Evaluation of the clinical, biochemical, and molecular spectrum of Cobalamin C (CblC) defect in 33 patients from Pakistan.

Background: Cobalamin C is the most common inborn error of intracellular cobalamin metabolism caused by biallelic pathogenic variants in the MMACHC gene, leading to impaired conversion of dietary vitamin B12 into its two metabolically active forms, methylcobalamin and adenosylcobalamin. Biochemical hallmarks are elevated plasma total homocysteine (HCYs) and low methionine accompanied by methylmalonic aciduria. This study aimed to evaluate the clinical, biochemical, and molecular analysis of Pakistani patients with CblC defect.

Methods: Medical charts, urine organic acid (UOA) chromatograms, plasma amino acid levels, plasma tHcy and MMACHC gene results of patients presenting at the Biochemical Genetics Clinic, AKUH from 2013-2021 were reviewed. Details were collected on a pre-structured questionnaire. SPSS 22 was used for data analysis.

Results: CblC was found in 33 cases (Male:Female 19:14). The median age of symptoms onset and diagnosis were 300 (IQR:135-1800) and 1380 (IQR: 240-2730) days. The most common clinical features were cognitive impairment (n = 29), seizures (n = 23), motor developmental delay (n = 20), hypotonia (n = 17), and sparse/hypopigmented scalp hair (n = 16). The MMACHC gene sequencing revealed homozygous pathogenic variant c.394C > T, (p.Arg132*) in 32 patients, whereas c.609G > A, (p.TRP203*) in one patient whose ancestors had settled in Pakistan from China decades ago. The median age of treatment initiation was 1530 (IQR: 240-2790). The median pre-treatment HCYs levels were 134 (IQR:87.2-155.5) compared to post-treatment levels of 33.3 (IQR: 27.3-44.95) umol/L.

Conclusions: Thirty-three cases of CblC defect from a single center underscores a significant number of the disorder within Pakistan. Late diagnosis emphasizes the need for increased clinical awareness and adequate diagnostic facilities.