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Mol Genet Genomic Med. 2018 Mar;6(2):160-170. doi: 10.1002/mgg3.342. Epub 2018 Jan 20.

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

Author information

1
Post-Graduation Program in Genetics and Molecular Biology, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil.
2
Laboratory of Basic Research and Advanced Investigations in Neurosciences (BRAIN), Hospital de Clínicas de Porto Alegre, Porto Alegre, Brazil.
3
Complexo Hospitalar Materno-Infantil do Maranhão, São Luis, Brazil.
4
Complexo Hospitalar Professor Edgard Santos, Universidade do Estado da Bahia, Salvador, Brazil.
5
Universidade do Estado da Bahia, Salvador, Brazil.
6
Hospital das Clínicas de Ribeirão Preto, Ribeirão Preto, Brazil.
7
Universidade de São Paulo, São Paulo, Brazil.
8
Medical Genetics Service, Hospital de Clínicas de Porto Alegre, Porto Alegre, Brazil.
9
Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil.
10
Universidade Estadual de Campinas, Campinas, Brazil.
11
Universidade Federal de Santa Catarina, Florianópolis, Brazil.
12
Universidade Federal de Minas Gerais, Belo Horizonte, Brazil.
13
Children's Hospital, Grupo Hospitalar Conceição, Porto Alegre, Brazil.
14
Genetics Unit, Hospital Materno-Infantil Presidente Vargas, Porto Alegre, Brazil.
15
Hospital de Apoio de Brasília, Brasília, Brazil.
16
Escola Superior de Ciências da Santa Casa de Misericórdia de Vitória, Vitória, Brazil.
17
Laboratory for Clinical Biochemistry and Metabolism, University Medical Center, Freiburg, Germany.

Abstract

BACKGROUND:

Classical 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.

RESULTS:

Parental 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.

CONCLUSIONS:

Most 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.

KEYWORDS:

CBS mutations; CβS deficiency; CβS expression; classical homocystinuria; homocysteine

PMID:
29352562
PMCID:
PMC5902399
DOI:
10.1002/mgg3.342
[Indexed for MEDLINE]
Free PMC Article

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