Humans throughout history have tried to understand the meaning behind a sneeze. Homer’s Odyssey proposed that a sneeze was a good omen, a viewpoint shared by the Greeks, Romans, and many pagan cultures. On the other hand, some cultures considered a sneeze to be a sign of impending doom, which was reinforced by the spread of the black plague. A more modern understanding has taught us that sneezing is actually a defense mechanism, capable of ejecting potential toxins or infectious agents out of the nasal cavity at speeds exceeding 100 miles per hour.
Is DNA to blame for seasonal allergies and ACHOO syndrome?
Sometimes, the sneeze will hit you before you even know it’s happening. Your face crinkles, your head cocks back, and you propel particles from your nose at speeds exceeding 100 miles per hour. Such a forceful reaction is more than funny (and slightly gross)—it’s the product of evolution. Our noses offer potentially harmful invaders a quick route into the body. To defend us from things like bacteria and fungi, we’ve evolved mucus to slow them down, and sneezing to expel them.
Among the top culprits for inducing a sneeze are seasonal allergies. In academic literature, you may see it referred to allergic rhinitis. As the name implies, this is a condition where a person is prompted to sneeze in reaction to some nearby pollen that’s a part of the natural seasonal cycle for plants. Aside from pollen, there’s a whole host of things that can promote a sneeze: cat dander, bacteria, fungi, pepper, and—for some—sunlight. People who sneeze in response to sunlight are said to have autosomal dominant heliopthalmic outburst syndrome, or ACHOO syndrome. People who have this condition know exactly what we’re talking about. For them, walking out from a shaded area into the bright sunlight might cause a sudden sneeze.
Studies looking into these conditions have noticed that both ACHOO syndrome and seasonal allergies may run in families, which suggests a possible genetic component to both of them. These studies have identified numerous variants in the DNA that may predispose someone to having one of these conditions, but it’s important to remember that DNA is just one piece of the puzzle—neither seasonal allergies nor ACHOO syndrome appear to be entirely caused by genetic factors. With complex traits like sneezing, there’s often a mixture of environmental factors and multiple, small-impact genetic factors that contribute to the final trait 1.
Do you sneeze when you look at a bright light? This gene might explain why
Anyone who’s had allergies knows that a number of airborne molecules can induce a sneeze, but one of the more mysterious of triggers is sunlight. Patterns have been observed that suggest a genetic factor contributes to this light-induced response, and a specific change in the DNA near the ZEB2 gene might be involved.
A number of researchers have explored the phenomenon of light-induced sneezing, also known as “photic sneeze.” Preliminary evidence suggests that sunlight-induced sneezing may be inherited and lead to the description of ACHOO syndrome—yes, ACHOO syndrome, which stands for Autosomal Dominant Compelling Helio-Ophthalmic Outburst Syndrome. These early indications are reinforced by a study using self-reported data in 2010 that indicated a single nucleotide polymorphism (a SNP, a change at a single position in the DNA sequence) near the ZEB2 gene may be involved.
ZEB2 is a very important gene because the protein it codes for controls many important developmental processes, particularly those involving neuronal development. However, the single base pair change associated with ACHOO syndrome is located between ZEB2 and another gene involved in organ development. It’s not clear exactly how this change can lead to the photic sneeze response, but research has suggested that ACHOO syndrome occurs when neurons misfire. Thus ZEB2’s involvement in neuronal development makes it a strong candidate behind the observed association with the photic sneeze reflex.
The link between sunlight-induced sneezing and neurons has been suggested by multiple research studies. The concept is that overstimulation of neurons in the eyes by a bright light will lead to misfiring of neurons in the brain, which ultimately leads to a sneeze. It’s also possible that sudden bright light stimulation leads the brain to respond with a non-specific flurry of signals (a little bit like smashing buttons on a keyboard instead of typing a cohesive message) that activates numerous neurons which go to places like the eyes and nose.
Though it’s not clear how this reflex develops, studies have shown that 15-35% of some populations experience photic sneezing!
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