Value of Amniotic Fluid Homocysteine Assay in Prenatal Diagnosis of Combined Methylmalonic Acidemia and Homocystinuria, Cobalamin C Type

Abstract Background: Combined methylmalonic acidemia and homocystinuria, cobalamin C type (cblC defect) is the most common inborn error of cobalamin metabolism. Even with rapid diagnosis and treatment, the long-term outcome remains poor. A reliable method for the prenatal diagnosis of cblC defect is needed for parental decisions regarding continuation of pregnancies of cblC defect fetuses. Results: The between-day and within-day imprecision of Hcy assay were 1.60%∼5.87% and 1.11%∼4.31%, respectively. For the 248 at-risk fetuses, there were 63 affected fetuses with cblC defect and 185 unaffected fetuses. The levels of Hcy in 63 affected fetuses were significantly higher than those in 185 unaffected fetuses, without overlap between the affected and unaffected groups. Sixteen fetuses displayed inconclusive genetic results of MMACHC variants, in which seven fetuses were determined to be affected with elevated amniotic fluid levels of Hcy, C3, C3/C2 and MMA. The remaining nine fetuses were considered unaffected based on a normal amniotic fluid metabolite profile. The diagnostic sensitivities of Hcy and other characteristic metabolites including propionylcarnitine (C3), ratio of C3 to acetylcarnitine (C2; C3/C2), methylmalonic acid (MMA), and methylcitrate acid (MCA) in amniotic fluid were 100%, 87.50%, 100%, 85.71%, and 28.57%, respectively. The respective specificities were 92.05%, 100%, 100%, 97.73%, and 99.43%. Conclusions: Hcy appears to be another sensitive characteristic metabolite biomarker for the prenatal diagnosis of cblC defect. The combination of Hcy assay with acylcarnitine and organic acid analysis is a fast, sensitive, and reliable prenatal diagnostic biochemical approach. This approach could overcome the challenge of the lack of genetic analysis for families with at-risk cblC defect fetuses.