Brain-derived neurotrophic factor (BDNF) as a potential marker of endometriosis: a systematic review and meta-analysis

BACKGROUND

The existing literature on the association between BDNF protein levels and endometriosis presents inconsistent findings. This systematic review and meta-analysis aim to synthesize the available evidence and evaluate the possible relationship between BDNF protein levels and endometriosis.

METHODS

Electronic databases (PubMed, Embase, Scopus, PsycINFO, and Web of Science) were used to conduct a comprehensive literature search from inception to June 2023. The search strategy included relevant keywords and medical subject headings (MeSH) terms related to BDNF, endometriosis, and protein levels. A random-effects model was used for the meta-analysis, and to explore heterogeneity subgroup analyses were performed. funnel plots and statistical tests were used for assessing the publication bias.

RESULTS

A total of 12 studies were included. The pooled standardized mean difference (SMD) of BDNF levels between women with endometriosis and controls was 0.87 (95% confidence interval [CI] 0.34 to 1.39, p = 0.001; I2 = 93%). The results showed that blood levels of BDNF are significantly higher in endometriosis patients (SMD: 1.13 95% CI 0.54 to 1.73, p = 0.0002; I2 = 93%). No significant publication bias was observed based on the results of Egger's regression test ((p = 0.15).

CONCLUSION

This study revealed a significant difference between patients diagnosed with endometriosis and healthy control in the level of BDNF. The results indicate that women with endometriosis have higher levels of BDNF. Further studies are needed to be undertaken to investigate the role of BDNF in endometriosis pathophysiology and the diagnostic value of BDNF in endometriosis.

Development and Validation of an Endometriosis Diagnostic Method Based on Serum Biomarkers and Clinical Variables

Endometriosis affects more than 10% of women of reproductive age, significantly impacting their quality of life. Diagnosis typically takes 4 to 11 years from symptom onset. The gold standard for diagnosing this disease, laparoscopy, is invasive, contributing to this delay in diagnosis. Two studies were conducted to develop a diagnostic test based on the combination of serum biomarkers and clinical variables. Study 1, the development study, aimed to: (i) confirm the ability of CA125, BDNF and clinical variables to differentiate between cases and controls, and (ii) develop a diagnostic algorithm based on these results. Study 2 validated the clinical performance of the developed in vitro diagnostic (IVD) test in diagnosing endometriosis. Serum samples and clinical variables extracted from psychometric questionnaires were obtained from the Oxford Endometriosis CaRe Centre biobank (UK). Case/control classification was performed based on laparoscopy and histological verification of the excised lesions. Studies 1 and 2 included n = 204 and n = 79 patients, respectively. Study 1 found a statistically significant difference between cases and controls for levels of both biomarkers. Of the assessed clinical variables from the patients' medical histories, six were found to be significantly different between endometriosis cases and controls. CA125, BDNF and these six clinical variables were combined into a multivariable prediction model. In Study 2, the IVD test demonstrated sensitivity and specificity values of 46.2% (25.5-66.8%) and 100% (86.7-100%), respectively. Due to its high specificity, this IVD test is a simple and accurate rule-in test for early disease identification, even in the presence of non-specific symptoms.

BDNF and TrKB expression levels in patients with endometriosis and their associations with dysmenorrhoea

BACKGROUND

Brain-derived neurotrophic factor (BDNF) is a known regulator of the development and maintenance of chronic pain in various chronic disorders. Together with its high-affinity tyrosine kinase type B (TrKB) receptor, BDNF is extensively expressed in the mammalian female reproductive system. However, BDNF and TrKB expression in different stages of endometriosis and the relationship between the expression of each in ectopic lesions and endometriosis pain remain unclear.

METHODS

Sixty-two women who underwent laparoscopic surgery were enrolled in this study: forty-six diagnosed with ovarian endometrioma (study group) and sixteen diagnosed with ovarian benign tumours (control group). Samples from eutopic endometrium and ovarian endometriotic lesions were obtained at laparoscopic surgery. BDNF and TrKB messenger RNA (mRNA) and proteins levels in the eutopic and ectopic endometrium of both groups were measured by real-time PCR and immunohistochemical staining, respectively. Before the surgery the visual analogue scale (VAS) was used to measure dysmenorrhoea.

RESULTS

BDNF and TrKB expression levels were higher in ovarian endometriotic lesions than in eutopic endometrium and normal endometrium (P < 0.05), and there was no cyclical change. Furthermore, their expression levels were higher in eutopic endometrium than in normal endometrium (P < 0.05), and BDNF and TrKB levels were higher in stage IV ovarian endometriotic lesions than in stage II and III lesions (P < 0.05), with their expression being non-significantly higher in stage III than in stage II (P > 0.05). Additionally, correlation coefficients for the association analysis between the mRNA expression of BDNF or TrKB in eutopic endometrium and the dysmenorrhoea VAS score were r = 0.52 and r = 0.56 for BDNF and TrKB, respectively (P < 0.05). The correlation coefficients for the associations between BDNF and TrKB in both the eutopic and ectopic endometrium were r = 0.82 and r = 0.66, respectively (P < 0.05).

CONCLUSIONS

BDNF and TrKB are closely related to dysmenorrhoea caused by endometriosis and may be important in the pathobiology or pathophysiology of endometriosis.

Brain-derived neurotrophic factor (BDNF) expression and function in the mammalian reproductive Tract

BACKGROUND

Neurotrophins of the nerve growth factor family are soluble polypeptides that are best known for their role in nerve growth, survival and differentiation in the central nervous system. A growing body of literature shows that neurotrophins and their receptors are also expressed throughout the reproductive tract.

OBJECTIVE AND RATIONALE

Neurotrophins are key regulatory proteins in reproductive physiology during development and throughout adult life. Of the neurotrophins, the literature describing the expression and function of brain-derived neurotrophic factor (BDNF) and its high-affinity receptor, neurotrophin receptor kinase-2 (NTRK2), has been expanding rapidly. We therefore conducted a systematic inductive qualitative review of the literature to better define the role of the BDNF in the reproductive tract. We postulate that BDNF and NTRK2 are central regulatory proteins throughout the reproductive system.

SEARCH METHODS

An electronic search of Medline (PubMed) and Web of Science for articles relating to BDNF and the reproductive system was carried out between January 2018 and February 2019.

OUTCOMES

In the ovary, BDNF expression and levels have been linked with follicle organisation during ovarian development, follicle recruitment and growth and oocyte maturation. In the endometrium, BDNF is involved in cell proliferation and neurogenesis. In contrast, literature describing the role of BDNF in other reproductive tissues is sparse and BDNF-NTRK2 signalling in the male reproductive tract has been largely overlooked. Whilst estradiol appears to be the primary regulator of BDNF expression, we also identified reports describing binding sites for glucocorticoid and myocyte enhancer factor-2, a calcium-response element through activation of an N-methyl-D-aspartate (NMDA) receptor, and aryl hydrocarbon receptor nuclear transporter protein-4 (ARNT) response elements in promoter regions of the BDNF gene. Expression is also regulated by multiple microRNAs and post-translational processing of precursor proteins and intracellular shuttling. BDNF-NTRK2 signalling is modulated through tissue specific receptor expression of either the full-length or truncated NTRK2 receptor; however, the functional importance remains to be elucidated. Dysregulation of BDNF expression and circulating concentrations have been implicated in several reproductive disorders including premature ovarian failure, endometriosis, pre-eclampsia, intra-uterine growth restriction (IUGR) and several reproductive cancers.

WIDER IMPLICATIONS

We conclude that BDNF and its receptors are key regulatory proteins central to gonadal development, ovarian regulation and uterine physiology, as well as embryo and placenta development. Furthermore, dysregulation of BDNF-NTRK2 in reproductive diseases suggests their potential role as candidate clinical markers of disease and potential therapeutic targets.

Factors affecting stability of plasma brain-derived neurotrophic factor

Circulating concentrations of brain-derived neurotrophic factor (BDNF) have been linked to cancer, neuropsychiatric, diabetes, and gynecological disorders. However, factors influencing plasma storage and subsequent BDNF quantification are incompletely understood. Therefore, the anticoagulant used in plasma separator tubes, storage-time, storage-temperature, and repeated freeze–thaw cycles on circulating BDNF concentrations was evaluated. Peripheral blood samples were collected from healthy women (n = 14) and men (n = 10) recruited prospectively from McMaster University (August 2014). Blood was collected from the cubital vein into plasma separator tubes containing five different anticoagulant systems [K2EDTA, Li-Hep, Li-Hep (gel), Na-Hep, Na-Hep (glass)], and placed on ice for transport to the lab for centrifugation. Plasma samples (n = 16) collected in K2EDTA tubes from women recruited to a previous study (April 2011 to December 2012) were used to determine the effect of multiple freeze–thaw cycles. Plasma BDNF was quantified using a commercially available ELISA kit. Plasma concentrations of BDNF were significantly affected by the type of plasma separator tube, storage-time, and number of freeze–thaw cycles. Storage temperature (− 20 vs. − 80 °C) did not significantly affect the quantity of BDNF measured as mean BDNF concentrations generally fell within our calculated acceptable change limit up to 6 months in the freezer. Our results suggest that for quantification of circulating BDNF blood collected in K2EDTA tubes and plasma stored up to 6 months at either − 20 or − 80 °C produces reproducible results that fall within an acceptable range. However, plasma samples stored beyond 6 months and repeated freeze–thaw cycles should be avoided.

The Impact of Endometriosis across the Lifespan of Women: Foreseeable Research and Therapeutic Prospects

In addition to estrogen dependence, endometriosis is characterized by chronic pelvic inflammation. The impact of the chronic pelvic inflammatory state on other organ systems and women's health is unclear. Endometriosis associated chronic inflammation and potential adverse health effects across the lifespan render it imperative for renewed research vigor into the identification of novel biomarkers of disease and therapeutic options. Herein we propose a number of opportunities for research and development of new therapeutics to address the unmet needs in the treatment of endometriosis per se and its ancillary risks for other diseases in women across the lifespan.

Defining future directions for endometriosis research: workshop report from the 2011 World Congress of Endometriosis In Montpellier, France

Endometriosis, defined as estrogen-dependent lesions containing endometrial glands and stroma outside the uterus, is a chronic and often painful gynecological condition that affects 6% to 10% of reproductive age women. Endometriosis has estimated annual costs of US $12 419 per woman (approximately €9579), comprising one-third of the direct health care costs with two-thirds attributed to loss of productivity. Decreased quality of life is the most important predictor of direct health care and total costs. It has been estimated that there is a mean delay of 6.7 years between onset of symptoms and a surgical diagnosis of endometriosis, and each affected woman loses on average 10.8 hours of work weekly, mainly owing to reduced effectiveness while working. To encourage and facilitate research into this debilitating disease, a consensus workshop to define future directions for endometriosis research was held as part of the 11th World Congress on Endometriosis in September 2011 in Montpellier, France. The objective of this workshop was to review and update the endometriosis research priorities consensus statement developed following the 10th World Congress on Endometriosis in 2008.(1) A total of 56 recommendations for research have been developed, grouped under 6 subheadings: (1) diagnosis, (2) classification and prognosis, (3) clinical trials, treatment, and outcomes, (4) epidemiology, (5) pathophysiology, and (6) research policy. By producing this consensus international research priorities statement, it is the hope of the workshop participants that researchers will be encouraged to develop new interdisciplinary research proposals that will attract increased funding support for work on endometriosis.

Proteomic identification of neurotrophins in the eutopic endometrium of women with endometriosis

OBJECTIVE

To evaluate neurotrophin (NT) expression in the endometrium of women with and without endometriosis.

DESIGN

Prospective, cross-sectional, translational study.

SETTING

Academic hospital.

PATIENT(S)

Thirty-three reproductive-age women undergoing laparoscopy for infertility, pelvic pain, intramural fibroids, or tubal ligation.

INTERVENTION(S)

Endometrial biopsies, protein microarrays, reverse transcriptase-polymerase chain reaction, ELISAs, and Western blotting.

MAIN OUTCOME MEASURE(S)

Neurotrophin proteins and mRNAs in eutopic endometrial biopsies.

RESULT(S)

Among seven neurotrophic proteins detected on the antibody microarrays, reverse transcriptase-polymerase chain reaction analysis confirmed nerve growth factor, NT-4/5, and brain-derived neurotrophic factor mRNAs in endometrial tissue. Quantitative ELISAs revealed that NT-4/5 (806 ± 701 vs. 256 ± 190 pg/100 mg protein) and brain-derived neurotrophic factor (121 ± 97 vs. 14 ± 11 ng/100 mg protein) concentrations were significantly higher in women with endometriosis. Nerve growth factor (100 ± 74 vs. 93 ± 83 pg/100 mg protein) levels did not differ between cases and controls.

CONCLUSION(S)

Neurotrophins are synthesized in situ within the endometrium. NT-4/5 and brain-derived neurotrophic factor proteins were more concentrated in biopsies from endometriosis cases than controls, whereas nerve growth factor levels were similar. We hypothesize that the local production of NTs induces sensory innervation of endometrium of women with endometriosis. These NTs represent novel targets for the diagnosis and treatment of endometriosis.