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

Abstract

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.

The Spectrum of Mutations of Homocystinuria in the MENA Region

“…The CBS gene is located on the long arm of chromosome 21 with 191 variants having been described [40] (Figure 2). The most frequent pathogenic and reported mutations in different countries around the world are p.G307S (31%), and p.I278T (24%) [41,42]. The p.G307S mutation is the most prevalent CBS deficiency mutation in Ireland and Australia [6,23].…”

“…On the other hand, the SIFT prediction software indicated that the p.T236A and p.L230G are damaging, while the p.L72I is a tolerated mutation (17). Similarly, a total of 35 samples were obtained from Brazilian patients with a biochemically confirmed diagnosis of classical homocystinuria [42]. Eight novel mutations [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] were found.…”

Three Main Causes of Homocystinuria: CBS, cblC and MTHFR Deficiency. What do they Have in Common?

“…Prenatal diagnosis can be achieved by molecular genetic analyses or the extraction and culture of cells from amniotic fluid to measure CBS activity. [11,42,43] Newborn screening has been performed, especially in countries with high incidences of HCU, such as Ireland and Qatar. [44,45] The most common method is the measurement of Met in dried blood spots, but a high proportion of false negatives have been obtained by using this method.…”

Homocystinuria: Literature Review and Clinical Case Description

“…Ген, кодирующий СBS, расположен на плече хромосомы 21q22.3, содержит 23 экзона [5]. К развитию классической формы заболевания приводят генетиче-ские варианты в гомозиготном или компаунд-гетерозиготном состоянии [4].…”