Why do ME patients with Orthostatic Intolerance have elevated BH4? Can our new findings lead to potential targets for treating OI?

1. Less utilization of oxygen in blood stimulates Non-oxidative Pentose Phosphate Pathway (PPP), which could produce high BH4.

Since ME bodies and cells are more likely to function in an anaerobic state (less oxygen use), we studied the effect of hypoxia - low oxygen - on the PPP pathway, BH4 and BH2. We found that cells grown with low oxygen produced more BH4 than those grown in normal oxygen levels.

2. Elevated biopterins (BH4, BH2) contribute to inflammation in microglial cells, producing nitric oxide, which in turn lowers blood pressure through vasodilation. 

Nitric oxide (NO) is a culprit in orthostatic intolerance, a condition where patients feel dizzy or faint when standing or sitting up. BH4 is known to be a co-factor enzyme for production of NO, meaning it is necessary for NO. In ME+OI patients, high levels of BH4 also correlated with high levels of NO. Nitric oxide widens blood vessels, which leads to lower blood pressure and less constriction of vessels upon standing. So, understanding the cause of elevated BH4 in ME+OI patients could lead to methods for regulating orthostatic intolerance.

3. An enzyme called transaldolase could control the upregulation of nitric oxide in microglial cells by upregulating BH4, possibly identifying a target for potential treatment. 

ME+OI patients had a higher level of transaldolase, which is an enzyme associated with Non-oxidative PPP pathway. We studied this enzyme to confirm that the PPP pathway was upregulated in ME+OI patients, compared to healthy controls. Then we wanted to learn how integral transaldolase is to the production of nitric oxide.

4. When transaldolase was silenced to not produce BH4, our team introduced samples from ME+OI patients with high BH4, which then stimulated inflammatory response in microglial cells, increasing reactive nitrogen species due to increased nitric oxide. Control samples did not.

Another creative experiment in this study demonstrated that transaldolase was important in the production of BH4 and nitric oxide. Our team silenced the taldo1 gene which reduced BH4 expression and production of nitric oxide in microglial cells in low-oxygen conditions. Then, they introduced serum from ME+OI patients with high BH4 and tested the production of nitric oxide. Serum with high BH4 restored nitric oxide production even with transaldolase silenced, while control samples did not. Remember, higher NO leads to vasodilation.