The present invention provides a synthetic MMACHC polynucleotide comprising a polynucleotide encoding MMACHC that is codon-optimized for expression in a human. Also provided is a polypeptide encoded by a synthetic MMACHC polynucleotide, an expression vector comprising a MMACHC gene sequence under the control of a chicken beta actin (CBA) promoter, and an expression vector comprising a synthetic MMACHC polynucleotide. Methods of treating cobalamin C deficiency and for detecting or tracking exogenous MMACHC are also provided.