31 May 2016

It has been known for some time that time in space affects astronauts’ vision. Post-flight vision changes have been reported by many astronauts after they spend time on the International Space Station. Many, but not all. But why are only certain people affected by spaceflight in this manner?’
Results of a new study involving 49 space station astronauts may help crack this case. The research points to differences in the way some individuals’ bodies operate at the cellular level which may influence whether they develop vision or eye issues in space.

The human body is complex. Intricate networks of chemical interactions within our cells convert basic nutrients from the foods we eat, into the molecules that keep us ticking. These chemical processes use specific pathways, called metabolic pathways, and are spurred by enzymes, each with its own job to do.
Scott M. Smith from the Biomedical Research and Environmental Sciences Division at NASA’s Johnson Space Center explains, “One of these pathways, called the one carbon metabolism pathway -- moves single carbon atoms from one compound to another in our cells. This is a nutrition and vitamin rich pathway that involves folate, vitamins B6, B12, and other vitamins, and is extremely important.”

In the one carbon pathway, enzymes help by pulling a single carbon atom from a folate or other carbon donor and attaching it to an amino acid, thus converting it to a different amino acid. If anything goes wrong – for example if this chain reaction doesn’t work properly, slows down, or backs up, the effect is felt down the line. Higher blood concentration of homocysteine in affected astronauts was the first clue that this team found. This led to the study of genetic differences in one carbon enzymes.
Smith says, “We found two genetic variances in the enzymes that facilitate the one carbon pathway. This may help explain why some astronauts’ vision is affected but not others.” This represents the first time researchers have identified a genetic marker associated with a spaceflight medical issue. In fact this is also the first time that NASA has used human genetic data directly in an investigation.

Smith continues, “We found that in addition to genetics, this effect was also associated with B vitamin status. A missing piece of the puzzle, however, is while we have shown an association between genetics and vision; we still don’t know the mechanism - or cause - of these effects. That’s one of the keys to figuring out how to prevent or treat this in affected crewmembers.”
This could be important on Earth, too, because differences in these same enzymes are associated with the occurrence of cardiovascular disease, stroke, and other medical issues. We are also performing research to identify similarities between characteristics of astronauts with vision issues and women with endocrine system disorders, specifically, polycystic ovary syndrome, or PCOS.” This beginning research is needed to document a possible tie between astronauts with vision issues and women with PCOS.

These study results could ultimately help researchers identify who will develop vision problems resulting from space flight, and importantly, help identify genetic related medical associations in the general population.
Scientists looking at this problem from different perspectives have more research to do before coming to a consensus. Smith notes, “This study merely scratches the surface toward understanding the genetics and apparent associated relationships.”