Elastic Fibers Link Pelvic Floor Disorders

Urinary incontinence and pelvic organ prolapse often occur simultaneously, but no genetic evidence had been found to link the two conditions. Building on recent findings that maintenance of elastic fibers in tissues depends on the protein lysyl oxidase-like 1 (LOXL1), new research connects these conditions, showing that a loss of elastic fibers causes both urinary disorders consistent with incontinence and pelvic organ prolapse. The findings appear in the February American Journal of Pathology.

Images courtesy of Tiansen Li

Out of shape. Dissected pelvic regions in wild-type mice (left) reveal well-formed and orderly organs compared with stretched vaginal walls, detached or pinched urethras, and free-moving or displaced bladders in mutant mice (center and right). Schematic diagrams below each image summarize the organ positions, shapes, and sizes.

This study represents the first use of a genetic model to investigate the role of elastic fiber defects in pelvic floor disorders. Xiaoqing Liu, HMS research fellow in ophthalmology at the Massachusetts Eye and Ear Infirmary and first author, compared Loxl1 knockout mice and wild-type mice before and after pregnancy. The Loxl1 knockout mice developed lower urinary tract dysfunction, urinating 10 times as often with one tenth the volume of wild-type mice, and severe organ prolapse after delivering their first or second litters. “The findings in the study come together to show that because of an elastic fiber defect, the pelvic tissue is left in a permanent overstretched status, and the damaged urethra cannot be closed tightly, ” said Liu.

It is the elastic fibers that give tissues flexibility (see Focus, Feb. 20, 2004). “The dogma is that elastic fibers are stable and turn over very slowly, but we start to lose them in old age,” said senior author Tiansen Li, HMS associate professor of ophthalmology at MEEI.

The reproductive system is an exception, however. Because pelvic tissue must accommodate the development of the fetus and delivery, it undergoes massive tissue remodeling, including elastic fiber degradation and regeneration. Liu and colleagues observed that in wild-type mice, LOXL1 expression decreased dramatically in the uterine cervix at the end of pregnancy in preparation for delivery, causing a softening of the tissue. The protein returns a few days after delivery and helps reconstruct elastic fibers. “Without LOXL1—the major workhorse for elastic fiber creation—the structural matrix can’t be rebuilt properly,” said Li.

Inspections of pelvic tissue of the Loxl1 knockout mice postpartum showed the effects of the tissue’s failure to reassemble its elastic fiber network. The vaginal walls were stretched, and the uterus collapsed into the vaginal wall. They observed displaced and pinched urethras, explaining frequent or irregular urination.

In addition to observing and measuring urinary behaviors and pelvic organ tissue health, Li and colleagues observed that LOXL1 levels also decline in aging wild-type female mice in pelvic organs. “This provides circumstantial evidence that diminishing LOXL1 may be associated with changes in reproductive hormones, suggesting an avenue for future research,” said Li.

To confirm that these findings apply to human pelvic floor disorders, Li and colleagues plan to work together with clinical researchers to demonstrate similar links in humans. A connection between the human disorders and LOXL1 would suggest new approaches to treatment, such as delivering LOXL1 to pelvic tissues or upregulating it to increase tissue elasticity.

—Elizabeth Dougherty

No Gene, No Pain: Single Transcription Factor Dictates Pain Sensitivity

Because chronic pain plagues so many people and responds to so few therapies, researchers have long scrutinized nociceptors, pain-sensing neurons, and have learned much about the structure and interactions of these cells. A new study in the Feb. 6 Neuron that examines how nociceptors grow to have vastly different pain-sensing capabilities finds that a single transcription factor, Runx1, coordinates their development.

Nociceptors are a heterogeneous lot, each type expressing a distinct set of ion channels and receptors. “We wanted to know whether individual nociceptors control their phenotypes independently, or if an intrinsic factor coordinates their development,” said Qiufu Ma, HMS associate professor of neurobiology at the Dana–Farber Cancer Institute.

The investigation showed evidence that Runx1 shapes pain-sensing neurons into specialized types by promoting genetic transcription that determines the types that develop. Initially, all pain-sensing neurons express the same basic nerve growth factor receptor and Runx1, but late in embryonic development and during early postnatal development, the selective elimination or maintenance of Runx1 causes these cells to segregate into two major subsets. In conditional Runx1 knockout mice, Ma and colleagues observed that the absence of Runx1 during development hindered such nociceptor differentiation.

The Runx1 knockouts did not develop a sensitivity to thermal pain or to pain caused by nerve damage, and developed a reduced sense of inflammatory pain. These mice did develop a normal sensitivity to pin pricks, however. Inspection of neurons showed that Runx1 knockout mice had impaired pain senses because they lacked many of the ion channels and receptors present in control mice. “Runx1 doesn’t control one receptor, it controls more like two dozen ion channels and receptors,” said Ma.

In addition to finding that Runx1 mediates nociceptor differentiation and the development of thermal and neuropathic pain sensitivity, this study shows that Runx1 also helps to guide the connections from peripheral pain-sensing neurons into the spinal cord to form specific neural pathways to the brain. “These findings demonstrate an effective core control system for both sense modality and target selection,” said Ma.

Among the receptors that Runx1 orchestrates are T cell receptors, making the protein a coordinator of pathogen detection in the immune system. “It is very interesting in terms of development and evolution that two host defense systems rely on the same transcription factor for receptor expression,” said Ma.

These findings may have implications for future chronic pain therapies. Rather than blocking individual ion channels and receptors, it is possible to envision therapies that block Runx1—the core control pathway—thereby blocking multiple nociceptor ion channels and receptors at once. The plausibility of such a therapy hinges on the role Runx1 plays beyond embryonic development, said Ma. “Our next goal is to understand how Runx1 functions in adults.”

—Elizabeth Dougherty

Molecular Weapon Hits Critical Target in Gastric Tumors

Some cancer cells rely on a single genetic defect for survival, a concept known as “oncogene addiction.” Their dependence on a single mutation creates an Achilles heel, making the cells vulnerable to molecularly targeted therapies that functionally disable the defect. “Tumors arise due to many mutations. According to the oncogene addiction model, not all of these genetic mistakes are equal. Some are critical for a cancer cell’s survival,” said co-author Daniel Haber, the Laurel W. Schwartz professor of oncology and director of the Massachusetts General Hospital Cancer Center (see Schwartz Professorship in Bulletin).

Haber and his colleagues have found such an Achilles heel in a subset of gastric cancers. In the Jan. 30 Proceedings of the National Academy of Sciences, the researchers report that those gastric cancers that have amplified copies of the gene for the MET growth factor receptor may be vulnerable to a new MET tyrosine kinase inhibitor.

Gromoslaw Smolen, HMS research fellow in medicine at MGH and lead author on the study, screened panels of gastric cancer cell lines for sensitivity to the MET tyrosine kinase inhibitor PHA-665752, in development by Pfizer. He found that cells with a genetic MET amplification showed extreme sensitivity to the drug, while it had no effect on those without it. “This gave us a hint that the mechanism of sensitivity is linked to MET amplification,” said Smolen.

MET amplification causes the MET growth factor receptor to dominate the cell signaling pathways to such an extent that it single-handedly prevents the tumor cells from undergoing apoptosis. In these cells, the receptor is always active, whether or not its suitable growth factor is present, and the receptor is directly responsible for activating critical downstream signals like the protein AKT, which prevents apoptosis. MET inhibition blocks the signals to AKT, disabling the cell’s defense against cell death. “Because MET is the primary survival signal for these cells, it becomes their Achilles heel,” said Smolen. In cells without MET amplification, inhibition of MET alone does not curtail the downstream survival signals, rendering the MET-inhibitor drug ineffective.

These findings have influenced the upcoming clinical trials of a related MET inhibitor at MGH. Researchers will screen patients with gastric cancer for MET amplification so they can correlate the genetic makeup of the cancer to the effectiveness of the drug. “We hope the same correlation we found in the lab will hold true in the clinical trial,” said Smolen. “The Holy Grail of targeted cancer therapy is understanding which group of patients is likely to respond to a specific drug and which have tumors that will be resistant to it.”

Targeted cancer therapies may challenge the accepted approaches to clinical drug trial design, according to Haber. Traditional cancer drugs tend to be toxic and do not discriminate among different varieties of cancer, such as gastric cancers with or without MET amplification. So Phase I studies typically seek to find the optimal dose of a drug by incrementally increasing the dose administered to study patients with many different types of cancer. “Targeted therapies such as kinase inhibitors are virtually nontoxic, and they appear to be very discriminating, depending on the genetic makeup of an individual tumor—either it works or it doesn’t. It may be that a trial’s first step should focus less on avoiding toxicity and more on finding susceptible subsets of cancer,” said Haber. “Because large clinical trials are so costly, we need preclinical approaches to identify the specific types of cancer that will respond to these very specific drugs. ”

—Elizabeth Dougherty

top