High Prevalence of the I278T Mutation of the Human Cystathionine β-Synthase Detected by a Novel Screening Application

Association of selected genetic variants in CBS and MTHFR genes in a cohort of children with homocystinuria in Sri Lanka

BACKGROUND

Homocystinuria is an inherited, inborn error of homocysteine metabolism, which leads to the abnormal accumulation of homocysteine and its metabolites in blood and urine, resulting in various complications. Variants in the cystathionine β-synthase (CBS) and methylenetetrahydrofolate reductase (MTHFR) genes interrupt the formation of the corresponding enzymes and prevent homocysteine from being metabolised; hence, the homocysteine levels in plasma increase than the optimum levels.

MATERIALS AND METHODS

In the current study, eight clinically confirmed children with homocystinuria were detected to study the chosen variants in the CBS gene (c.833 T>C and c.19del) and in the MTHFR gene (c.665 C>T, c.1286 A>C) using SNaPshot mini-sequencing and direct sequencing.

RESULTS

After screening eight patients, none had the c.833T>C, but four patients were in the homozygous state for the c.19del variant in the CBS gene. Furthermore, seven were heterozygous for c.1286A>C, while one patient was heterozygous for c.665C>T in the MTHFR gene.

CONCLUSION

According to the results, c.19del is common in the studied cohort of Sri Lankan children, while c.833T>C is absent, whereas c.1286A>C was more frequent than c.665C>T. To our knowledge, the current study was the first report to discuss the genetic impact of homocystinuria in Sri Lanka; further comprehensive studies are necessary with a larger sample size to establish the association of these variants with the disease in Sri Lanka, which can be beneficial in enhanced patient care and for prospective studies.

Long-Term Outcomes of Adult Patients with Homocystinuria before and after Newborn Screening

BACKGROUND

Homocystinuria (HCU) is a rare inherited metabolic disease. In Japan, newborn screening (NBS) for HCU (cystathionine β-synthase deficiency) was initiated in 1977. We compared the outcomes between patients detected by NBS (NBS group) and clinically detected patients (non-NBS group).

METHODS

We administered questionnaires about clinical symptoms and social conditions to 16 attending physicians of 19 adult HCU patients treated with methionine-free formula.

RESULTS

Eighteen patients (nine patients each in the NBS and non-NBS groups) participated. The frequency of patients with ocular, vascular, central nervous system, and skeletal symptoms in the NBS group was lower than that in the non-NBS group. Intellectual disability was observed in one and eight patients in the NBS and non-NBS groups, respectively. Concerning their social conditions, all patients in the NBS group were employed or still attending school, while only two patients in the non-NBS group were employed. Three of the four patients who discontinued treatment presented some symptoms, even in the NBS group.

CONCLUSION

The social and intellectual outcomes of adult Japanese patients with HCU detected by NBS were favorable. However, even in the patients in the NBS group, some symptoms might not be preventable without continuous treatment.

Estimated prevalence of moderate to severely elevated total homocysteine levels in the United States: A missed opportunity for diagnosis of homocystinuria?

Classical homocystinuria (HCU) is a genetic disorder caused by mutations in the cystathionine beta synthase gene, which results in impaired metabolism of the sulfur-bearing amino acid homocysteine and its accumulation in blood and tissues. Classical HCU can be detected via newborn screening in the United States, but the test is widely acknowledged to miss many patients. While severely elevated homocysteine levels (>100 μmol /L) frequently lead to a classical HCU diagnosis, intermediate levels (>30 to 100 μmol /L), though linked to many of the known complications of HCU, are not always recognized as associated with HCU. We aimed to identify and describe potentially undiagnosed classical HCU patients using a nationally-representative database of administrative claims and laboratory results. We estimated the national prevalence of patients with homocysteine >30 μmol /L, and compared their demographic and clinical characteristics to those of patients with homocysteine levels ≤30 μmol/L. Among 57,580 patients with a homocysteine test result, 1.8% had a value >30 μmol /L. Patients with homocysteine >30 μmol /L were more frequently diagnosed with hypothyroidism (39.2% vs. 20.7%, p < .001) and renal disease (9.7% vs. 5.5%, p < .001), and were more likely to have a prescription for an anxiolytic/antidepressant (44.5% vs. 38.9%), opioid (58.4% vs. 53.1%), steroid (46.4% vs. 42.5%), or thyroid hormone (38.8% vs. 18.8%), compared to patients with homocysteine ≤30 μmol /L (all p < .05). Both groups were equally likely to have a diagnosis of homocystinuria or another disorder of sulfur-bearing amino acid metabolism (3.8% vs. 4.0%, p = .752). The age-adjusted national prevalence of homocysteine >30 μmol /L was estimated at 33,068 (95% CI: 1033 - 35,104). These findings suggest that thousands of people in the US may be living with intermediate to severely elevated homocysteine levels and may require further evaluation for the presence of classical HCU.

Classical homocystinuria: A common inborn error of metabolism? An epidemiological study based on genetic databases

BACKGROUND

Biallelic pathogenic variants in CBS gene cause the most common form of homocystinuria, the classical homocystinuria (HCU). The worldwide prevalence of HCU is estimated to be 0.82:100,000 [95% CI, 0.39-1.73:100,000] according to clinical records and 1.09:100,000 [95% CI, 0.34-3.55:100,000] by neonatal screening. In this study, we aimed to estimate the minimal worldwide incidence of HCU.

METHODS

The 25 most common pathogenic alleles of HCU were identified through a literature review. The incidence of HCU was estimated based on the frequency of these common pathogenic alleles in a large genomic database (gnomAD).

RESULTS

The minimum worldwide incidence of HCU was estimated to be ~0.38:100,000, and the incidence was higher in Europeans non-Finnish (~0.72:100,000) and Latin Americans (~0.45:100,000) and lower in Africans (~0.20:100,000) and Asians (~0.02:100,000).

CONCLUSION

Our data are in accordance with the only published metanalysis on this topic. To our surprise, the observed incidence of HCU in Europeans was much lower than those described in articles exploring small populations from northern Europe but was similar to the incidence described on the basis of neonatal screening programs. In our opinion, this large dataset analyzed and its population coverage gave us greater precision in the estimation of incidence.

Molecular and biochemical investigations of patients with intermediate or severe hyperhomocysteinemia

A discrepancy has been identified between numbers of expected and identified patients with homocystinuria due to cystathionine beta-synthase (CBS) deficiency. Patients homozygous for the frequent c.833T>C (p.I278T) are most often responsive to vitamin B6, and can present with a total-homocysteine (tHcy) <100 μM on a normal diet. In Denmark, patients with tHcy <100 μM are not routinely sequenced for CBS(2) mutations. This study investigated the prevalence of CBS mutations and the common methylenetetrahydrofolate reductase (MTHFR) c.677C>T polymorphism in patients with tHcy ≥ 50 μM and the association with clinical manifestations. We studied a cohort of patients with intermediate and severe hyperhomocysteinemia (tHcy ≥ 50 μM) determined between 1996 and 2011. Among the 413 eligible patients, 184 (45%) patients agreed to participate in the present follow-up study. A MTHFR(3)c.677TT genotype was found in 49% of the patients. Eight patients were found to have mutations in CBS(2). Of those, two were homozygous for c.833T>C (p.I278T), and four were compound heterozygous for c.833T>C. One c.833T>C (p.I278T) compound heterozygote was identified by lowering the threshold for sequencing from tHcy at 100 μM to 50 μM. The most prominent clinical presentation among patients with a CBS(2) mutation was thrombosis presenting at a median age of 25 years. In case of arterial or venous thrombosis without any explanation in individuals below 40 years, tHcy should be part of the thrombophilia screening. When tHcy is between 50 and 100 μM genotyping for the MTHFR(3) c.677TT is relevant, and when tHcy >100 μM CBS should be genotyped.

Heterozygous carriers of classical homocystinuria tend to have higher fasting serum homocysteine concentrations than non-carriers in the presence of folate deficiency

BACKGROUND & AIMS

Many studies have reported that serum total homocysteine (tHcy) levels in cystathionine-beta-synthase (CBS) carriers are usually normal and only elevated after a methionine load. However, the amount of methionine required for a loading test is non-physiological and is never reached with regular feeding. Therefore, CBS carriers do not seem to be at an increased risk of cardiovascular diseases. However, the risk of cardiovascular diseases of CBS carriers with folate deficiency has not been studied. We recently found an extraordinarily high carrier rate (1/7.78) of a novel CBS mutation (p.D47E, c.T141A) in an Austronesian Taiwanese Tao tribe who live in a geographic area with folate deficiency. We evaluated if the CBS carriers tend to have higher fasting serum tHcy concentrations than non-carriers in presence of folate deficiency.

METHODS

The serum tHcy and folate levels before and after folate replacement were measured in 48 adult Tao carriers, 40 age-matched Tao non-carriers and 40 age-matched Han Taiwanese controls.

RESULTS

The serum tHcy level of the Tao CBS carriers (17.9 ± 3.8 μmol/l) was significantly higher than in Tao non-carriers (15.7 ± 3.5 μmol/l; p < 0.008) and Taiwanese controls (11.8 ± 2.9 μmol/l; p < 0.001). Furthermore, a high prevalence of folate deficiency in the Tao compared with the Taiwanese controls (4.9 ± 1.8 ng/ml vs. 10.6 ± 5.5 ng/ml; p < 0.001) was also noted. Of note, the difference in tHcy levels between the carriers and non-carriers was eliminated by folate supplementation. (carriers:13.65 ± 2.13 μmol/l; non-carriers:12.39 ± 3.25 μmol/l, p = 0.321).

CONCLUSIONS

CBS carriers tend to have a higher tHcy level in the presence of folate deficiency than non-carriers. Although many reports have indicated that CBS carriers are not associated with cardiovascular disease, the risk for CBS carriers with folate deficiency has not been well studied. Owing to a significantly elevated level of fasting tHcy without methionine loading, it is important to evaluate the risk of cardiovascular disease in CBS carriers with folate deficiency.

Homocystinuria in Taiwan: an inordinately high prevalence in an Austronesian aboriginal tribe, Tao

The newborn screening of homocystinuria in Taiwan has never been formally reported before. Since 1984, out of 5 million newborns screened, only 3 newborns (Han Taiwanese) suffering from homocystinuria were detected in this newborn screening program. Four mutations (p.R121L [c.362G>T], p.E176K [c.526G>A], p.V320G [c.959T>G] and p.G259D [c.776G>A]) were identified in these 3 patients. Unexpectedly, we recently found 8 patients presenting with homocystinuria in an Austronesian Taiwanese Tao tribe. Out of them, three patients participated in the newborn screening program but were unidentified by the current newborn homocystinuria (using methionine as a marker) screening. All the Tao patients are homozygous for a new p.D47E (c.141T>A) mutation. Among the 428 adult islanders screened for the D47E mutation, approximately 1 in 7.78 is a carrier of the mutation, and an estimated 1 in 240 islanders suffered from homocystinuria. This is the highest known prevalence of homocystinuria worldwide. The result of expression studies of all the mutations identified in Taiwan revealed that, except for p.D47E mutation, all mutations were severely limited in their ability to form functional tetramers. The clinical manifestations of the Tao patients varied widely, despite sharing the same mutation and very similar genetic and environmental backgrounds. Comparisons of clinical and biochemical phenotypes of these patients were presented in this report.

Hypermethioninemias of genetic and non-genetic origin: A review

This review covers briefly the major conditions, genetic and non-genetic, sometimes leading to abnormally elevated methionine, with emphasis on recent developments. A major aim is to assist in the differential diagnosis of hypermethioninemia. The genetic conditions are: (1) Homocystinuria due to cystathionine β-synthase (CBS) deficiency. At least 150 different mutations in the CBS gene have been identified since this deficiency was established in 1964. Hypermethioninemia is due chiefly to remethylation of the accumulated homocysteine. (2) Deficient activity of methionine adenosyltransferases I and III (MAT I/III), the isoenzymes the catalytic subunit of which are encoded by MAT1A. Methionine accumulates because its conversion to S-adenosylmethionine (AdoMet) is impaired. (3) Glycine N-methyltrasferase (GNMT) deficiency. Disruption of a quantitatively major pathway for AdoMet disposal leads to AdoMet accumulation with secondary down-regulation of methionine flux into AdoMet. (4) S-adenosylhomocysteine (AdoHcy) hydrolase (AHCY) deficiency. Not being catabolized normally, AdoHcy accumulates and inhibits many AdoMet-dependent methyltransferases, producing accumulation of AdoMet and, thereby, hypermethioninemia. (5) Citrin deficiency, found chiefly in Asian countries. Lack of this mitochondrial aspartate-glutamate transporter may produce (usually transient) hypermethioninemia, the immediate cause of which remains uncertain. (6) Fumarylacetoacetate hydrolase (FAH) deficiency (tyrosinemia type I) may lead to hypermethioninemia secondary either to liver damage and/or to accumulation of fumarylacetoacetate, an inhibitor of the high K(m) MAT. Additional possible genetic causes of hypermethioninemia accompanied by elevations of plasma AdoMet include mitochondrial disorders (the specificity and frequency of which remain to be elucidated). Non-genetic conditions include: (a) Liver disease, which may cause hypermethioninemia, mild, or severe. (b) Low-birth-weight and/or prematurity which may cause transient hypermethioninemia. (c) Ingestion of relatively large amounts of methionine which, even in full-term, normal-birth-weight babies may cause hypermethioninemia.

Cystathionine beta-synthase null homocystinuric mice fail to exhibit altered hemostasis or lowering of plasma homocysteine in response to betaine treatment

Cystathionine beta-synthase (CBS) deficient homocystinuria is an inherited metabolic defect that if untreated typically results in mental retardation, thromboembolism and a range of connective tissue disturbances. A knockout mouse model has previously been used to investigate pathogenic mechanisms in classical homocystinuria (Watanabe et al., PNAS 92 (1995) 1585-1589). This mouse model exhibits a semi-lethal phenotype and the majority of mice do not survive the early neonatal period. We report here that the birth incidence of cbs (-/-) mice produced from heterozygous crosses is non-Mendelian and not significantly improved by treatment with either the Hcy lowering compound betaine or the cysteine donor N-acetylcysteine. Betaine treatment did improve survival of cbs (-/-) mice and restored fertility to female cbs (-/-) mice but did so without significantly lowering Hcy levels. Surviving cbs (-/-) mice failed to show any alteration in coagulation parameters compared to wild-type controls. Moribund cbs (-/-) mice exhibited severe liver injury and hepatic fibrosis while surviving cbs (-/-) mice although less severely affected, still exhibited a level of severe liver injury that is not found in the human disease. The hepatopathy observed in this model may offer an explanation for the failure of cbs (-/-) mice to respond to betaine or exhibit a hypercoagulative phenotype. We conclude that although this model provides useful data on the biochemical sequelae of classical homocystinuria, it does not successfully recapitulate a number of important features of the human disease and its use for studying mechanisms in homocystinuria should be treated with caution as the hepatopathy produces changes which could influence the results.

Genetic variants of folate and methionine metabolism and PCNSL incidence in a German patient population

Functional genetic polymorphisms involved in folate and methionine metabolism play an important role in both DNA synthesis and methylation, and affect the risk of various malignancies including lymphoproliferative disorders such as systemic non-Hodgkin's lymphoma. In a retrospective analysis of 185 immunocompetent patients with primary central nervous system lymphoma (PCNSL) and 212 population controls we therefore investigated eight genetic polymorphisms affecting methionine metabolism for potential association with the development of PCNSL. We observed underrepresentation of the G-allele of the methyltetrahydrofolate homocysteine S-methyltransferase (MTR) c.2756A > G (D919G) missense polymorphism among PCNSL patients (P = 0.045; odds ratio (OR) = 0.65; 0.43-0.99). Furthermore, for the methylenetetrahydrofolate reductase (MTHFR) c.1298A > C (E429A) polymorphism the mutated C-allele was found more frequently among PCNSL patients than among population controls (P = 0.026; OR = 1.57; 1.05-2.34). There were no associations of the other polymorphisms investigated (MTHFR c.677C > T, transcobalamin 2 (Tc2) c.776C > G, cystathionin beta-synthase (CBS) c.844_855ins68, reduced folate carrier-1 (RFC-1) c.80G > A, thymidylate synthase (TYMS) 28-bp repeat, and dihydrofolate reductase (DHFR) c.594 + 59del19 bp) and the presence of PCNSL. This analysis is the largest to date to evaluate associations between genetic variants of folate and methionine metabolism and PCNSL. Our results suggest the hypothesis that folate and methionine metabolism is relevant to susceptibility to PCNSL.

A revisit to the natural history of homocystinuria due to cystathionine beta-synthase deficiency

We review the evidence that in Denmark and probably certain other European countries the number of individuals identified with homocystinuria due to homozygosity for the widespread c.833T>C (p.I278T) mutation in the gene that encodes cystathionine beta-synthase (CBS) falls far short of the number of such individuals expected on the basis of the heterozygote frequency for this mutation found by molecular screening. We conclude that the predominant portion of such homozygotes may be clinically unaffected, or may be ascertained for thromboembolic events occurring no sooner than the third decade of life. If so, there was significant ascertainment bias in the time-to-event curves previously published describing the natural history of untreated CBS deficiency Mudd et al. and these curves should be used with care.

Birth prevalence of homocystinuria in Central Europe: frequency and pathogenicity of mutation c.1105C>T (p.R369C) in the cystathionine beta-synthase gene

OBJECTIVES

To estimate the frequency of the cystathionine beta-synthase deficiency caused by c.1105C>T mutation in Central Europe compared to Norway, and to examine the pathogenicity of the corresponding p.R369C mutant enzyme.

STUDY DESIGN

Mutation c.1105C>T was analyzed in 600 anonymous Czech newborn blood spots. Catalytic activity and quaternary structure of the p.R369C mutant was evaluated after expression in 2 cellular systems.

RESULTS

Population frequency of the c.1105C>T mutation was 0.005, predicting the birth prevalence of homocystinuria of 1:40000, which increased to 1:15500 in a model including 10 additional mutations. In Escherichia coli the p.R369C mutant misfolded, and its activity was severely reduced, and expression in Chinese hamster ovary cells enabled proper folding with activity decreased to 63% of the wild-type enzyme. This decreased activity was not due to impaired K(m) for both substrates but resulted from V(max) lowered to 55% of the normal cystathionine beta-synthase enzyme.

CONCLUSIONS

The c.1105C>T (p.R369C) allele is common also in the Czech population. Although the p.R369C mutation impairs folding and decreases velocity of the enzymatic reaction, our data are congruent with rather mild clinical phenotype in homozygotes or compound heterozygotes carrying this mutation.

Bone tissue and hyperhomocysteinemia

Bone tissue quality is determined not only by multiple architectural variables, but also by the mechanical properties of collagen type 1. Homocysteinuria is a genetic disease whose manifestations include severe hyperhomocysteinemia and decreased bone strength. The effects of smaller homocysteine elevations on bone tissue are difficult to demonstrate in clinical studies. Studies in animals and in humans suggest that homocysteine may weaken collagen crosslinks and, if present in large amounts, interfere with bone remodeling. Whether routine homocysteine assays should be performed to detect bone frailty remains unclear. In clinical practice, the focus should be on identifying patients with potential causes of homocysteine elevation (e.g., medications), who should then be given vitamin D and folic acid supplementation if needed. This approach may improve not only bone health, but also vascular and general health.

Diversity of cystathionine beta-synthase haplotypes bearing the most common homocystinuria mutation c.833T>C: a possible role for gene conversion

Homozygosity or compound heterozygosity for the c.833T>C transition (p.I278T) in the cystathionine beta-synthase (CBS) gene represents the most common cause of pyridoxine-responsive homocystinuria in Western Eurasians. However, the frequency of the pathogenic c.833C allele, as observed in healthy newborns from several European countries (q_c.833C ≊ 3.3 × 10^–3), is ∼20-fold higher than expected on the basis of the observed number of symptomatic homocystinuria patients carrying this mutation (q_c.833C ≊ 0.18 × 10^–3), implying clinical underascertainment. Intriguingly, the c.833C mutation is also present in combination with a 68-bp insertion, c.[833C; 844_845ins68], in a substantial proportion of chromosomes from nonhomocystinuric individuals worldwide. We have sought to study the relationship between the pathogenic and nonpathogenic c.833C-bearing chromosomes and to determine whether the pathogenic c.[833C; −] chromosomes are identical-by-descent or instead arose by recurrent mutation. Initial haplotype analysis of 780 randomly selected Czech and sub-Saharan African wild-type chromosomes, employing 12 intragenic markers, revealed 29 distinct CBS haplotypes, of which 10 carried the c.[833C; 844_845ins68] combination; none carried an isolated c.833C or c.844_845ins68 mutation. Subsequent examination of 69 pathogenic c.[833C; −] chromosomes, derived from homocystinuria patients of predominantly European origin, disclosed three unrelated haplotypes that differed from their wild-type counterparts by virtue of the presence of c.833C, thereby indicating that c.833T>C transition has occurred repeatedly and independently in the past. Since c.833T does not reside within an obvious mutational hotspot, we surmise that the three pathogenic and comparatively prevalent c.[833C; −] chromosomes may have originated by recurrent gene conversion employing the common nonpathogenic c.[833C; 844_845ins68] chromosomes as templates. Hum Mutat 28(3), 255–264, 2007. Published 2006 Wiley-Liss, Inc.†

Homocysteine metabolism in families from southern Italy with neural tube defects: role of genetic and nutritional determinants

OBJECTIVE

To evaluate the role of different polymorphic gene variants involved in homocysteine metabolism and plasma levels of homocysteine, folate and vitamin B12 in families from southern Italy with neural tube defects (NTDs).

METHODS

Eighteen fathers, 15 NTD children and 60 women who had conceived NTD foetuses were investigated. A group of 100 adults and 43 apparently healthy children was used as control. At the time of blood draw, none were taking vitamin pills or nutritional supplements.

RESULTS

Among controls, 79 (55.2%) were heterozygous for C677T MTHFR variant and 26 (18.2%) were TT homozygous. Among the cases, 35 (61.4%) out of 57 mothers and 7 (38.9%) out of 18 fathers carried the T allele; 12 (21.1%) mothers and 2 (11.1%) fathers had the TT genotype. Four (26.7%) out of 15 probands were TT homozygous and 11 (73.3%) were heterozygous (Fisher exact test p = 0.025). No significant difference between groups was observed for the 1298C MTHFR variant and CBS haplotypes. Median homocysteine in NTD children was significantly higher (10.0 micromol/L) than that of controls (median 4.5 micromol/L, Mann-Whitney p < 0.05). Folate and B12 were not different among groups.

CONCLUSIONS

The T677 MTHFR allele is significantly associated with the occurrence of NTDs; no significant association has been observed with other genetic determinants analysed. Homocysteine levels in children with NTDs are significantly higher than those of the paediatric population from the same geographical area.

Expression study of mutant cystathionine beta-synthase found in Japanese patients with homocystinuria

Cystathionine beta-synthase (CBS) deficiency is the most common cause of homocystinuria. More than 130 pathogenic mutations, mostly in the Caucasian populations, have been described. Recently, our group reported a mutation analysis of Japanese homocystinuric patients. In the present paper, we report an expression study of several mutant CBS enzymes in Escherichia coli, i.e., R121H, G148R, G151R, S217F, H232D, R266G, 1591delTTCG, and K441X. All of the mutants except K441X exhibited severely decreased activity, and the capability to form tetramers of most mutants was severely impaired. The K441X mutant, on the other hand, exhibited relatively high activity (63% of the wild type activity). This was probably due to two factors. First, the high abundance of the full-length CBS protein, a likely K441Q mutant, which was produced through suppression of the amber termination codon by glutamine tRNA in E. coli. And second, the presence of a C-terminally truncated protein, which was previously shown to be constitutively activated. Patient-derived lymphocytes, however, showed no detectable CBS subunits. As previously hypothesized, the increased aggregation of mutant CBS subunits might be a common pathogenic mechanism in CBS deficiency.

Cystathionine beta-synthase T833C/844INS68 polymorphism: a family-based study on mentally retarded children

Background

Cystathionine β-synthase (CBS) mediates conversion of homocysteine to cystathionine and deficiency in enzyme activity may lead to hyperhomocysteinemia/homocystinuria, which are often associated with mental retardation (MR). A large number of polymorphisms have been reported in the CBS gene, some of which impair its activity and among these, a T833C polymorphism in cis with a 68 bp insertion at 844 in the exon 8 is found to be associated with mild hyperhomocysteinemia in different ethnic groups.

Methods

The present study is aimed at investigating the association between T833C/844ins68 polymorphism and MR. One hundred and ninety MR cases were recruited after psychometric evaluation. Hundred and thirty-eight control subjects, two hundred and sixty-seven parents of MR probands and thirty cardiovascular disorder (CVD) patients were included for comparison. Peripheral blood was collected after obtaining informed written consent. The T833C/844ins68 polymorphism was investigated by PCR amplification of genomic DNA and restriction fragment length polymorphism analysis, followed by statistical analysis.

Results

The genotypic distribution of the polymorphism was within the Hardy-Weinberg equilibrium. A slightly increased genotypic frequency was observed in the Indian control population as compared to other Asian populations. Both haplotype-based haplotype relative risk analysis and transmission disequilibrium test reveled lack of association of the T833C/844ins68 polymorphism with MR; nevertheless, the relative risk calculated was higher (>1) and in a limited number of informative MR families, preferential transmission of the double mutant from heterozygous mothers to the MR probands was noticed (χ² = 4.00, P < 0.05).

Conclusion

This is the first molecular genetic study of CBS gene dealing with T833C/844ins68 double mutation in MR subjects. Our preliminary data indicate lack of association between T833C/844ins68 polymorphism with MR. However, higher relative risk and biased transmission of the double mutation from heterozygous mothers to MR probands are indicative of a risk of association between this polymorphism with mental retardation.

Common genetic variants of homocysteine metabolism in ischemic stroke: a case-control study

Hyperhomocysteinemia is a risk factor for ischemic stroke. We investigated five functional polymorphisms involved in homocysteine metabolism in each 159 stroke patients and controls. The folate-sensitive polymorphism methylenetetrahydrofolate reductase (MTHFR) c. 677 C > T (A222V) referred a non-significant risk of ischemic stroke (odds ratio: 1.20) in all patients, and homozygosity for MTHFR c. 677 C > T was associated with an earlier onset of stroke selectively in patients younger than 60 years (38 +/- 3 years vs. 45 +/- 1 years; P = 0.043). This study suggests that the investigated polymorphisms are no major risk factors for stroke, although MTHFR c. 677 C > T could be a minor factor of vulnerability especially in young patients (TT genotype), which might be helpful for the clinical work-up of stroke cases and for preventive dietary strategies.

Detection and allele-frequencies of the 833T>C, 844ins68 and a novel mutation in the cystathionine beta-synthase gene

BACKGROUND

The most common 833T>C/844ins68 in cis double mutation in the cystathionine beta synthase (CBS) gene probably is non-pathogenic because the 68-bp insertion eliminates the 833T>C mutation due to alternative splicing. However, allele frequency and effects of the isolated 833T>C mutation are unclear.

METHOD

DNA was isolated from 500 volunteers and used directly for PCR-RFLP of CBS gene exon-8. A new primer design was developed to create annealing sites upstream and downstream of exon-8 for forward and reverse primers, respectively. The design was made to exclude sequence homology of the forward primer with the insert fragment and to introduce an internal Bsr1 digestion site.

RESULTS

A new 9276G>A mutation was found in intron 8. Because of this mutation, an extra Bsr1 digestion site is created in intron 8. In Caucasian volunteers, the following allele frequencies were found: 833T>C=0.2%, 833T>C/844ins68=10.2%, and 9276G>A=0.2%.

CONCLUSION

The developed PCR-RFLP method is able to detect the 833T>C mutation, the 833T>C/844ins68 polymorphism as well as a new 9276G>A mutation in intron 8. Further study should explore the effect of the isolated 9276G>A mutation.

Screening for Serum Total Homocysteine in Newborn Children

Background: Newborn screening for total homocysteine (tHcy) in blood may identify babies with vitamin B12 (B12) deficiency or homocystinuria, but data on the causes of increased tHcy in screening samples are sparse.

Methods: Serum concentrations of tHcy, cystathionine, methionine, folate, and B12 and the methylenetetrahydrofolate reductase (MTHFR) 677C&gt;T polymorphism were determined in 4992 capillary blood samples collected as part of the routine screening program in newborn children. Methylmalonic acid (MMA), gender (SRY genotyping), and the frequency of six cystathionine β-synthase (CBS) mutations were determined in 20–27% of the samples, including all samples with tHcy &gt;15 μmol/L (n = 127), B12 &lt;100 pmol/L (n = 159), or methionine &gt;40 μmol/L (n = 154).

Results: The median (5th–95th percentile) tHcy concentration was 6.8 (4.2–12.8) μmol/L. B12 status, as determined by serum concentrations of B12, tHcy, and MMA, was moderately better in boys than in girls. tHcy concentrations between 10 and 20 μmol/L were often associated with low B12, whereas tHcy &gt;20 μmol/L (n = 43) was nearly always explained by increased methionine. tHcy did not differ according to folate concentrations or MTHFR 677C&gt;T genotypes. None of the babies had definite CBS deficiencies, but heterozygosity led to low cystathionine, increased methionine, but normal tHcy concentrations.

Conclusion: Increased tHcy is a common but not specific finding in newborns. The metabolite and vitamin profiles will point to the cause of hyperhomocysteinemia. Screening for tHcy and related factors should be further evaluated in regions with high prevalence of homocystinuria and in babies at high risk of B12 deficiency.

The methionine synthase polymorphism D919G alters susceptibility to primary central nervous system lymphoma

Primary central nervous system lymphomas (PCNSL) frequently reveal genomic instability. We analysed different functional genetic variants affecting the folate and homocysteine metabolism important for DNA integrity in 31 PCNSL patients and 142 controls. We found significantly less carriers of the methionine synthase c.2756A>G (D919G) missense polymorphism among the patients (0.16 vs 0.42; odds ratio 0.26, CI(95%): 0.09-0.74; P=0.005), suggesting a protective function of the G allele. These data stimulate further epidemiological and functional studies focusing on the role of homocysteine and folate metabolism in lymphoma tumorigenesis.

Cystathionine?-synthase deficiency in Georgia (USA): Correlation of clinical and biochemical phenotype with genotype

Cystathionine beta-synthase (CBS) deficiency is a rare autosomal recessive disorder that is the most frequent cause of clinical homocystinuria. Patients not treated in infancy have multi-systems disorders including dislocated lenses, mental deficiency, osteoporosis, premature arteriosclerosis, and thrombosis. In this paper, we examine the relationship of the clinical and biochemical phenotypes with the genotypes of 12 CBS deficient patients from 11 families from the state of Georgia, USA. By DNA sequencing of all of the coding exons we identified mutations in the CBS genes in 21 of the 22 possible mutant alleles. Ten different missense mutations were identified and one novel splice-site mutation was found. Five of the missense mutations were previously described (G307S, I278T, V320A, T353M, and L101P), while five were novel (A226T, N228S, A231L, D376N, Q526K). Each missense mutation was tested for function by expression in S. cerevisiae and all were found to cause decreased growth rate and to have significantly decreased levels of CBS enzyme activity. The I278T and T353M mutations accounted for 45% of the mutant alleles in this patient cohort. The T353M mutation, found exclusively in four African American patients, was associated with a B(6)-nonresponsive phenotype and detection by newborn screening for hypermethioninemia. The I278T mutation was found exclusively in Caucasian patients and was associated with a B(6)-responsive phenotype. We conclude that these two mutations occurred after ethnic socialization and that the CBS genotype is predictive of phenotype.

Isolated thrombosis due to the cystathionine beta-synthase mutation c.833T>C (1278T)

Hereditary homocystinuria due to cystathionine beta-synthase (CBS) deficiency is a rare disease (about 1:20000 in Germany) often complicated by thromboembolism. Single mutations, which affect the C-terminal region of the CBS enzyme, lead to isolated thrombosis without further symptoms typical for homocystinuria such as atherosclerosis, psychomotor retardation, and dislocation of the ocular lenses. In this study, DNA samples of patients with stroke (n = 225) and sinus thrombosis (n = 46) were screened for the most common homocystinuria mutation, CBS 1278T. In each group one homozygous patient was identified. Thus, not only C-terminal mutations but also the most common mutation in classical homocystinuria, CBS 1278T, can lead to isolated thrombophilic events. These data support the hypothesis that homocystinuria is an underdiagnosed disease.

Homocysteine: An aetiological contributor to peripheral vascular arterial disease

Surgeons are increasingly being exposed to the term 'hyperhomocysteinaemia' but few understand this condition that affects up to 10% of the population, or its pathological sequelae. Hyperhomocysteinaemia has been identified as an important and independent risk factor for atherosclerosis. There is increasing evidence that, in addition to coronary disease, hyperhomocysteinaemia is also associated with an increased risk of developing peripheral arterial disease. Causes of elevated homocysteine levels include inherited enzyme deficiencies and acquired vitamin deficiencies. Detection of hyperhomocysteinaemia is particularly relevant in patients with early onset atherosclerosis. Effective lowering of elevated homocysteine levels is possible with folate, vitamin B6 and B12 supplementation and may reduce the incidence and sequelae of atherosclerotic peripheral arterial disease.