PAX8

PAX8 (Paired Box Gene 8) encodes a transcription factor belonging to the PAX family, essential for organogenesis of the thyroid gland, kidneys, and Müllerian duct-derived structures (uterus, fallopian tubes, ovaries, upper vagina) during embryogenesis. In the thyroid, PAX8 regulates expression of thyroglobulin, thyroid peroxidase, and the sodium-iodide symporter, making it essential for thyroid hormone synthesis.

Clinically, PAX8 is widely used as an immunohistochemical (IHC) marker in tumor diagnosis because it is expressed in a restricted set of tissues and their neoplasms. It is positive in: thyroid carcinomas (papillary, follicular, medullary - though medullary is variably positive), renal cell carcinomas (clear cell, papillary, chromophobe subtypes), and gynecologic tumors (ovarian epithelial carcinomas - especially serous and endometrioid types, endometrial carcinomas, and cervical adenocarcinomas). It is notably negative in most other carcinomas, including those of lung, breast, colon, and prostate, which helps distinguish primary tumors from metastases in cases of unknown primary.

PAX8 is particularly valuable in distinguishing: (1) renal cell carcinoma metastases from other clear cell tumors, (2) ovarian/Müllerian origin tumors from gastrointestinal or other mimickers, and (3) thyroid-origin tumors in the setting of metastatic disease of unknown primary. A key clinical pearl: PAX8 positivity in a tumor of unknown primary should prompt consideration of thyroid, renal, or gynecologic origin. Germline PAX8 mutations are associated with congenital hypothyroidism due to thyroid dysgenesis.