It was commonly observed in subjects with chronic progressive external ophthalmoplegia (cPEO) and with primary myopathy without cPEO, but also-although less frequently-in multisystem phenotypes such as MELAS, MERFF, Kearns Sayre syndrome, NARP, MNGIE and Leigh syndrome.
Mutations in thymidine phosphorylase gene (TP) are linked to mitochondrial neurogastrointestinal encephalomyopathy (MNGIE), an autosomal recessive disorder in which PEO is associated with gastrointestinal dysmotility and leukodystrophy.