The DPYD gene provides instructions for making an enzyme called dihydropyrimidine dehydrogenase, which is involved in the breakdown of molecules called uracil and thymine when they are not needed. Uracil and thymine are pyrimidines, which are one type of nucleotide. Nucleotides are building blocks of DNA, its chemical cousin RNA, and molecules such as ATP and GTP that serve as energy sources in the cell.
Dihydropyrimidine dehydrogenase is involved in the first step of the breakdown of pyrimidines. This enzyme converts uracil to another molecule called 5,6-dihydrouracil and converts thymine to 5,6-dihydrothymine. The molecules created when pyrimidines are broken down are excreted by the body or used in other cellular processes.
More than 50 mutations in the DPYD gene have been identified in people with dihydropyrimidine dehydrogenase deficiency. DPYD gene mutations interfere with the breakdown of uracil and thymine and result in excess quantities of these molecules in the blood, urine, and the fluid that surrounds the brain and spinal cord (cerebrospinal fluid). It is unclear how the excess uracil and thymine are related to the specific neurological problems that affect some people with dihydropyrimidine dehydrogenase deficiency.
Mutations in the DPYD gene also interfere with the breakdown of drugs with structures similar to the pyrimidines, such as the cancer drugs 5-fluorouracil and capecitabine. As a result, these drugs accumulate in the body and cause the severe reactions that can occur in people with dihydropyrimidine dehydrogenase deficiency.
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- dihydropyrimidine dehydrogenase [NADP+]
- dihydrothymine dehydrogenase
- dihydrouracil dehydrogenase