Patient 3 was homozygous for a novel missense, c.427T>C (p.Cys143Arg), mutation in ADK, a known gene of adenosine kinase deficiency leading to hypermethioninemia.
Therefore, the currently available evidence shows that hypermethioninemia due to heterozygous MAT1A mutations such as Arg264His is a mild condition for which no treatment is necessary.
The T353M mutation, found exclusively in four African American patients, was associated with a B(6)-nonresponsive phenotype and detection by newborn screening for hypermethioninemia.