS gene, SSCP of exon 8 allows screening of the heterozygous carrier state of these mutations in a large population, to determine the importance of heterozygosity of CBS mutations as the cause of mild hyperhomocyst(e)inemia associated with premature vascular diseases.