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We generally expect the human body to grow in a pretty predictable pattern.  Little clumps of cells turn into tiny bodies, then those infants grow into chunky toddlers and those toddlers eventually hit puberty and become adults, but at the end of the 20th century, doctors were mystified by one American girl who essentially remained a toddler until her death at age 20.  She would become famous in medical circles as the first documented case of a condition we know call neotenic complex syndrome, or NCS for short, a rare disease where infants just don't grow up, and while scientists still don't really know how this mysterious condition happens, further research into it can help us better understand human development.

When the girl was born in 1993, she had multiple physical anomalies.  Both of her hips were dislocated, for example, and she had some breathing and gut issues.  Her doctor presumed these body parts didn't form as expected in utero because of large scale genetic mutations, like chunks of her chromosomes had been rearranged, but according to tests, her chromosomes were typical.  That was the first sign that something was strange about her case, and things only got more puzzling from there.

As the years went by, this young girl just didn't grow up.  At her death at age 20, she still had the height, weight, and facial features of a toddler.  Also, her brain hadn't developed past the toddler stage and she never hit puberty.  Certain aspects of her had aged though.  When she was 16, for example, she had some adult teeth, but not all of them, more like what you'd expect of an 8 year old, and her bones had begun developing into adult bones, but hadn't finished.  

This research, published in 2009, also looked at her telomeres, little caps on the tips of chromosomes that shorten as you get older.  They were the length normally seen in someone older, but that could have been a side effect of her body staying infant-like, so there was nothing about them that indicated developmental delays.  In the end, doctors were unable to figure out what was causing her neoteny, or childlike appearance, but her seemingly unique condition led to a lot of media attention, and as her story spread, more parents came forward with stories of kids seemingly stuck in toddler-hood.  

Thanks to this, doctors were able to find five similar cases for a 2015 study.  Just like with the first girl, none of the participants showed anything out of the ordinary in genetic tests for known conditions.  The only big thing they found was that the age of their blood cells lined up with their actual age, so these patients seemed to be aging but not developing.  Unfortunately, the results didn't really shed light on why, so scientists were still stumped.

Determined to get to the bottom of this mystery, researchers individually sequenced each and every participant's entire genome, thinking that there had to be some clues hidden somewhere, and finally, they did find something.  As detailed in a study published in 2018, five of the six participants had never before seen mutations in 5 potentially relevant genes.  They weren't exactly a smoking gun, but they provided some intriguing clues about the ultimate cause of the syndrome, like some of these mutations were in genes that seemed to be pretty important in development, because they're associated with other developmental disorders, but what was truly revealing was that most of these mutated genes were involved in one way or another with regulating transcription.

That's the process where DNA is read to make RNA, an important step in turning genetic blueprints into proteins.  This pointed the team back to something they suspected years ago: epigenetics.  See, at any given moment, only some genes are actively being transcribed and turned into proteins, what geneticists refer to as being expressed.  The gene expression can differ from person to person or even in the same parts of the same person at different times in their life because modifications to all sorts of cellular machinery can influence how a DNA sequence is read, so even if a gene sequence is unchanged, mutations to other parts of the genome can affect the amount of protein that's produced from that gene or even the protein's shape or function, since many genes contain information for more than one form of a particular protein, and that's what all the genes with mutations in the NCS patients have in common.  

They're all involved in some way in the processes that govern gene expression.  Specifically, many of the genes play a part in modifying histones, proteins which help our long strands of DNA bundle up into compact packages, and how a particular section of DNA is packaged can influence whether or not the genes it in get transcribed.  Even still, the researchers don't think these expression-influencing mutations are enough to explain NCS all by themselves.

There seems to be something scientists are missing, whether it's other mutations, environmental factors, or something they haven't thought of yet, and figuring it all out will ultimately depend on finding more people to study, as spotting things like epigenetic changes generally requires a fairly large sample size.  With a condition this rare, that will take time, so that's where things are at right now.  

Scientists have put over a decade of research into neotenic complex syndrome and still don't know exactly what causes it, but they're pretty sure it comes down to how different genes end up being expressed.  Now the challenge is to figure out precisely which genes are being turned on and off, how, and why these changes prevent these infants from growing up.  Figuring all that out will reveal a lot about development that we don't currently understand, and it might even point toward treatments for NCS and other developmental disorders. 

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