Some people are born with the innate ability, or better said disability, of not feeling pain - whatsoever. It's not that they can't feel anything, quite on the contrary - they still have a sense of texture, they feel pressure, they can feel a hug or handshake just like anyone, they experience warmth or coldness and so on - it's just that any sensation that passes a certain threshold, effectively becoming pain, isn't registered with these people.
That's really weird, right? For a bit of insight into what's it like living without physical pain, I'd recommend you read this NY Times piece on Ashlyn Blocker, a normal looking teenager from a small town called Patterson, GA who at first glace isn't significantly different from any other kid her age. In the article, however, you'll find out how she never cried as a baby, how she first burned her hand when she was only two years old or how later on in her teens she constantly bruised herself and had broken bones.
"The girl who feels no pain was in the kitchen, stirring ramen noodles, when the spoon slipped from her hand and dropped into the pot of boiling water. It was a school night; the TV was on in the living room, and her mother was folding clothes on the couch. Without thinking, Ashlyn Blocker reached her right hand in to retrieve the spoon, then took her hand out of the water and stood looking at it under the oven light. She walked a few steps to the sink and ran cold water over all her faded white scars, then called to her mother, “I just put my fingers in!” Her mother, Tara Blocker, dropped the clothes and rushed to her daughter’s side. “Oh, my lord!” she said — after 13 years, that same old fear — and then she got some ice and gently pressed it against her daughter’s hand, relieved that the burn wasn’t worse." excerpt from the NY Times article.
Pain, like all sensations we experience, is there for a purpose. It's there to protect us, to warn us if we've strayed too far, it lets us know that we're in danger and we need help, and best of all it tells us that we need to stop whatever it is we're doing that causes the pain. Some people, however, are on the other side of fence. There are millions of people in the world suffering from diseases that induce chronic, unbearable pain. Some of these people need to live with an acute sensation of pain for the rest of their lives, and most of the time painkillers don't cut it or induce side effects that turns the patient into an emotional train wreck (see painkiller epidemic in America).
X-men - Subject 1 - mutation: "can feel no pain"
A team of European researchers recently analyzed the genome of an anonymous girl or woman from Germany, dubbed "Index Subject 1", that in most respect is just like Ashlyn - she can't feel any pain. Photos published in a paper about her show severe injuries to her head, face and knee, the latter of which she has fractured multiple times. After comparing her genome with several human genome databases, such as "1,000 Genomes," which includes all the genes of 1,092 people from 14 populations, the researchers identified a specific gene mutation in Index Subject 1 that couldn't be found in in any of the human genome databases they scoured.
This almost unique mutation (the researchers have since found a male also carrying it Index Subject 2 - to no surprise, the man can't also feel pain) affected a gene called SCN11A that makes a protein that controls how much sodium goes in and out of cells in the human body. Sodium channels are indispensable for relaying information back and forth nerve endings in the body, and apparently the particular type of sodium channel that was mutated in Index Subject 1 is abundant in nociceptors, the types of nerves that sense pain. The pieces of puzzles seem to fit nicely, however intuitive how this may all seem, the scientists still had to test this assumption.
They introduced the mutated gene into 101 lab mice. Of these, 11 gave themselves self-inflicting wounds suggesting that they couldn't feel the pain. Also, to be sure, the researchers injected the mice with a chemical that caused their paws to swell up. The mice which underwent the successful mutation didn't try to protect the swollen paw like the other, normal mice did. When subjected to high temperature, the mutated mice showed no sign of discomfort, again compared to normal mice. Also, as another important indicator, mutated mice bowels didn't move as well as normal mice - digestive issues, sometimes severe, are common among people who can't experience pain.
The findings published in Nature are nothing short of remarkable, hinting that indeed this single gene mutation is what causes some people, like Ashlyn, to live in world without physical pain. By better understanding the sodium channel mechanics, it's foreseeable that one day researchers might be able to craft painkillers that exploit the symptoms of Index Subject 1, but with temporary effects. Remember, pain is good.