Heteroplasmy, mtDNA Copy Number, Uridine, and Folic Acid

There was this interesting line of thinking in the treatment of genetically-based mitochondrial disorders [I don't say inherited, because deletions in the mitochondrial genome aren't really inherited, usually, and apparently are thought to result from some bottleneck effect on the availability of nucleotides for mtDNA replication (or maybe some abberant process of mitochondrial biogenesis) in the maternal oocytes that leads to heteroplasmy (http://www.ncbi.nlm.nih.gov/pubmed/9718339)], and the idea was, or ideas were, that, because uridine biosynthesis is coupled to the activity of the mitochondrial respiratory chain, deficient activity of complex I or complex II or whatever would lead to a deficiency of uridine and all other pyrimidines, that this pyrimidine depletion would impair mitochondrial DNA replication and reduce the mitochondrial DNA copy number, and that this impairment in mtDNA replication (or also transcription) could be potentially overcome by administering exogenous uridine. Part of the rationale for this is that treatment of cultured cells with ethidium bromide, an inhibitor of the mitochondrial isoform of DNA polymerase (polymerase-gamma), produces uridine auxotrophy, a condition in which the cells can't grow without an external supply of uridine and pyruvate (rho-naught cells, etc.). The authors of this article discuss this (the fulltext is available there, too):

http://scholar.google.com/scholar?hl=en&lr=&cluster=6806894149759193037

I think there were issues with the use of uridine and uridine prodrugs in treating the true mitochondrial disorders, but the concept has been applied to the prevention of mtDNA depletion in response to some antiretroviral drugs (as they discussed above, in that article). I think there are issues with that conceptual model and with high-dose uridine as a way to treat heteroplasmy or reductions in the mitochondrial DNA copy number, which can be acquired with aging and in disease states, etc. Heteroplasmy is the presence of more than one mtDNA genome sequence, among the many copies (as many as 10,000 in oocytes, I think, but I may be wrong), in a single mitochondrion or a single cell. Heteroplasmy can be tissue-restricted (tissue-specific) and only occur in the brain or whatever other "diseased" tissue.

I think one reason may be the absence of purines in combination with the uridine, because, although uridine has pretty significant neurotrophic effects on the brain (and some of this has been applied to the treatment of psychiatric disorders), its effects would be expected to ultimately depend on an adequate pool of purines (I don't feel like looking for the references now):

http://www.ncbi.nlm.nih.gov/pubmed/16055952

http://farmakoloji.uludag.edu.tr/pdf/derleme-mehmet.pdf
(pubmed id: http://www.ncbi.nlm.nih.gov/pubmed/16769123)

But it's interesting that supplementation with folate or reduced folates has been shown to prevent decreases in the mtDNA copy numbers in response to chemotherapy, in animals, and aging or another experimental model:

http://www.ncbi.nlm.nih.gov/pubmed/17381984

http://www.ncbi.nlm.nih.gov/pubmed/17709439

Some of that can be explained by the effect that folate repletion has on reducing uracil misincorporation into mtDNA (by first decreasing the intramitochondrial dUMP/dTMP ratio), given that uracil misincorporation can produce large deletions in mtDNA and nuclear DNA (many refs). I think the purine side is relevant, though, because other articles have shown that hypoxanthine, in combination with thymidine (or hypoxanthine alone in some cases), can rescue cells deprived of folate.

Here's an interesting article on "cerebral folate deficiency," a condition in which a reduction in the cerebrospinal fluid 5-methyltetrahydrofolate level, sometimes in the presence of "normal" serum folate, produces severe neurological damage:

http://www.bh4.org/pdf/pineda.pdf
(pubmed id: http://www.ncbi.nlm.nih.gov/pubmed/16365882)

Some of those cases (it's an "etiologically-heterogeneous" disorder) can apparently be caused by autoantibodies to the folate receptor, a protein that transports 5-MTHF (more than folic acid) into cells, I think. They apparently don't know the full cause of the case they report on, but the authors mention that cerebral folate deficiency is a generic diagnosis and tends to be associated with large deletions in the mitochondrial genome. Treating it with reduced folates, such as folinic acid (racemic leucovorin, probably), can apparently partially reverse the reductions in mtDNA copy number and effectively reduce the clinical manifestation of the mtDNA copies containing the deletions. I don't think it's possible to repair large deletions. The repair process occurs through the replication of wild-type mtDNA copies and the gradual over-representation of those wild-type copies, in relation to the mutant copies (I think).

It's interesting because folic acid deficiency has almost never been associated with neurological symptoms in adults, but B12 deficiency has, of course, been. It's possible that that rule of thumb, that folate deficiency doesn't produce neurological symptoms, may not be correct. They may have only been looking at serum folate and not CSF folate (both are mainly 5-MTHF, not folic acid, in the serum and CSF) or something. There are lots of articles on that cerebral folate deficiency.

Reductions in the mtDNA copy number accumulate in a lot of degenerative diseases like Alzheimer's disease and atherosclerosis, etc. (http://scholar.google.com/scholar?hl=en&lr=&q=mtDNA+%22copy+number%22+Alzheimer%27s). Some of the research on those abnormalities is controversial in the context of Alzheimer's, but it depends how strictly one defines it. There's one study, at least, showing that, in humans, serum folate or homocysteine correlates, positively or inversely, respectively, with mtDNA copy number in peripheral lymphocytes or monocytes or something, but I don't feel like looking for it now.

The idea is that a reduction in mtDNA content, usually reflecting a reduction in the mtDNA copy number, can eventually lead to reductions in ATP or vulnerability to the collapse of mitochondrial activity in response to an ischemic insult, etc.