Abstract: Objective：To analyze the mutation type and mutation frequency of methylmalonic acidemia and homocystinemia in children, and to explore the relationship between genotype and phenotype. Methods: A total of 113 810 newborns in Shijiazhuang city were screened for acylcarnitine spectrum by tandem mass spectrometry, and analyzed by gas chromatography-mass spectrometry (GC-MS) and vitamin B12 absorption test. The MMACHC and HCCFC1 gene mutations were analyzed using second-generation sequencing technology, and further characterized using Sanger sequencing. Results: 25 newborns with methylmalonic acidemia were diagnosed, of which 11 cases were genetically tested, 3 cases were of MMA simple type and 8 cases combined type. A total of 11 gene mutation sites were detected, of which 8 mutation sites have been reported: C.666C＞A, C.482G＞A, C.656_658delAGA, c.609G＞A, C.80A＞G, C.394C＞T, C.440_441del and C.599G＞A. 3 novel mutations sites were identified: C.239A＞G, C.4535A＞T and C.4442C＞T. The mutation frequency analysis showed that C. 482G＞A was more common, and mutation frequency was 0.25. The follow-up visit showed that 1 case of 8 patients died, the genotype was C.656_658delAGA/C.440_441del, and that 3 cases stunted, the genotypes were C.599G＞A/C.656_658delAGA/C.4535A＞T, C.609G＞A/C.609G＞A，C.80A＞G/C. 394C＞T. The remaining cases showed no abnormalities. Conclusions: The mutations of MMACHC and HCFC1 were analyzed in 8 cases with combined MMA, and C.482G＞A was more common. Combined with clinical symptoms and prognosis, the C.656_658delAGA /C.440_441del genotype is more pathogenic, the C.599G＞A/C.656_658delAGA/C.4535A＞T and C.80A＞G/C.394C＞T have poor prognosis. The gene mutation site C.656_658delAGA is highly pathogenic and poor prognosis. The prognosis of the mutation site C. 482G＞A is relatively better. Neonatal tandem mass spectrometry screening can detect children with methylmalonic acidemia early, and the identification of gene mutations can provide evidence for family reproductive guidance, prenatal diagnosis and prognosis.

Key words: Neonatal screening, Methylmalonic acid, Metabolism, inborn errors, Genes, Mutation