Asymmetric activation of ANKRD1-CAND1-Atrogin-1 signaling in mature myofibers promotes adolescent idiopathic scoliosis progression by inducing asymmetric paravertebral muscle atrophy

The human spine is a marvel of engineering, a flexible column of bone that supports the body while allowing us to move, bend, and twist. Yet, for many teenagers, this structure begins to curve sideways in a condition known as adolescent idiopathic scoliosis. The word "idiopathic" is the medical term for "of unknown cause," and for decades, scientists have struggled to pinpoint exactly why this happens. While the spine itself bends, the story of this deformity may not begin in the bone at all, but in the muscles that wrap around it. These paraspinal muscles act like a dynamic suspension system, constantly adjusting their tension to keep the spine straight. When these muscles on one side of the body become weaker or smaller than those on the other, the balance is lost, and the spine can begin to twist and curve. Understanding how and why this muscular imbalance occurs is the key to unlocking the mystery of the disease.

A team of researchers at Nanchang University has now uncovered a specific molecular chain reaction that drives this muscular imbalance, offering a new explanation for how scoliosis starts and worsens. By studying tissue samples from patients and conducting experiments in the lab and in mice, they identified a protein called ANKRD1 as the primary culprit. In healthy muscle, this protein helps cells respond to stress, but in the patients with scoliosis, it becomes dangerously overactive on the side of the spine that is curving inward. This overactivity triggers a cellular cleanup crew to work too hard, breaking down essential muscle proteins and causing the muscle fibers to shrink. The researchers found that this process is not random; it is driven by a precise interaction between ANKRD1 and another protein called CAND1, which together supercharge a machine inside the cell responsible for tagging and destroying muscle components.

To understand the mechanics of this discovery, one must first look at the patients themselves. The researchers collected small samples of muscle from the backs of teenagers undergoing surgery to correct their scoliosis. They took samples from two specific locations: the side of the curve where the muscles are stretched (the convex side) and the side where the muscles are compressed (the concave side). When they examined these tissues under a microscope, the difference was stark. The muscles on the compressed, concave side were significantly thinner and showed signs of internal damage, with their structural fibers falling apart. In contrast, the muscles on the stretched side appeared much healthier. To find the genetic cause of this difference, the team used a powerful technique called single-nucleus RNA sequencing. This method allows scientists to read the genetic instructions inside individual cell nuclei to see which genes are turned on or off. The analysis revealed that in the damaged muscles on the concave side, a specific gene for the ANKRD1 protein was turned on to a much higher level than in the healthy muscles. This increase was specific to the mature muscle fibers and was not seen in patients with other types of spinal deformities or in people without scoliosis, suggesting it is a unique feature of this specific disease.

The researchers then moved to the laboratory to prove that this protein was indeed the cause of the muscle wasting, rather than just a symptom of it. They used a common muscle cell line and genetically engineered some of the cells to produce extra ANKRD1, while others were modified to produce less of it. The results were immediate and clear. The cells with too much ANKRD1 failed to grow into strong, thick muscle fibers; instead, they remained small and weak. Conversely, the cells with reduced levels of the protein grew larger and stronger. This confirmed that the protein directly controls the size and health of the muscle. To see if this held true in a living organism, the team created a special model using young mice that were trained to stand upright on their hind legs, mimicking the vertical posture of humans. They injected a virus carrying the ANKRD1 gene into the muscles on only one side of the mice's spines. Within a few weeks, the mice developed a visible curve in their spines, mirroring the condition seen in humans. The side with the extra protein had atrophied, or shrunk, while the other side remained normal, proving that an imbalance in this single protein is enough to trigger the spinal deformity.

The next step was to figure out exactly how ANKRD1 causes the muscle to break down. Inside every cell, there is a system known as the ubiquitin-proteasome system, which acts as a waste disposal unit. It tags unwanted or damaged proteins with a molecular label called ubiquitin, marking them for destruction by a large molecular machine called the proteasome. The researchers discovered that ANKRD1 hijacks this system. It physically binds to a protein called CAND1, which normally helps regulate the assembly of the cellular machines that do the tagging. By attaching to CAND1, ANKRD1 forces these machines to assemble more frequently and work more efficiently, specifically targeting a set of vital muscle proteins for destruction. The study showed that when ANKRD1 is active, the levels of these essential muscle proteins drop because they are being broken down faster than the cell can replace them. This process is not a general failure of the cell but a targeted attack on the muscle's structural integrity, driven by the ANKRD1-CAND1 partnership.

Having identified the mechanism, the researchers asked if it could be stopped. They turned to a drug called bortezomib, which is already approved by the Food and Drug Administration for treating certain types of blood cancer. This drug works by blocking the proteasome, the machine that breaks down the tagged proteins. In the lab, when they treated the muscle cells with this drug, the wasting caused by the excess ANKRD1 was reversed; the muscle fibers grew back to a normal size. They then tested this in the mice with the induced spinal curves. By injecting the drug into the weakened muscles, they were able to stop the muscle from shrinking and, crucially, slow down the progression of the spinal curve. The mice treated with the drug maintained a straighter spine compared to those that received a placebo. This suggests that the pathway identified by the researchers is a viable target for treatment.

The findings provide a clear, step-by-step explanation for a long-standing medical mystery. The study rules out the idea that the muscle weakness is simply a result of the spine curving and pinching the tissue; instead, it shows that the molecular imbalance in the muscle can actually drive the curve itself. The researchers demonstrated that the ANKRD1 protein acts as a pathological amplifier, taking normal mechanical signals and turning them into a destructive force that dismantles the muscle from the inside out. While the study does not yet explain why some teenagers have this protein become overactive in the first place, it establishes a direct link between a specific molecular error and the physical deformity of the spine. By pinpointing the exact interaction between ANKRD1 and CAND1, the research opens the door for future therapies that could target this specific connection, potentially offering a way to protect the muscles and prevent the spine from curving in the first place. The work transforms the understanding of scoliosis from a purely structural bone problem into a complex disease of muscle protein regulation, offering new hope for precision treatments that address the root cause rather than just the symptoms.