cystathionine synthase

CBS catalyzes the condensation of homocysteine and serine to form cystathionine, using pyridoxal phosphate (vitamin B6) as a cofactor. This is a key regulatory step in the transsulfuration pathway, which converts methionine-derived homocysteine into cysteine, ultimately allowing synthesis of glutathione and other sulfur-containing compounds. Homocysteine can either be remethylated back to methionine (via methionine synthase, requiring B12 and folate) or irreversibly converted to cystathionine by CBS.

Cystathionine β-synthase deficiency is the most common cause of homocystinuria, an autosomal recessive disorder leading to accumulation of homocysteine and methionine in blood and urine. Clinical features include: - Ectopia lentis (downward and inward lens dislocation, contrasting with upward dislocation in Marfan syndrome) - Marfanoid body habitus (tall stature, long limbs, arachnodactyly) but without aortic root dilation - Intellectual disability and developmental delay - Thromboembolic events (due to endothelial damage and hypercoagulability from elevated homocysteine) — a major cause of morbidity/mortality - Osteoporosis

Diagnosis involves elevated plasma/urine homocysteine and methionine. Treatment includes high-dose vitamin B6 (pyridoxine) supplementation (some patients are B6-responsive), dietary methionine restriction, cysteine supplementation, and betaine (which promotes alternative remethylation of homocysteine to methionine via betaine-homocysteine methyltransferase).

Differentiating from other causes of elevated homocysteine: Methylenetetrahydrofolate reductase (MTHFR) deficiency and vitamin B12/folate deficiency also cause hyperhomocysteinemia but through defects in the remethylation pathway rather than transsulfuration. Elevated homocysteine is also a risk factor for atherosclerosis and venous thromboembolism.

High-yield board association: Homocystinuria is a classic differential for Marfan syndrome — distinguishing features are lens dislocation direction, intellectual disability, and thrombosis risk vs. cardiovascular (aortic) complications in Marfan syndrome.