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I grew up with a neighborhood kid who was a nice guy but burst into tears at even the tiniest of scrapes--heck, even if he fell down--so we always called him a crybaby. He seemed to be very tuned to his sensory environment--in our gang's perspective, overly tuned--hence the epithet for his behavior. The many unjust cruelties of childhood notwithstanding, was that really a fair accusation? There is growing evidence that may supply a solid molecular answer to this question. It may reveal the accusation to be not only emotionally unkind but also biologically unsound.
In this column, I will explore remarkable new data describing the molecular underpinnings of pain sensitivity. The results are not enough to fully explain responses to such famous clinical questions as, "On a scale of 1 to 10, please describe your discomfort." Yet something exciting is on the horizon. Recent research suggests that someday (maybe soon) there will exist a pain-sensitivity test that is based not on a person's subjective self-report but on his or her chromosomal makeup.
To describe these interesting findings, I will review some pain perception basics and then move on to the molecular biology of a most extraordinary protein: catechol-O-methyltransferase (COMT). Feel free to skip to the "Genetic variation" section if nociceptors and stem-loop RNA structures are working parts of your vocabulary.
It has not always been easy to conduct pain research, especially pain research ending with the identification of genes. The ability to interpret a molecular research finding is determined in large part by which type of pain is being studied. This can be frustrating because the pain experienced in a puncture wound is different from the pain experienced when biting into a hot pepper, and both are different from the pain of grief. Remarkable progress has been made, but many basic issues remain to be settled. For example, researchers have yet to isolate the ion channel that mediates aversive mechanical stimuli. All of this makes the isolation of the COMT gene and its well-characterized role in pain sensitivity all the more remarkable.
The input of environmental circumstance is another hurdle when researching the experience of pain. Pain perceptions are modifiable by stress, previous knowledge, anticipation, excitement, and even religious belief. Any comprehensive description of pain must also deal with its remarkably subjective characteristics. Most researchers divide these cognitive aspects into separable components--the actual sensation at the pain source and one's affective response to the experience. Noninvasive imaging studies have been helpful, functioning as a cartographer in an exploratory journey. It is increasingly possible to isolate specific regions of the brain that are responsible for the more subjective components.
Although a comprehensive description of how pain is processed would take volumes, it is possible to divide the process into 2 steps. Taking the simplest example of a hammer hitting the thumb, here is a brief review.
The first step involves the initial response to the pain source. Somatosensory pain begins with signals derived from nociceptors in the skin making contact with the hammer. Nociceptors are primary sensory neurons and come in several types. A-delta fibers, which are fully myelinated, respond mostly to pressure and heat. C-fibers, which are unmyelinated, are sensitive to more intense stimulations such as temperature but are also involved in pressure and noxious chemicals. Regardless of type, the cell bodies of all nociceptors lie in the dorsal root or trigeminal ganglia, and their exposed peripheral terminals end in the skin (mostly in the epidermis). The sequential activation of the faster-responding A-delta fibers, followed by the slower-processing C-fibers, may be partially responsible for the 2 stages observed in most thumb-whackings: a sharp sensation followed by intense throbbing.
The second step involves the routing and interpretation of the signals received from the nociceptors. These signals arrive at the spinal column and are shuttled to the brain--easily the most complex part of the story but surprisingly well characterized. These routed signals lead to a suite of activation patterns in the brain that are so predictable that the pattern has been given its own name, the canonical pain matrix.
Different areas of the matrix are involved in different aspects of the pain response from being hit on the thumb with the hammer. For example, the S1 and S2 regions (primary and secondary somatosensory cortices) mediate the ability to evaluate the intensity of a painful stimulus. They also participate in helping the brain determine the location of the stimuli. The frontal cortex, anterior cingulate cortex, and anterior insula regions appear to be involved more in the subjective components of the pain experience. Other regions are also associated with pain responses, but in a less-reliable fashion. These include the amygdala and entorhinal cortex, which may be associated with both memory and anxiety responses to pain.
Where does COMT protein fit into this complex series of responses? The neurons mediating persistent pain all use neurotransmitters, and the big 3 (dopamine, norepinephrine, and epinephrine) have pervasive involvement. COMT has been shown to mediate many human behaviors of clinical relevance, including sensitivity to pain thresholds; the protein performs this vital function in part by metabolizing catecholamines. Dopamine appears to play a particularly prominent role.
Characterizing COMT's role has been one of the great success stories in molecular pain research. In what started as a research trickle and ended with a flurry of papers, the relationship was shown to be quantitative: the less COMT you had, the more sensitivity to pain you were likely to experience; the more COMT you had, the less sensitive you were. This turned out to be a relationship involving a wide variety of pain issues, from managing temporomandibular joint disorder (TMJD) to fibromyalgia. More recently, COMT biology has been implicated in mental health issues and even addictive behavior.
As you might suspect, characterizing the genetic structure behind this protein became a top priority. The human gene is responsible for encoding 2 very different proteins. One is a soluble form, S-COMT, which is free-floating in the cytosol. The other is tethered to a specific region--the cell membrane--and is appropriately termed "MB-COMT."
COMT is a member of the class II gene family, which means it encodes the information that is necessary to make protein. Since the protein-manufacturing site is located in the cytosol and class IIs are locked tight in the nucleus, COMT has an information transfer problem to solve. Like all class II genes, it does this through the manufacture of a mobile messenger, mRNA, which can leave the nucleus, find the protein-manufacturing apparatus, and coax it to make its protein.
The mRNA is constructed as a linear transcription of the gene, forming a kind of snake made of letters, but does not usually stay in a serpentine form. Once manufactured, the transcript folds into particular configurations called secondary structures. Some of these secondary structures transform the mRNA into lollipop-like configurations; others force the transcript into complex, dumbbell-like shapes (Figure). These secondary structures can also shift; exactly what shape they assume depends on the sequence possessed by the nucleotides in combination with the cellular microenvironment the mRNA inhabits. These configurations are very important in determining whether a given transcript will be translated into a protein. And that event is critically important for pain perception.
Where the transcript wants to find itself once its creation is complete is safe in the molecular arms of the cytosolic ribosome. (A ribosome is a group of molecules that when assembled scan the mRNA and make a protein from the instructions they encounter.) The mRNA leaves the nucleus, often under molecular escort, finds a willing ribosome, and begins the process. The ribosome assembles directly onto the transcript. Looking something like a meatball on top of a Frisbee, this assembly will only occur if the mRNA is in the proper configuration. Certain secondary structures will allow the self-assembly process to proceed, others will not. When inhibition occurs, the ribosomal assembly collapses and no protein is made. Since pain sensitivity is dependent on the presence of COMT protein, pain sensitivity also is dependent on the secondary structure of COMT mRNA.
To complete this story, one would only need to find mutations within the COMT gene that changed the secondary structure of its mRNA, show that these changes inhibited COMT translation, and then demonstrate this. Going in almost the reverse order, that is exactly what happened.
Genes are composed of individual subunits--nucleotides--that in groups of 3 encode the information to make protein. Mutations exist that can alter the sequence of these nucleotides. Variations in the sequence of a gene at a single nucleotide site are called single- nucleotide polymorphisms (SNPs). Although seemingly innocuous (a typical gene can easily have more than 1000 nucleotides), SNPs can exert a profound influence on the function of the protein that is eventually encoded.
The human COMT gene was shown to have a number of SNP mutational combinations. These fell into 3 haplotypes that were shown in a statistically robust fashion to vary with pain sensitivity. In this case a haplotype is a particular combination of alleles in a defined chromosomal region that is both the necessary and sufficient condition to produce a given phenotype. Serving as a miniaturized version of the genotype, a haplotype encompasses much larger regions of DNA and could easily possess a collection of SNPs.
Further characterizations of these haplotypes revealed the presence of critical SNPs. In fact, one SNP makes the gene and its cognate enzyme seem almost monstrous. Called val158met (a valine/methionine interchange at position 158), this SNP has been associated with changes in cognition, addiction, an increased risk of affective disorders, and most important, changes in pain sensitivity. Simply put, if you have this mutation you are much more sensitive to pain. It has been associated with TMJD, fibromyalgia, headache, and even irritable bowel syndrome.
Exactly how does this monster work? Recent data suggest the culprit has to do with the mRNA's secondary structure. Here is how it works:
• An SNP, or combination of SNPs, is acquired by a particular chromosomal region (for any number of reasons, heritability being one).
• SNP error is transcribed faithfully into mRNA.
• This SNP causes the mRNA to fold into an aberrant secondary structure (as the lollipops discussed above).
• The mRNA leaves the nucleus and finds the ribosome.
• The aberrant structure of the mRNA does not allow the ribosomal apparatus to successfully assemble onto the mRNA during the initial stages of translations.
• The mRNA remains untranslated, and COMT is not made (or is made at such reduced levels it becomes biologically irrelevant).
• This loss translates into increased susceptibility to pain.
This is a bombshell of a finding. Not only is a particular gene structure strongly associated with something as subjective as pain sensitivity, but the molecular mechanism in which this sensitivity starts has revealed its secrets.
As with all good research, these findings produce more questions than they answer. Mathematically, haplotype associations with pain sensitivity are more robust than the single monster mutation described above. That probably means the monster is getting assistance from other neighborhood SNPs. How do these interact with the problem at position 158 to create the offending secondary structure?
At the 40,000-foot level, more tantalizing questions remain. What does catecholamine elevation do to surrounding neural tissues to mediate such complex behaviors as pain perception? Exactly how does the inhibition of the COMT protein result in differences in pain sensitivity? How do these mutations interact with the more subjective components of the pain matrix? These data only begin to address the "black box" of brain biology, the strange realm in which a neural input goes in and a conscious perception comes out. It is a great foothold on the question, but it is only a foothold.
Nonetheless, it is a strong foothold, sufficiently robust to have clinical implications, some of which might even carry ethical baggage. With the elucidation of the molecular components of pain sensitivity in hand, it is possible to create a genetic test for use in a clinical setting. If the person has the offending haplotype, the clinician will eventually be able to predict his sensitivity to pain reactions with some certainty--and do so simply by ordering a blood test.
Taken together, these data point to the sobering realization that there is a strong genetic component to pain sensitivity. There are people who really do extract more noxious feelings than others from a painful experience and do so through very little fault of their own. I keep wondering if my childhood acquaintance was one of these people.