(Inside Science) -- Nearsightedness, or myopia, has reached epidemic levels worldwide. In some populations, up to 80 percent of young adults have the condition. While the causes of myopia are still hotly debated, scientists have long suspected a combination of genes and environment.
Now researchers zeroing in on that interaction have identified a genetic variation that appears to make children who interact with their environment in a particular way -- namely, by reading a lot -- susceptible to myopia. But the variation is rare, and not everyone is convinced reading is related to being nearsighted.
"This is the first experimental proof that myopia is underlain by genes and environment and that there's an interaction between these two factors," said Andrei Tkatchenko, a molecular geneticist at Columbia University in New York, who led the new study.
Identifying genes involved in myopia, Tkatchenko says, could eventually lead to the development of drugs to treat or prevent it.
"We can see some treatment options for myopia on the horizon," he said. "We need to do more research, but at least we know it's a treatable condition."
Nearsightedness has skyrocketed around the world. In the U.S., 44 percent of the population has myopia, compared to 25 percent just 30 years ago. In some parts of East Asia, 80 percent of young adults are nearsighted. Researchers estimate that by 2020, a third of the world's population -- 2.5 billion people -- could have myopia, a condition that increases the risk of potentially blinding diseases such as cataracts, glaucoma and retinal detachment.
An eye becomes myopic when it's elongated, causing the lens to focus light in front of the light-sensing retina rather than directly on it. Scientists have long known that the condition can be inherited, but the rise in recent decades has been too fast to be due to simply genes. Environmental factors -- such as being indoors too much, or reading and other kinds of "near work" -- may trigger myopia in people who already have a genetic susceptibility to the condition.
From studying how certain genes turn on and off in the retinas of myopic monkeys, Tkatchenko and his team had previously identified APLP2 as a gene related to myopia. While scientists have linked a couple dozen other genes to myopia, APLP2 is now the first for which evidence suggests an interaction with an environmental factor: reading.
That evidence, the new study published in PLOS Genetics, comes from the genomes of nearly 4,000 children and 46,000 adults. Researchers combed the genomes in search of variants of APLP2 that could cause myopia. Specifically, the researchers discovered that children with the variant who also read more than an hour a day were five times more likely to develop myopia than those who read less.
Both mice and humans have two APLP2 genes, and by studying mice with both, one, and neither of their APLP2 genes switched off, the researchers learned that when the eye had less of the APLP2 protein -- which is important in the development of the retina -- the mice were less susceptible to myopia. That was true not only in mice that had the particular variant of APLP2 that increased myopia risk in children, but in mice with any variation of the gene.
Scientists are still a long way from human treatments, but the mice experiments suggest that drugs that limit the production of the APLP2 protein could potentially treat myopia.
Still, while APLP2 does seem important, it's probably not the main gene behind myopia, says Donald Mutti, a myopia expert at The Ohio State University's College of Optometry in Columbus.
"This is one of many variants related to myopia that seem to have perhaps a decent effect size, but are too rare to be an important source of myopia in the population," he said.
Indeed, only one percent of the population has this particular variant, and hundreds more of yet-to-be identified genes are likely involved in myopia. But even if APLP2 alone isn't responsible for myopia, Tkatchenko says, it has a much stronger effect than any of the other known genes linked to myopia. And as the mice studies suggest, the APLP2 protein itself may increase the risk for myopia whether or not this particular variant is present.
But Mutti is also skeptical of the link between APLP2 and reading. The study's conclusions are based on a single question asking mothers to estimate how many hours their children read.
"The question is crude, and the way they treat the variable is crude," he said. "It's difficult to tell whether the effect is due to the act of reading itself, or something related to reading, like personality or some cognitive variability."
In a 20-year longitudinal study of children, Mutti found no link between near work and risk for myopia. Instead, he and other researchers propose that the lack of bright, outdoor lights causes myopia.
Tkatchenko disagrees, citing the new study and previous animal research that suggest myopia happens when the eye tries to focus on nearby objects. Regardless, most studies agree that outdoor time is important to myopia prevention.
"We can argue whether it's the effect of light or reading, but the fact is if you spend two hours outside, it protects you from myopia," he said.