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

Proteomic exploration of cystathionine β-synthase deficiency: implications for the clinic

Introduction: Homocystinuria due to cystathionine β-synthase (CBS) deficiency, the most frequent inborn error of sulfur amino acid metabolism, is characterized biochemically by severely elevated homocysteine (Hcy) and related metabolites, such as Hcy-thiolactone and N-Hcy-protein. CBS deficiency reduces life span and causes pathological abnormalities affecting most organ systems in the human body, including the cardiovascular (thrombosis, stroke), skeletal/connective tissue (osteoporosis, thin/non-elastic skin, thin hair), and central nervous systems (mental retardation, seizures), as well as the liver (fatty changes), and the eye (ectopia lentis, myopia). Molecular basis of these abnormalities were largely unknown and available treatments remain ineffective. Areas covered: Proteomic and transcriptomic studies over the past decade or so, have significantly contributed to our understanding of mechanisms by which the CBS enzyme deficiency leads to clinical manifestations associated with it. Expert opinion: Recent findings, discussed in this review, highlight the involvement of dysregulated proteostasis in pathologies associated with CBS deficiency, including thromboembolism, stroke, neurologic impairment, connective tissue/collagen abnormalities, hair defects, and hepatic toxicity. To ameliorate these pathologies, pharmacological, enzyme replacement, and gene transfer therapies are being developed.

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.

Sex affects N-homocysteinylation at lysine residue 212 of albumin in mice

The modification of protein lysine residues by the thioester homocysteine (Hcy)-thiolactone has been implicated in cardiovascular and neurodegenerative diseases. However, only a handful of proteins carrying Hcy on specific lysine residues have been identified and quantified in humans or animals. In the present work, we developed a liquid chromatography/mass spectrometry targeted assay, based on multiple reaction monitoring, for quantification of N-Hcy-Lys212 (K212Hcy) and N-Hcy-Lys525 (K525Hcy) sites in serum albumin in mice. Using this assay, we found that female (n = 20) and male (n = 13) Cbs^−/− mice had significantly elevated levels of K212Hcy and K525Hcy modifications in serum albumin relative to their female (n = 19) and male (n = 17) Cbs^+/− littermates. There was significantly more K212Hcy modification in Cbs^−/− males than in Cbs^−/− females (5.78 ± 4.21 vs. 3.15 ± 1.38 units, P = 0.023). Higher K212Hcy levels in males than in females were observed also in Cbs^+/− mice (2.72 ± 0.81 vs. 1.89 ± 1.07 units, P = 0.008). In contrast, levels of the K525Hcy albumin modification were similar between males and females, both in Cbs^−/− and Cbs^+/− mice. These findings suggest that the sex-specific K212Hcy modification in albumin might have an important biological function in mice that is not affected by the Cbs genotype.

Novel cystathionine β-synthase gene mutations in a Filipino patient with classic homocystinuria

BACKGROUND

Classic homocystinuria due to cystathionine β-synthase (CBS) deficiency is an autosomal recessive disorder of sulfur metabolism. Clinical manifestations include mental retardation, dislocation of the optic lens (ectopia lentis), skeletal abnormalities and a tendency to thromboembolic episodes. We present the first mutational analysis of CBS in a Filipino patient with classic homocystinuria.

METHODS

Genomic DNA was extracted from peripheral blood collected from a diagnosed Filipino patient with classic homocystinuria. The entire coding region of CBS (17 exons) was amplified using polymerase chain reaction and bidirectionally sequenced using standard protocols.

RESULTS

The patient was found to be compound heterozygous for two novel mutations, g.13995G>A [c.982G>A; p.D328K] and g.15860-15868dupGCAGGAGCT [c.1083-1091dupGCAGGAGCT; p. Q362-L364dupQEL]. Four known single-nucleotide polymorphisms (rs234706, rs1801181, rs706208 and rs706209) were also detected in the present patient's CBS. The patient was heterozygous for all the identified alleles.

CONCLUSIONS

This is the first mutational analysis of CBS done in a Filipino patient with classic homocystinuria who presented with a novel duplication mutation and a novel missense mutation. Homocystinuria due to CBS deficiency is a heterogeneous disorder at the molecular level.

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.

High prevalence of cerebral venous sinus thrombosis (CVST) as presentation of cystathionine beta-synthase deficiency in childhood: molecular and clinical findings of Turkish probands

Classical homocystinuria is the most commonly inherited disorder of sulfur metabolism, caused by the genetic alterations in human cystathionine beta-synthase (CBS) gene. In this study, we present comprehensive clinical findings and the genetic basis of homocystinuria in a cohort of Turkish patients. Excluding some CBS mutations, detailed genotype-phenotype correlation for different CBS mutations has not been established in literature. We aimed to make clinical subgroups according to main clinical symptoms and discussed these data together with mutational analysis results from our patients. Totally, 16 different mutations were identified; twelve of which had already been reported, and four are novel (p.N93Y, p.L251P, p.D281V and c.829-2A>T). The probands were classified into three major groups according to the clinical symptoms caused by these mutations. A psychomotor delay was the most common diagnostic symptom (n=12, 46.2% neurological presentation), followed by thromboembolic events (n=6, 23.1% vascular presentation) and lens ectopia, myopia or marfanoid features (n=5, 19.2% connective tissue presentation). Pyridoxine responsiveness was 7.7%; however, with partial responsive probands, the ratio was 53.9%. In addition, five thrombophilic nucleotide changes including MTHFR c.677 C>T and c.1298 A>C, Factor V c.1691 G>A, Factor II c.20210 G>A, and SERPINE1 4G/5G were investigated to assess their contributions to the clinical spectrum. We suggest that the effect of these polymorphisms on clinical phenotype of CBS is not very clear since the distribution of thrombophilic polymorphisms does not differ among specific groups. This study provides molecular findings of 26 Turkish probands with homocystinuria and discusses the clinical presentations and putative effects of the CBS mutations.

Cystathionine β-synthase-deficient mice thrive on a low-methionine diet

Cystathionine β-synthase (CBS) deficiency is a recessive inborn error of metabolism characterized by elevated serum total homocysteine (tHcy). Previously, our laboratory developed a mouse model of CBS deficiency, TgI278T Cbs(-)/(-) (abbreviated as Cbs(-/-)), characterized by low weight, low adiposity, decreased Scd-1 expression, facial alopecia, and osteoporosis. To determine the potential benefit of a methionine-restricted diet (MRD), we fed Cbs(-/-) and Cbs(+/-) control mice either an MRD or a regular diet (RD) from weaning till 240 d of age. Cbs(-/-) mice fed the MRD had a 77% decrease in tHcy, 28% increase in weight, 130% increase in fat mass, 82% increase in Scd-1 expression, and 10.6% increase in bone density and entirely lacked the alopecia phenotype observed in age-matched Cbs(-/-) mice fed the RD. At the end of the study, Cbs(-/-) mice fed the MRD were phenotypically indistinguishable from Cbs(+/-) mice fed the RD. Notably, whereas the MRD diet was highly beneficial to Cbs(-/-) mice, it had nearly opposite effect on Cbs(+/-) mice. These studies show that a low-methionine diet can correct the phenotypic consequences of loss of CBS and provide a striking example of how genotype and diet can interact to influence phenotype in mammals.