A Remarkable Mechanism of Pain Insensitivity

Wife–Husband Dinner Conversations Open Up New Pathway to Combat Pain

BY THE NIH CATALYST STAFF

A few years ago, NIMH Clinical and Translational Neuroscience Branch Chief Karen Berman began seeing alarming reports from parents about their children in a clinical trial she was conducting. The children appeared to be insensitive to pain that would have others shrieking.

One child presented with skin red from hot bath water. Another boy was severely bitten by a dog, but he didn't feel pain, even while receiving stitches. Yet another child got banged in the jaw, lost a tooth, but complained only of the bitter taste of the blood in his mouth.

Aside from apparent pain insensitivity, these children share one other trait: They have a rare genetic alteration called Dup7, which is short for 7q11.23 duplication syndrome. The condition is related to, but in some ways the polar opposite of, Williams syndrome, which stems from the same chromosome region.

Berman and her team in NIMH study both Williams syndrome and Dup7, the latter of which was first described only about 20 years ago. When they and their colleague Carolyn Mervis at the University of Louisville in Kentucky heard these first-hand reports of incredible—if not dangerous—pain insensitivity, they couldn't help but wonder if there was a deeper, biological connection.

As chance would have it, Berman is married to an NIH pain researcher, Michael Iadarola, a senior research scientist in the CC’s Department of Perioperative Medicine. Over many an evening dinner and drive to campus, they would talk about this rare pain insensitivity. The conversations drove Iadarola to search for an underlying genetic mechanism within the 7q11.23 region, which contains 25 genes.

And bam! The reason popped out almost instantly. Combined efforts between the Berman and Iadarola groups found that people with Dup7 also have an overexpression of syntaxin-1A (STX1A), a protein involved in the release of neurotransmitters specifically in pain-sensing sensory neurons. Too much syntaxin essentially smothers pain-signal synaptic transmission. Iadarola, Berman, and their colleagues published this finding in January (PMID: 38261410).

But this story is just getting going.

A special pair of cohorts

Berman has studied Williams syndrome for 15 years and has examined scores of volunteers with the syndrome. She first encountered the syndrome while in medical school many years earlier, when it was referred to in textbooks as elfin facies syndrome, because one external characteristic is a person’s fairylike face, kindly demeanor, and gregarious behavior.

However, serious conditions include mild to moderate intellectual disability; visual-spatial disabilities; and due to an underexpression of the protein elastin, a narrowing of the aorta just above the aortic valve; low muscle tone; and hernias. The rare Williams-elfin facies syndrome, affecting 1 in 7,500 to 20,000 individuals, is caused by a deletion of genetic material on the chromosomal region 7q11.23.

Dup7, as the name implies, occurs when the same section on the chromosome is duplicated, and this duplication is equally as rare. So, whereas Williams syndrome stems from hemideletion of a section of chromosome 7 (leaving one copy of affected genes, instead of the typical two copies), Dup7 results from duplication of the same section of chromosome 7 (resulting in three copies of the same genes).

People with Dup7 can have characteristics closer to some individuals with autism, such as social avoidance, social anxiety, attention-deficit/hyperactivity disorder, and language delays. Yet they have relative strength in spatial skills, including math.

Berman also has seen scores of Dup7 individuals. Many study participants with both Dup7 and Williams syndrome, along with their families, have returned every two years for over a decade to engage with ongoing research study protocols. Berman and her team also work closely with Duplication Cares, an advocacy group for families of individuals with Dup7.

Michael Gregory, a staff clinician in the NIMH Clinical and Translational Neuroscience Branch, was among the first to note the similarities that Dup7 individuals had in regard to pain insensitivity, as reported by their parents. Pain insensitivity is not something that was easy to test in the clinic (i.e., they would not want to deliberately induce pain), but the anecdotal stories began to pile up. Some stories would give you the shivers: Think, car door slam followed by a child saying, “Hey, Mom. Look!”

Add a dash of pepper

And so, with morsels of information fed to him over dinners at home through Berman’s table talk about what Gregory and others were hearing from Dup7 parents, Iadarola began to formulate a research project to identify a cause for this profound insensitivity. His expertise in pain and pain management dates back to 1986, when he first came to the NIH with a background in opioid neuropeptides. He then led the NIDCR Neurobiology and Pain Therapeutics Section, where his research focused on basic and translational research on pain mechanisms, pain molecular neurobiology, and the development of new treatments.

“We always joke that I study things from the neck up, and he studies things from the neck down,” Berman said about her research compared with her husband’s. “Our work overlaps enough that we understand what each other does, but not enough to drive each other crazy.”

Iadarola reasoned that the diversity of organ systems involved in these anecdotal reports (e.g., inflamed intestines, broken bones, lost teeth, lacerated skin) implied that the inability to sense pain in people with Dup7 must be a whole-body phenomenon. He also understood that pain insensitivity, exceedingly rare but seen in some people without Dup7, usually results from loss-of-function mutations in genes expressed in pain-sensing dorsal root ganglion (DRG) neurons that connect the body to the spinal cord.

Using a dash of capsaicin, the chemical in chili peppers that makes them spicy, Iadarola set out to induce pain signals in a culture of rat cells genetically modified to have the 7q11.23 Williams syndrome/Dup7 swath. He hypothesized that the candidate insensitivity gene in the 7q11.23 region would exhibit enriched expression in the pain-sensing population. With next-generation sequencing machines at the NIH Intramural Sequencing Center, Iadarola’s lab analyzed every gene expressed in pain-sensing and non-pain-sensing dorsal root ganglion neurons.

The aha moment came when he plotted gene expression concentrations in the two populations. One gene stood out like a nail through a plank, STX1A, which yielded a sixfold expression of the pain-sensing neurons compared with the non-pain-sensing population. “What was most amazing,” he said, was that “this particular gene made intrinsic sense; somehow, overexpression was inhibiting synaptic transmission, interrupting communication with the next neuron in the pain pathway.”

Specifically, his team was looking at transient receptor potential vanilloid 1 (TRPV1)–expressing neurons because, from one of their clinical trials, they knew that interrupting transmission from TRPV1 neurons produced potent and long-lasting analgesia. Their experiments showed that overexpressing STX1A in cultured DRG neurons inhibited transmitter release from TRPV1 neurons. Syntaxin 1A is part of the “SNARE” complex that captures and opens synaptic vesicles to cause transmitter release upon stimulation.

Follow-up studies using human dorsal root ganglion tissue, recovered from organ donors, indicated that human nociceptive neurons naturally contain both TRPV1 and syntaxin. The TRPV1-STX1A genes’ co-localization places STX1A directly in the same neuron in humans, further implying that too much syntaxin is well positioned to mute pain in humans, as seen—albeit not yet tested directly—in the Dup7 population.

“[Syntaxin] must be very effective to make people like the child with the dog bite and the others insensitive to pain,” Iadarola said.

Pathway to pain relief

Pain is both a blessing and a curse. Although unpleasant, the pain sensation serves as an essential bodily function to help us avoid danger, seek care, and promote healing. For the Dup7 children, there is no treatment on the horizon that would restore sensitivity to pain. For now, this discovery by the Berman and Iadarola labs have raised awareness in the Dup7 community for parents to be diligent about any irregularity such as fever, an unusual gait, or clamminess that might indicate internal tissue damage.

There is hope that the syntaxin pathway could lead to the discovery of nonopioid analgesic treatment strategies, although that path is not straightforward, Iadarola said. Through a simple injection, certain opioids and the calcium channel blocker ziconotide can act at the same synaptic site at which an overabundance of syntaxin is assumed to act, although via different mechanisms. But for syntaxin to work to relieve chronic pain, a type of localized gene or protein interaction therapy would need to be administered.

“Turning this into a medicine is a challenge, but that doesn’t mean it can’t be done," Iadarola said. He added that the molecule syntaxin has been known for quite a long time as part of the synaptic vesicle machinery, but no useful medical phenotype has been associated with it until now.

Berman noted that the deletion and duplication of 7q11.23, causing Williams syndrome and Dup7, respectively, involve the same genes in nearly 96% of affected individuals, and thus is very well defined. Studies of these populations hold great promise for numerous studies of the syndromes’ other characteristics, such as changes in cognition, which Berman’s lab also is pursuing.