Abstract
Incidence and mortality disparities between males and females exist in many diseases including bladder cancer, but the mechanisms remain unclear. To adequately address this issue, researchers must design experiments appropriately, appreciate that sex and gender are not synonymous concepts and understand that the role of both sex and gender in disease need to be elucidated.
Interest in understanding the role of sex and gender differences in cancer incidence, prevalence and severity is growing. To promote communication and collaboration of this issue in bladder cancer, Cedars-Sinai organized the first national symposium on this topic in February 2022. A panel of experts (TABLE 1) spoke on research spanning molecular biology to social determinants of health, epidemiological data to electronic health records, and preclinical to clinical studies that all pointed to a clear conclusion — sex and gender as biological variables deserve a permanent seat at the table of future research efforts in bladder cancer.
Table 1 |.
Expert speakers at the Cedars-Sinai Gender Medicine Symposium
| Presenter | Title of presentation |
|---|---|
| Art Arnold, PhD | X and Y chromosome agents of sexual dimorphism |
| Berenice A. Benayoun, PhD | Lifelong sex-dimorphisms in neutrophils |
| Paul C. Boutros, PhD, MBA | Sex differences in cancer drivers and evolution |
| Chawnshang Chang, PhD | Androgen receptor roles in bladder cancer |
| Charles G. Drake, MD, PhD | The effects of sex on response to immune checkpoint blockade |
| Maurice Garcia, MD | Gender-affirming care: the reconstructive urologist’s perspective |
| Helen Goodridge, PhD | Monocyte heterogeneity, sex differences and ageing |
| Sabra L. Klein, PhD | SeXX matters for respiratory viral pathogenesis and responses to vaccines |
| Simon Knott, PhD | The molecular consequences of androgen activity in the human breast |
| Xue Li, PhD | The sex-biasing role of KDM6A histone demethylase in bladder cancer |
| Zihai Li, MD, PhD | Immunological mechanisms of sex bias in cancer |
| Cathy Mendelsohn, PhD | PPARγ signaling controls bladder cancer subtype and immune exclusion |
| Joshua Rubin, MD, PhD | Sex differences in cancer mechanisms: hallmarks |
| Shu-Yuan Yeh, PhD | Multi-faces of oestrogen receptor signaling in urothelial cancer |
“cancer genomics has highlighted sex differences in cancer-driving molecular pathways ”
Bladder cancer incidence and outcomes
Bladder cancer is well known to be associated with a sex bias; the disease occurs three to five times more commonly in men than in women globally1. This sex difference persists even when known risk factors such as cigarette smoking are taken into consideration, strongly suggesting the critical roles of sex as a biological variable (SABV) in bladder cancer risk. Experimental and clinical evidence also implicate the involvement of androgens and the androgen receptor (AR). However, not all clinical studies have reached unanimous conclusions.
Most bladder cancers arise from the urothelium — the epithelial lining of the bladder lumen. As initial symptoms are reminiscent of urinary tract infections, female patients are often treated with several rounds of antibiotics before they receive a full urological investigation. This delay in diagnosis might partially contribute to the poorer prognosis seen in females than in similar male patients. However, the increased bladder cancer mortality in women with muscle-invasive tumours cannot be explained by clinicopathological factors alone; these disparities are probably due to both hormonal and non-hormonal biological factors. Thus, further investigation of sex differences in therapeutic procedures and outcomes, including complications of muscle-invasive bladder cancer, must be performed. In addition, these clinical observations highlight the need to increase awareness for bladder cancer in women and to develop low-cost urinary molecular screening tests for patients with concerning symptoms.
Sex hormones and sex chromosomes
The biology of sex is organized by the unequal composition and effects of sex hormones (oestrogens versus androgens) and the sex chromosomes (XX versus XY). Biasing effects of sex hormones are undeniable, and their mechanism of action seems to be dependent on specific cancer types. Androgens operate through both classical AR2 and non-classical3 pathways to promote bladder cancer development. Data suggest that targeting AR could decrease bladder cancer invasion4 and also reduce expression of CD44, a gene associated with aggressive behaviour of bladder tumours5. Targeting AR with the antiandrogen enzalutamide increases prostate cancer cell invasion but decreases bladder cancer cell invasion4. Oestrogens bind to one of the two structurally and functionally distinct nuclear receptors (ERα and ERβ) — ERα suppresses whereas ERβ promotes bladder cancer initiation6, which is in contrast to their roles in breast cancer. However, both ERs promote bladder tumour progression. Thus, effects of sex hormones on bladder cancer development probably differ from those on other cancers, such as prostate and breast cancer. This difference suggests that effects of hormonal therapies used for prostate or breast cancer might not be fully recapitulated in bladder cancer, especially when sex or gender of the patient is not always considered, and that bladder cancer clinical trials must include enough female patients in the study population.
The impact of the sex chromosomes on cancer risk has also been established. In general, men have a higher risk of cancer than women; however, somewhat paradoxically, loss of the Y chromosome increases cancer risk and cancer-related deaths in men. Furthermore, smoking is a risk factor for several cancers, and mosaic chromosome Y loss in leukocytes is moderately associated with increased incidence of some solid tumours7. The mechanisms underlying these findings remain unknown. By contrast, the X chromosome seems to be protective against bladder cancer. Females with Turner syndrome — a chromosomal disorder in which a copy of the X chromo some is missing or partially missing — exhibit a signi ficant increase in bladder cancer risk; whereas males with Klinefelter syndrome — a chromosomal disorder yielding an extra copy of the X chromosome — have a decreased risk of solid tumours8. Patients with Turner syndrome also have ovarian dysfunction, so the increased cancer risk was initially attributed to the reduction in oestrogens. As the sex chromosomes and sex hormones are coupled confounding variables, quantitatively assessing their independent and interactive sex-biasing activities based on the association studies using human epidemio logical data is challenging. In order to overcome this obstacle, the ‘four-core genotype (FCG)’ mouse model was used to study bladder cancer9. FCG mice can be one of four sex types: two testis-bearing sex types (XYM and XXM) and two ovary-bearing sex types (XXF and XYF). By comparing mice with the same gonadal sex type but different chromosomal sex types, one study demonstrated clearly that an extra copy of the X chromosome is protective against bladder cancer and also that the X chromosome has sex-biasing effects independent of the sex hormones and, moreover, that the interaction between the sex chromosomes and sex hormones amplifies sex differences in bladder cancer9. These findings are important because of the potential for the mechanism of interaction to be targeted to reduce bladder cancer risk without directly targeting sex hormones.
“Current trends in data reporting still have not adopted a routine practice of differentiating between male and female data ”
Dosage compensation and the sex epigenome
X chromosome inactivation (XCI) is a female-specific dosage compensation mechanism that equalizes the copy number difference of X between sexes. However, a subset of X-linked genes escape XCI, resulting in higher expression in females than in males, which would protect females from single chromosome mutations9. The gene encoding histone lysine demethylase 6A (KDM6A) is a notable XCI escapee that functions as a versatile epigenetic regulator to activate downstream gene expression with several roles. KDM6A has intrinsic demethylase activity antagonizing polycomb repressive complex 2 by catalysing the removal of methyl groups from histone H3 lysine 27 (H3K27), making it available for acetylation. It also demonstrates demethylase-independent activity, in which KDM6A partners with the COMPASS (complex of proteins associated with SET1), which bears methyltransferase activity to methylate H3K4. Conditional knockout of mouse Kdm6a significantly increases bladder cancer risk in females but not in males, suggesting that Kdm6a functions as a prototypical sex-biasing tumour suppressor of bladder cancer9. These findings further suggest that sex-specific epigenetics — also known as the sex epigenome — might have a previously unknown role in driving sex differences. Defining the sex epigenome in both physiological and pathophysiological conditions is, therefore, needed to advance our understanding of sex and gender disparities in cancers.
Immunity and ageing
SABV could operate in tumour cells and/or in the tumour microenvironment, such as in immune cells. Given the advances in immuno-oncology in cancer therapy, as well as the fact that autoimmunity is a female-dominant condition, interrogating how immunity contributes to sex differences in cancers is timely.
CD8+ T cells are mediators of the adaptive immunity and are important for killing cancerous or virally infected cells. Multiple preclinical models have shown the sex-biasing effect of CD8+ T cells, which results in faster bladder tumour growth in males than in females10. Further studies have revealed that the tumour microenvironment in females has increased numbers of effector CD8+ T cells, whereas the microenvironment in males has higher levels of exhausted CD8+ T cells10. In addition to T cells, neutrophil biology and chromatin architecture differ by sex and age — a notable finding because neutrophils are highly involved in both tumour biology and oncogenic inflammation. Evidence of monocyte gene expression being specific to age and sex might also indicate an age-specific and sex-specific monocyte response to tumour biology and treatment. Understanding how sex hormones, chromosomes and epigenetics affect the sex-related and age-related differences in both innate and adaptive immunity, and how SABV can be integrated into immunotherapeutic decisions about cancer management are important areas of study for the future.
Concluding remarks
The advent of cancer genomics has highlighted sex differences in cancer-driving molecular pathways and mutational processes in non-reproductive organ cancers. Despite immense progress, challenges remain in this field, including insufficiently powered statistical models to analyse genes encoded by the X and Y chromosomes and difficulties in controlling for confounding variables, such as behavioural risks and the natural history of the diseases. Current trends in data reporting still have not adopted a routine practice of differentiating between male and female data, grossly limiting opportunities to identify sex-differentiating correlates.
At the close of the Cedars-Sinai Gender Medicine Symposium, a resounding call for action echoed across institutions and disciplines to increase visibility of sex differences by improving surveys and data reporting in disease and outcomes research, for interdisciplinary collaboration, to reframe the problem of sex differences as involving more factors than just sex hormones or environmental exposures, and to innovate holistic frameworks that better account for how any given individual lies within a spectrum of sex-specific molecular and social differences.
Footnotes
Competing interests
X.L., D.T. and Z.L. are supported by NIH grants. Z.L. sits on the scientific advisory boards for Alphamab, Henlius, Ikonisys, Heat Biologics and HanchorBio.
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