The CBS Gene Encodes Cystathionine Beta-Synthase
The CBS gene (located on human chromosome 21q22.3) encodes the enzyme cystathionine beta-synthase. 1
Gene Structure and Organization
The human CBS gene spans at least 23 kb and consists of 17 exons 2. The gene's organization differs notably from its rat homolog, particularly in the 5'-untranslated region (5'-UTR). In humans, the first ATG (translation start codon) is located in exon 3, and the 5'-UTR contains two different versions of exon 1 (E1a and E1b) that link with exon 2 2.
Chromosomal Location
- Human: The CBS gene maps to the subtelomeric region of band 21q22.3 on chromosome 21 1
- Mouse homolog (Cbs): Located on the proximal half of mouse chromosome 17 1
This chromosomal location is clinically significant because CBS resides on chromosome 21, making it relevant to Down syndrome pathophysiology where three copies of this gene exist 3.
Alternative Splicing
The CBS gene undergoes alternative splicing, producing multiple mRNA variants. In rats, four distinct mRNA types (I-IV) have been identified, encoding proteins of varying sizes (61.5,39,60, and 52.5 kDa) 4. Types I and III are the predominant forms in liver tissue and both exhibit enzymatic activity 4.
Enzyme Function
The CBS gene encodes L-serine hydrolyase, which catalyzes the condensation of serine with homocysteine using pyridoxal-5'-phosphate (vitamin B6) as a cofactor to form cystathionine 5. This enzyme sits at the critical intersection of transmethylation, transsulfuration, and remethylation pathways 6.
Clinical Significance
CBS deficiency causes classical homocystinuria, characterized by severe hyperhomocysteinemia with plasma homocysteine levels exceeding 100 μmol/L 7. The enzyme's deficiency fundamentally blocks homocysteine degradation, an essential step in glutathione synthesis, leading to retinovascular thromboembolism, lens dislocation, and vascular cognitive impairment 6.