A New Drug Target for Endometriosis Treatment?

Endometriosis is a complex disease that affects 5-10% of women of reproductive age – approximately 176 million women globally. It is characterized by endometrial-like cells – tissue that is similar to the tissue lining the uterus – existing outside of the uterus. These cells induce inflammatory reactions, which can cause scarring, pain and fertility issues. The aetiology of endometriosis is not yet fully understood, and subsequently it is challenging to diagnose. On average, a woman waits eight years from the onset of symptoms to receiving a clinical diagnosis. While a number of screening tools and diagnostic approaches have been tested, they are not validated to predict an individual's risk of developing endometriosis.

The complex pathophysiology of endometriosis also makes its treatment challenging. Currently available therapeutic options are purely symptomatic, as no cure exists for the disease. Consequently, living with endometriosis can have a negative effect on an individual's physical and mental health and ultimately their quality of life; it is an area of women's health that has a great unmet need.

Krina Zondervan is professor of Reproductive & Genomic Epidemiology and head of the Nuffield Department of Women’s & Reproductive Health. In addition, she is also the co-director of the Oxford Endometriosis CaRe Centre. Zondervan's research group combines genomic, molecular and environmental epidemiological research methods to further our understanding of women's health disorders. The team focus particularly on endometriosis, and Zondervan co-founded the International Endometriosis Genome Consortium (IEGC) that has conducted some the largest genome-wide association studies (GWAS) of the disease to date.

Genetic analyses in individuals and families with endometriosis have revealed novel insights into the heritable nature of the disease, which is now estimated at 50%. The identification of specific gene variants associated with endometriosis provides novel opportunities for the development of biomarkers and targeted treatments.

In a new body of work, published in Science Translational Medicine, Zondervan and a collaboration of colleagues sequenced DNA samples from 32 families in which at least three women had been diagnosed with endometriosis. They identified that individuals with a more severe form of the disease possessed variants in a gene known as NPSR1, a finding that was also present in a GWAS of 849 non-human primates with endometriosis. In mouse models of the disease, an inhibitor of the NPSR1 protein – known as SHA 68 – reduced pain and inflammation. The authors suggest that the NPSR1 system may be a potential non-hormonal target for treating endometriosis.

Technology Networks had the pleasure of speaking with Zondervan and Dr. Thomas Tapmeier, a senior research fellow at Monash University and visiting fellow in the Nuffield Department of Women’s and Reproductive Health, and corresponding author of the paper, to learn more about how the research was conducted and the clinical relevance of the findings. 

Q: For our readers that may be unfamiliar, can you explain what spontaneous endometriosis is?

A: We use the term "spontaneous endometriosis" only in the context of endometriosis in animals. All endometriosis in women is "spontaneous", i.e., it arises without anyone actively trying to induce it. The endometriosis we observed and studied in the rhesus macaque, a non-human primate, is also spontaneous. In contrast, there are mouse models for endometriosis where endometrium is transplanted into the mouse, and this creates a model of endometriosis to study.

Q: What inspired the researchers to adopt a genetic sequencing approach to explore the root cause of endometriosis?

A: The work started with the long-standing observation that endometriosis can be "heritable", i.e., can run in families. Back in 2007 we published work using DNA from families with at least 3 women diagnosed with endometriosis, a so-called "genetic linkage" study and showed that an area on chromosome 7 was more often shared between affected women than expected by the laws of inheritance alone. This focused our interest on this area as it could harbor genes and variants implicated in familial endometriosis. However, the area contained many genes. In this study we narrowed down the area further and then successfully used next generation genome sequencing to identify genetic variants that were over-represented in familial cases (mostly stage III/IV disease) vs. controls, in the gene NPSR1.

Q: Why did the study explore genes in both humans and non-human primate models?

A: We have a long-standing collaboration to study endometriosis in the rhesus macaque with Professor Joseph Kemnitz at the Wisconsin National Primate Research Center, University of Wisconsin-Madison and Professor Jeffrey Rogers of the Human Genome Sequencing Center, Baylor College of Medicine. Through genealogical information, we had access to a large, complex pedigree of 850+ animals containing many female animals with endometriosis. Rhesus macaques menstruate just like women do, and they can show similar symptoms associated with endometriosis, such as infertility and period pain. We used DNA from the animals in the pedigree to conduct a similar linkage analysis as we had done for human families, which – unexpectedly – replicated the signal we saw on human chromosome seven. This was extremely exciting and led our joint teams to further study both humans and rhesus macaques.

Q: How representative was the human sample?

A: The sequencing study focused on familial endometriosis, i.e., women diagnosed with endometriosis in families with three or more endometriosis cases. This led us to identify three relatively rare (frequency of around two percent in the general population) "coding" DNA variants that directly impact how the NPSR1 protein is produced. Families participating in this study came from all over the world, although most came from the UK and USA. We also observed an association with a more common variant in NPSR1 (frequency around 10%) using DNA from women unselected for a family history of endometriosis. Most women in this analysis came from the UK and Australia. Worth noting is the results were seen predominantly for women with stage III/IV disease. This is consistent with our previous observations that stage III/IV disease (characterized by more extensive disease typically involving the ovaries and wide-spread adhesions) is more "genetically driven" than stage I/II (characterized typically by superficial disease in the abdominal cavity without widespread adhesions).

Q: You identified a gene that is linked to endometriosis. Can you tell us more about this gene and its function?

A: The gene, NPSR1, encodes the protein Neuropeptide S receptor 1. NPSR1 is a membrane protein that bind NPS (Neuropeptide S), named after its role as a neurotransmitter in the brain – S, because its amino acid sequence starts with a serine. Variants in this gene have previously been associated with other inflammatory conditions such as asthma and inflammatory bowel disease, but also with psychological traits such as anxiety, panic and sleep disorders. We showed that NPSR1 is expressed in epithelial cells within the endometrium, and also on certain white blood cells (monocytes) in peritoneal fluid. Peritoneal fluid is present in the abdominal cavity of all women, and in women with endometriosis commonly contains cells that play a role in the inflammatory response characteristic of endometriosis. As indicated above, NPSR1 is expressed at high levels in certain brain regions, raising the possibility that, in addition to involvement in inflammatory response, it could be involved in pain processing in relation to endometriosis. We do not currently have a good understanding of the functional mechanistic link between NPSR1 (variants) and endometriosis, and this needs to be further explored in specific cellular and tissue-based assays.

Q: You identified an inhibitor of the receptor, known as SHA 68R, which reduced inflammation and abdominal pain in a mouse model. What research exists for this protein, and has it been used in any existing therapeutic strategies/ explored in other research?

A: The inhibitor we used is what is termed a "tool compound", i.e., a molecule known to inhibit the function of a target (in this case, the NPSR1 receptor). We used SHA 68R purely to show the effect of NPSR1 inhibition, but the compound itself is unlikely to be therapeutically applicable. The next step is to start designing more specific compounds similar to SHA 68R that are clinically active and applicable. (R designates the R-enantiomer of the racematic compound, which is the active ingredient.)

Q: Are there any limitations to this work you wish to highlight?

A: I think this work presents a very exciting new avenue for the development of non-hormonal therapeutics in endometriosis, targeting the inflammation that is characteristic of endometriosis. Inevitably, questions remain, such as described above: we need to better understand the mechanistic link between genetic variation in NPSR1 and tissue-specific function of the receptor and the protein it binds (NPS), and we need to work on therapeutics that target this pathway in a specific and targeted manner with minimal side effects.

Krina Zondervan and Thomas Tapmeier were speaking to Molly Campbell, Science Writer for Technology Networks.