Homocysteine accumulation in serum and urine is the biochemical hallmark of inborn errors of sulfur amino acid metabolism, a heterogeneous group of disorders characterized by multi-systemic involvement and considerable morbidity if not aggressively treated. These patients also display abnormal levels of the amino acid methionine, low in remethylation disorders, such as methylenetetrahydrofolate reductase (MTHFR) deficiency, or high in cystathionine-beta-synthase (CBS) deficiency (classic homocystinuria). To reduce homocysteine toxicity and normalize methionine, patients are treated with various modalities, including hydroxocobalamin (vitamin B12) injections and supplementation with the methyl-donor betaine, although there is no consensus on dosage or drug combinations used [1, 2].

Monitoring total homocysteine and methionine levels in plasma is recommended for long-term follow-up [1-4]. However, most studies in the last twenty years focused on clinical aspects of these disorders, reporting only limited laboratory testing results in response to treatment. Attempting to fill the knowledge gap, we have evaluated the results of total homocysteine and amino acids testing in 812 plasma samples from 123 patients with different disorders of sulfur amino acid metabolism, including MTHFR and CBS deficiencies [5].

Overall, our analysis of biochemical profiles in view of different treatment modalities demonstrated the utility of biochemical testing in long-term monitoring these patients. We confirmed that total homocysteine levels seldom normalize regardless of the specific metabolic defect or treatment used. Methionine levels return to normal in most patients with remethylation disorders, but not in CBS deficiency patients. Interestingly, patients on betaine therapy had significantly higher concentrations of the amino acid sarcosine than patients on other treatments. Betaine supplementation offers an alternative route for homocysteine clearance producing sarcosine, which is now commonly reported as part of liquid chromatography-tandem mass spectrometry-based amino acid analysis, a test routinely employed to evaluate both metabolic and non-metabolic conditions. In our cohort, sarcosine increased significantly on betaine therapy: 19-fold in remethylation disorders and 3-fold in CBS deficiency on average, compared to pre-betaine levels. Sarcosine levels correlated inversely with total homocysteine and positively with methionine with the strongest correlation seen in MTHFR deficiency (rs=-0.68, p<0.0001 and rs=0.71, p<0.0001, respectively). Moreover, a sarcosine cut-off >5mmol/L demonstrated 97% sensitivity and 95% specificity to betaine treatment in patients with remethylation disorders, superior to the traditional biomarkers such as total homocysteine and methionine. The area under the ROC curve (AUC), which measures a parameter’s capacity to distinguish between two conditions (in our case, treated versus non-treated with betaine), was higher for sarcosine (0.98; p<0.0001), than for homocysteine and methionine (0.51; p=0.8254; 0.83 p<0.0001, respectively).

In summary, we demonstrated that sarcosine is a sensitive and specific biomarker to monitor compliance to betaine therapy in patients with disorders of sulfur amino acid metabolism. With the increased utilization of betaine to lower the risk of vascular disease associated with mild homocystinuria in non-metabolic patients, our findings could aid therapeutic monitoring in broader patient populations.


  1. Morris, A.A., et al., Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency. J Inherit Metab Dis, 2017. 40(1): p. 49-74.
  2. Huemer, M., et al., Phenotype, treatment practice and outcome in the cobalamin-dependent remethylation disorders and MTHFR deficiency: Data from the E-HOD registry. J Inherit Metab Dis, 2019. 42(2): p. 333-352.
  3. Huemer, M., et al., Guidelines for diagnosis and management of the cobalamin-related remethylation disorders cblC, cblD, cblE, cblF, cblG, cblJ and MTHFR deficiency. J Inherit Metab Dis, 2017. 40(1): p. 21-48.
  4. Valayannopoulos, V., et al., Betaine anhydrous in homocystinuria: results from the RoCH registry. Orphanet J Rare Dis, 2019. 14(1): p. 66.
  5. De Biase, I., et al., Laboratory evaluation of homocysteine remethylation disorders and classic homocystinuria: Long-term follow-up using a cohort of 123 patients. Clin Chim Acta, 2020. 509: p. 126-134.