Mitochondrial toxicity can be increased by genetic factors

Researchers are one step closer to understanding why some people taking NRTIs suffer the potentially fatal side-effect of mitochondrial toxicity. A new gene mutation linked to the side-effect has been identified in a Thai woman treated with stavudine (Zerit) for a year.

NRTIs form the backbone of current HAART regimens but are associated with a variety of possible side-effects when used long term.

One of the most common and potentially serious problems is mitochondrial toxicity -when the body’s mitochondria become damaged or destroyed. Mitochondria are structures found in most human cells which are essential for energy production.

This can cause both mild and severe problems, including muscle weakness (myopathy), loss of feeling in fingers and toes (peripheral neuropathy) or pancreatitis.

At its most severe it can cause lactic acidosis, a build-up of lactic acid in the body, which can be fatal.

Not all patients treated long-term with NRTIs develop mitochondrial toxicity, suggesting genetic factors could be involved.

NRTIs are thought to cause mitochondrial toxicity by inhibiting an enzyme called polymerase γ which is involved in DNA replication within the mitochondria.

Japanese and Thai researchers isolated DNA from eleven patients with a history of raised lactic acid levels induced by stavudine and sequenced the polymerase γ gene. They also took DNA from five people with normal lactic acid labels despite long-term stavudine use.

In two of the patients with raised lactic acid levels they found a previously reported polymorphism.

But in one 34 year-old woman they found a completely novel mutation in the polymerase γ which resulted in the amino acid arginine being replaced by cysteine at position 964 in the enzyme protein. The mutation has been termed R964C.

When the researchers added stavudine to cultures containing cells with the mutated gene they found mitochondrial DNA levels dropped significantly.

They also wanted to find out whether this mutation affected the activity of the enzyme itself. So they isolated both mutant and normal (or “wild type”) versions of the enzyme and found the mutant polymerase γ had just 14% of the activity of the wild type enzyme (Yamanaka 2007).

The researchers believe that before exposure to stavudine the R964C mutation compromised the woman’s mitochondrial activity - but to just above the threshold where it could cause problems.

But once she started taking the NRTI her mitochondrial activity fell below this threshold and started to cause acute mitochondrial failure and lactic acidosis.

Because she was the only patient with lactic acidosis to have this mutation in the polymerase γ gene there must be other factors involved in NRTI- induced mitochondrial toxicity.

But the researchers says this is the first time a direct correlation has been made between this side-effect, the function of a genetically mutated form of polymerase γ and the action of stavudine.

The mutation was not seen in 26 other Thai HIV-infected people studied subsequently or in 100 healthy Thai individuals.

But five of 27 relatives of the woman tested had one copy of the mutated gene. The woman had both mutated gene copies.

An accompanying editorial in JID says: ‘Like all important findings, [this] observation opens a new door for unravelling the puzzle of mitochondrial toxicity”.

It adds that this is one step closer to individualised antiretroviral therapeutics but adds: “Such an undertaking is challenging but not overly ambitious because of the intrinsic power it offers with respect to patient care” (Lewis 2007).

Stavudine used to be a commonly used NRTI in first line HAART regimens but became less so when it was found to be more likely to cause mitochondrial toxicity than other NRTIs.

But it is still a component of combination treatments GPO-vir and Triomune both often used in resource-limited countries.