P-338 TSPO is a novel molecular target for noninvasive detection of endometriosis
Authors: Casalechi M , Assis W. A , Cruz C. Dela , Francisco A. M. C , Reis F. M 
Published: June 2022

Study question

Is TSPO expressed in endometriotic lesions and negative in adjacent tissues, being eligible as a molecular target for noninvasive detection of endometriosis?

Summary answer

TSPO is detected in endometriotic lesions and its presence is mostly negative in adjacent tissue. TSPO-based molecular imaging is potentially feasible for noninvasive endometriosis detection.

What is known already

TSPO is a 18KDa protein located in the outer mitochondrial membrane, mostly in neurons, but also in inflammatory sites outside the brain. There are specific TSPO ligands - such as PBRR111 and PK11195 - that can be conjugated to radioisotopes to allow non-invasive imaging of tissues rich in TSPO, using positron emission tomography (PET). Studies have shown that TSPO is present in the rat uterus, therefore we hypothesized that it might also be expressed in the human endometrial-like tissue, being a target for molecular imaging of endometriosis.

Study design, size, duration

Prospective cross-sectional study including women with endometriosis (n = 24) and endometriosis-free controls (n = 12). All participants were enrolled in this study from March 2017 to September 2018 and signed an informed consent approved by the IRB.

Participants/materials, setting, methods

Endometriosis lesions (n = 83) and endometrium from endometriosis patients (n = 28) were obtained during laparoscopy, while endometrial samples from control women (n = 12) were collected by Pipelle® at hysteroscopy examination. TSPO mRNA expression was evaluated by quantitative reverse-transcription real time PCR (RT-PCR) in samples of endometrium and lesions, while TSPO protein expression was evaluated by immunohistochemistry (IHC) with a monoclonal antibody anti-human TSPO.

Main results and the role of chance

TSPO mRNA was detected in endometrium from control women and from endometriosis patients, as well as in endometriotic lesions. The RT-PCR results were homogeneous for amplification of the S26 reference gene. The duplicates of the samples were concordant and the melting curves (Tm) showed a single peak for TSPO and S26 genes, demonstrating the reliability of the results. TSPO protein was localized in endometrium and in several types of endometriotic lesions. The immunostaining index (on a 0 to 6 scale) was higher in the glandular epithelium (median 4.5, interquartile range 4 to 5) than in the stroma (median 2, interquartile range 2 to 3, p &lt; 0.001). Endometriotic glands had TSPO immunostaining index considerably higher than their adjacent tissue (median 0, interquartile range 0 to 2, p &lt; 0.001). Positive controls had the expected staining pattern and negative controls had no staining, demonstrating the reliability of the results.

Limitations, reasons for caution

Endometriosis samples were obtained from patients undergoing laparoscopy surgery, therefore women with early or asymptomatic disease may not be represented in the study sample. TSPO expression may vary between endometriosis phenotypes, which will require a larger sample to be stratified according to the type of lesion.

Wider implications of the findings

The limitations of current imaging methods to detect small or superficial endometriotic lesions prompts the search for new molecular targets, such as TSPO. The presence of TSPO in endometriotic lesions (but not in adjacent areas) opens the perspective of using this molecular target to improve the noninvasive detection of endometriosis.

Trial registration number

not applicable