Advances in Anatomic Pathology:
Value of PAX 8 Immunostaining in Tumor Diagnosis: A Review and Update
Ordóñez, Nelson G. MD
The University of Texas M.D. Anderson Cancer Center, Houston, TX
The author has no funding or conflicts of interest to disclose.
Reprints: Nelson G. Ordóñez, MD, The University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030 (e-mail: firstname.lastname@example.org).
PAX 8 is a transcription factor involved in the regulation of organogenesis of the thyroid gland, kidney, and Müllerian system. Recent studies have shown that, among tumors, PAX 8 is commonly expressed in epithelial tumors of the thyroid and parathyroid glands, kidney, thymus, and female genital tract. Although PAX 8 expression has also been reported in certain neuroendocrine tumors, including well-differentiated pancreatic neuroendocrine tumors, and duodenal and rectal carcinoids, as well as in B-cell lymphomas, it has recently been shown that the PAX 8 positivity reported in these tumors was due to a cross-reactivity of the antibody used with the N-terminal region of PAX 6 and PAX 5, respectively. Owing to its restricted expression, PAX 8 has proved to be a useful immunohistochemical marker with a wide range of diagnostic applications in surgical pathology, some of which are briefly reviewed.
PAX 8, also known as paired box protein 8, is one of the 9 members of the paired box (PAX) family of transcription factors that regulates organogenesis in a variety of organs during fetal development and plays an important role in maintaining the normal function of certain cells after birth.1–4 PAX proteins are defined by the presence of a highly conserved domain of 128 amino acids, the paired box, which recognizes a specific DNA-binding site, initiates transcription, and controls gene regulation.5,6 On the basis of their structural similarities, the PAX family has been divided into 4 subgroups. Subgroup II includes PAX 2, PAX 5, and PAX 8.4 During embryogenesis, PAX 8 plays an important role in the development of several organs, including the thyroid gland, kidney, certain areas of the central nervous system, inner ear, eye, and organs derived from the mesonephric (Wolffian) duct and those related to the Müllerian duct.7–9 The PAX 8 protein is comprises 450 amino acids, has a molecular weight of ∼48 kDa, and is encoded by a gene located on chromosome 2q13. The PAX 8 gene produces 6 isoforms, A to F, that are generated by alternative splicing and differ in their carboxic terminal regions. Mutation of the PAX 8 gene is associated with thyroid dysgenesis.8,10–12 Recently, it has been reported that PAX 8 could serve as a useful immunohistochemical marker for epithelial tumors of the thyroid, kidney, and thymus and some gynecologic (Müllerian) neoplasms. The purpose of this article is to present a review of the current literature on PAX 8 expression in tumors and on the application of PAX 8 immunostaining in diagnostic pathology.
EXPRESSION OF PAX 8 IN THYROID AND PARATHYROID TUMORS
During development, PAX 8 complexes with thyroid transcription factor-1 (TTF-1) and thyroid transcription factor-2 to induce thyroid hormone biosynthesis by regulating the expression of thyroglobulin, thyroperoxidase, sodium iodine symporter, and thyrotropin receptor.13 In normal mature thyroid, PAX 8 has been reported to be diffusely expressed in follicular cells and focally expressed in C cells and in thyroid C-cell hyperplasia.14 PAX 8 is frequently expressed in thyroid tumors, with the percentage of positivity ranging from 33% to 100% in follicular adenomas,14–19 38% to 100% in follicular carcinomas,14–19 31% to 100% in papillary carcinomas,14–19 75% to 100% in poorly differentiated (insular) carcinomas,14,18 and 0% to 80% in anaplastic carcinomas14,15,17,18,20,21 (Figs. 1A, B). This wide range of positivity reported in different series has largely been attributed to differences in the anti-PAX 8 antibodies used in the studies.14 When compared with other thyroid associated markers, PAX 8 sensitivity for follicular adenomas, follicular carcinomas, papillary carcinomas, and poorly differentiated (insular) carcinomas is similar to that of thyroglobulin and TTF-1. It has an advantage over both of the latter markers, however, in that PAX 8 is frequently expressed in anaplastic thyroid carcinomas, whereas these tumors rarely express thyroglobulin and TTF-1.14,22,23 In a recent immunohistochemical study on PAX 8 expression in anaplastic thyroid carcinomas, 26 (74%) of the 34 cases were found to be positive for this marker, including 16 (100%) of 16 squamous cell variants, 7 (57%) of 12 giant cell/pleomorphic variants, and 3 (50%) of 6 spindle cell variants.21 Another advantage that PAX 8 has over TTF-1 is that it is not expressed in lung adenocarcinomas and, because of this, immunostaining for PAX 8 can assist in distinguishing thyroid carcinomas from lung adenocarcinomas. Although more specific for thyroid tumors than PAX 8, thyroglobulin has the drawback of having the tendency to leak out of the cytoplasm of the follicular cells and diffuse into the adjacent tissues where it is incorporated into the cytoplasm of other cell types (eg, in metastatic carcinomas), which can cause a false-positive interpretation of an immunoreaction.24 As PAX 8 expression is nuclear, this does not occur with this marker.
PAX 8 has been reported to be positive in over 50% of the parathyroid adenomas investigated.18,19,25 In contrast, TTF-1 is not expressed in parathyroid tumors and, because of this, it is more useful in discriminating between thyroid and parathyroid adenomas, especially in small specimens where these tumors can sometimes be difficult to distinguish from each other. The immunohistochemical results reported in studies on PAX 8 expression in thyroid and parathyroid tumors are shown in Table 1.
EXPRESSION OF PAX 8 IN RENAL TUMORS
PAX 8 is a nephritic lineage transcription factor that, together with PAX 2 has an important function in renal organogenesis.4,26 Both transcription factors are expressed throughout the 3 stages of embryonic kidney development, the pronephrous, mesonephrous, and metanephrous stages, and they control the gene expression program responsible for mesenchymal-epithelial conversion involved in nephrogenesis. Mutation of the PAX 2 gene is associated with renal malformation, whereas PAX 8 null mice have no obvious kidney abnormalities, perhaps due to PAX 2 compensation.4,26 The PAX 2 and PAX 8 double mutants have a complete lack of kidney formation.4 In the normal mature kidney, PAX 8 is expressed in the nuclei of epithelial cells of the proximal and distal tubules, loops of Henle, collecting ducts, and parietal epithelial cells of the Bowman capsule.27 Although the number of published studies on PAX 8 expression in renal epithelial tumors is still limited, the results of this investigation indicate that PAX 8 is commonly expressed in all types of renal cell carcinomas, Wilms tumors, renal cell oncocytomas, and nephrogenic adenomas.25,28 The percentage of PAX 8 positivity reported in various types of primary renal cell carcinomas has ranged from 91% to 98% for clear cell,19,25,27 71% to 100% for papillary,19,25,27 57% to 88% for chromophobe,19,25,27 71% to 100% for collecting duct,19,25,29 and 0% to 24% for transitional cell carcinomas of the kidney.25,27,29 It has also been reported to be expressed in the vast majority (83% to 100%) of metastatic clear cell renal cell carcinomas investigated regardless of their degree of differentiation19,27,30–32 (Figs. 1C, D). One large study that was published comparing PAX 8 and PAX 2 expression in renal cell carcinomas32 found 152 (83%) of 184 and 91 (49%) of 185 metastatic clear cell renal cell carcinomas to be PAX 8 and PAX 2 positive, respectively. Although the authors of the study suggested that PAX 8 was a marker with a much higher sensitivity than PAX 2 for these tumors, this finding is rather unusual and difficult to explain given that PAX 8 is always coexpressed with PAX 2 in embryonal renal tissue and also that the percentage of PAX 2 expression reported in several large series has ranged from 84% to 93%33–39 for primary clear cell renal cell carcinomas and from 77% to 88% for metastatic clear cell renal cell carcinomas.31,39–41 More comparative studies are needed, however, to fully determine whether PAX 8 is a more sensitive marker than PAX 2 for the different types of renal cell carcinomas. In addition, when compared with other kidney-associated markers that are commonly used to assist in the differential diagnosis of metastatic clear cell renal cell carcinoma, PAX 8 has a sensitivity (83% to 100%)19,27,30–32 that is similar to that of CD10 (77% to 80%)32,42,43 and carbonic anhydrase IX (87% to 100%).44–46 The specificity of both of the latter markers, however, is lower than that of PAX 8 as they have been reported to be expressed in a wide variety of nonrenal tumors. When compared with the so-called renal cell carcinoma marker (RCC Ma), PAX 8 is more sensitive, as the percentage of positivity reported for RCC Ma in metastatic clear cell renal cell carcinomas has ranged from 18% to 55% of the cases.32,41,43,47–50 Similar to the previously mentioned markers, RCC Ma is not specific for renal cell carcinoma as it has been reported to be expressed in a wide variety of nonrenal tumors, including carcinomas of the breast,49,51,52 clear cell and serous carcinomas of the ovary,40,53,54 serous carcinomas of the endometrium,54 hepatocellular carcinomas,40 adrenal cortical carcinomas,49 lung carcinomas,40,49,54 prostatic adenocarcinomas,49 germ cell tumors,40,52,55 and mesotheliomas.48,56 The results of the published studies on PAX 8 expression in renal tumors are shown in Table 2.
EXPRESSION OF PAX 8 IN TUMORS OF THE FEMALE GENITAL TRACT
PAX 8 is thought to be a crucial transcription factor for organogenesis of the Müllerian system, the precursor of the female genital tract.8 Deletion of the PAX 8 gene in mice results in the lack of a functional uterus with absence of the endometrium and poor development of the myometrial tissue, as well as absence of the vaginal opening.8 The fallopian tubes, cervix, and upper parts of the vagina, however, are not affected, thus suggesting that other transcription factors, such as PAX 2, play a compensatory role in Müllerian organogenesis. Immunohistochemical studies have shown that, in the normal female genital tract, PAX 8 is strongly expressed in the epithelial cells of the endocervix and endometrium, as well as in the nonciliated epithelial cells of the fallopian tubes and in simple ovarian epithelial inclusion cysts.25,57,58 The demonstration of PAX 8 expression in the surface epithelial cells of the ovary has been less consistent, with some authors indicating that these cells do not express this marker,57 whereas others have reported weak focal positivity.58 Among ovarian tumors, PAX 8 is expressed in the vast majority of nonmucinous carcinomas, including serous carcinomas (79% to 100%), endometrioid carcinomas (38% to 92%), clear cell carcinomas (76% to 100%),14,18,19,57–62 and transitional cell carcinomas (67% to 100%;18,58 Fig. 1E). In a combined review of 8 published studies, 648 (91%) of 711 serous carcinomas, 149 (60%) of 248 endometrioid carcinomas, 131 (80%) of 163 clear cell carcinomas, and 15 (65%) of 23 transitional cell carcinomas investigated were reported to be PAX 8 positive.14,18,19,57–60,62 In general, the percentage of PAX 8 positivity in mucinous carcinomas is lower (0% to 50%) than in the nonmucinous group18,19,57–60,62–64 (Table 3). In a combined review of 201 cases from 9 published studies, 53 (26%) mucinous carcinomas were reported to express PAX 8 and the staining was often described as focal and weak.18,19,57–60,62–64 None of the 7 small cell carcinomas of the ovary that were investigated for PAX 8 expression were positive for this marker.18
Only a few studies have been published on PAX 8 expression in carcinomas of the endometrium and uterine cervix.18,58,62,64,66 The results of these investigations indicate that endometrioid, serous, and clear cell endometrial carcinomas are usually PAX 8 positive. The immunohistochemical results published on PAX 8 expression in other neoplasms of the endometrium, as well as those of the uterine cervix, are more variable and difficult to interpret due to the limited number of cases that have been investigated (Table 4).
To my knowledge, only 1 study has been published comparing PAX 8 and PAX 2 expression in primary and metastatic Müllerian epithelial tumors.58 Among the primary neoplasms, PAX 8 and PAX 2 expression was reported in 98% and 55% of serous tumors, 94% and 25% of endometrioid tumors, 100% and 19% of clear cell tumors, 67% and 11% of transitional/undifferentiated tumors, and 22% and 10% of mucinous tumors, respectively. PAX 8 and PAX 2 expression was demonstrated in 98% and 38% of the metastatic carcinomas, respectively. In all instances, the immunoreactivity for PAX 8 was described as being stronger than that for PAX 2, and no case that reacted only for PAX 2 was observed. On the basis of these results, the authors concluded that PAX 8 was a better marker for Müllerian tumors than PAX 2. More studies are needed, however, to confirm this observation and to determine whether or not these differences in reactivity may have been due to a lack of sensitivity of the antibody to PAX 2 that was used in the study.
EXPRESSION OF PAX 8 IN NEUROENDOCRINE TUMORS
Two members of the PAX family, PAX 4 and PAX 6, are known to be essential in the organogenesis and function of the pancreas.4 Experimental animal studies have shown that deletion of PAX 4 in mice results in the absence of both β and δ islet cells and an increase in α cells67; whereas, deletion of PAX 6 results in the absence of α cells with a concomitant decrease of other pancreatic endocrine cells.68 In 2010, Long et al,69 using a commercially obtained polyclonal antibody raised against the N-terminal region of PAX 8, reported PAX 8 expression in pancreatic islet cells and in 42 (67%) of 63 and 9 (50%) of 18 primary and metastatic well-differentiated endocrine tumors, respectively, as well as in the majority of duodenal (100%) and rectal (85%) carcinoids and in a minor subset of appendiceal (21%) and gastric (20%) carcinoids included in the study. Because all carcinoid tumors of the lung and ileum investigated were PAX 8 negative, the authors concluded that immunostaining for this marker could be useful for assisting in determining the primary site of a metastatic well-differentiated neuroendocrine tumor from an occult primary. Subsequent studies by other groups of investigators using the same PAX 8 polyclonal antibody confirmed this observation.25,70,71 More recently, however, using the PAX8R1 monoclonal antibody generated against the more variable C-terminal region of PAX 8, Lorenzo et al72 were unable to demonstrate PAX 8 positivity in either the developing pancreas or in mature islet cells, or in any of the pancreatic endocrine tumors included in their study. They also found that PAX 8 mRNA in mouse islets was undetectable while human islets exhibited low levels. In addition, they demonstrated that the PAX 8 polyclonal antibody used in most published studies also recognizes the islet-enriched PAX 6 protein, both by Western blotting and immunohistochemistry. On the basis of these observations, it was concluded that PAX 8 is not expressed in the pancreas and that the PAX 8 positivity reported in nonneoplastic islets and in pancreatic endocrine tumors was the result of a cross-reactivity of the anti-PAX 8 polyclonal antibody used in those studies with PAX 6, thus casting doubt on the value of PAX 8 as a pancreatic endocrine marker. In Table 5 are summarized the results of the published studies on PAX 8 expression in neuroendocrine tumors.
EXPRESSION OF PAX 8 IN MISCELLANEOUS TUMORS
Immunohistochemical studies have shown a differential expression pattern for PAX 8 in the urothelium of the upper and lower urinary tract. Although PAX 8 is expressed in the full thickness of the urothelium of the renal papillae, the number of positive cells decreases in the renal pelvis and ureter and are absent in the urinary bladder and urethra.27 In 2006, Pellizzari et al73 published the first immunohistochemical study on PAX 8 expression in urothelial carcinomas of the bladder. Using a noncommercial polyclonal antibody, they reported positivity for this marker in 28 (93%) of the 30 urothelial bladder carcinomas investigated. However, 4 subsequent studies, using a different anti-PAX 8 polyclonal antibody that was obtained from the same commercial source, were unable to demonstrate any immunoreactivity in any of the 106 bladder urothelial carcinomas included in those investigations.14,27,28,61 Some authors have attributed the differences in the results obtained in the different studies to the fact that the antibody used by Pellizzari and colleagues was raised against a short segment (amino acids 291 to 308) of the PAX 8 protein, whereas that used in the other investigations was raised to the N-terminal fragment of a recombinant PAX 8 from amino acids 1 to 212.28 More recently and using a different commercially obtained polyclonal antibody than was used in the previously mentioned investigations, Tacha et al19 reported PAX 8 positivity in 5 (10%) of the 50 urothelial carcinomas of the bladder. The results of all of these studies indicate that PAX 8 expression, if it occurs, seems to be uncommon in urothelial bladder carcinomas.
PAX 8 expression has been reported to occur in the epithelial cells in the thymus. The first study on the expression of this marker in epithelial tumors of the thymus was by Sangoi et al,46 who reported positivity in 2 of the 4 thymomas investigated. A subsequent study by Laury et al18 demonstrated PAX 8 expression in 8 (89%) of the 9 thymomas and 4 (80%) of the 5 thymic carcinomas. The largest study was by Weissferdt and Moran,74 who confirmed the observation that both thymomas and thymic carcinomas can express PAX 8. In that investigation, 58 (97%) of the 60 thymomas and 24 (77%) of the 31 thymic carcinomas were found to express this marker. This finding indicates that, because PAX 8 is frequently expressed in thymic epithelial tumors, immunostaining for this marker can be useful in assisting in the diagnosis of these tumors (Fig. 1F).
In addition to all previously mentioned tumors, PAX 8 expression has been reported in rare cases of pancreatic adenocarcinomas,18 solid pseudopapillary tumors of the pancreas,19 adenocarcinomas of the esophagus,18 and peritoneal mesotheliomas.75 All adenocarcinomas of the breast,18,19,60,61,64 prostate,14,18,19,25,28 stomach,14,18,19,64 colon,14,18,19,64 bladder,19,28,64 salivary gland,18 bile duct25 and ampulla,64 hepatocellular carcinomas,14,18,19,25 adrenal cortical tumors,18,19,32 acinar cell carcinomas of the pancreas,71 and all types of lung carcinomas,14,18,19,25,61 including adenocarcinomas, squamous cell carcinomas, and neuroendocrine carcinomas, that have been investigated have been consistently PAX 8 negative. Among the nonepithelial tumors, the information regarding PAX 8 expression is rather limited. Some studies have, on occasion, reported PAX 8 positivity in testicular germ cell tumors, including yolk sac tumors,46 seminomas,14 and mixed germ cell tumors,18 but the number of cases that have been investigated is too small to draw any conclusion regarding whether or not PAX 8 immunostaining has any value in the diagnosis of these tumors. All pheochromocytomas,19,25 olfactory neuroblastomas,18 and melanomas19,25 that have been investigated have been reported to be PAX 8 negative.
Some studies have also reported PAX 8 expression in lymphoid cells of B-cell lineage and B-cell lymphomas,25,46 and the PAX 8 positivity in lymphocytes has been used as an internal positive control for immunostaining for this marker.18 A recent study has shown, however, that the PAX 8 expression reported in normal and neoplastic B cells is the result of a cross-reactivity of the antibody used in those studies with PAX 5 and that PAX 8 is not expressed in normal B cells or in B-cell lymphomas.76
DIAGNOSTIC APPLICATIONS OF PAX 8 IMMUNOSTAINING
Thyroid carcinomas frequently metastasize to the lung and, as these tumors have morphologic features resembling those of lung carcinomas, the differential diagnosis can sometimes be very difficult, especially in those instances in which the thyroid carcinoma is either papillary, poorly differentiated, or anaplastic. Because carcinomas of the lung, regardless of their histologic subtype, do not express PAX 8, immunostaining for this marker has proven to be very useful in assisting in this differential diagnosis. PAX 8 immunostaining is also useful in assisting in establishing the origin of squamoid tumors of the neck. This differential diagnosis includes primary anaplastic thyroid carcinoma and advanced squamous cell carcinomas of the larynx or pharynx. In a recent study by Bishop et al,21 PAX 8 expression was demonstrated in all 16 squamoid anaplastic thyroid carcinomas investigated, whereas none of the 118 head and neck squamous cell carcinomas included in the study were positive for this marker. Similar results were obtained by Nonaka et al14 who reported PAX 8 positivity in 12 (92%) of the 13 anaplastic thyroid carcinomas showing squamoid features, but in none of the 29 primary squamous cell carcinomas of the lung investigated by these authors. The results of these studies indicate that when a poorly differentiated carcinoma with squamoid morphology is encountered in the thyroid, cervical lymph nodes, or lung, the finding of PAX 8 positivity is a strong indication for thyroid origin.21 Another possible application of PAX 8 immunostaining is in discriminating between anaplastic thyroid carcinoma exhibiting either spindle cell or pleomorphic/giant cell features and true soft tissue sarcomas of the neck. At present, the latter tumors have not been systematically investigated for PAX 8 expression, and only a few soft tissue sarcomas have been included in published studies.14,19 With the exception of rhabdomyosarcomas, which were reported to be positive in 40% of the cases in 1 study,19 other sarcomas, including leiomyosarcomas, have been negative.19,21
As renal cell carcinomas can present a broad range of morphologic appearances, they can potentially be confused with a wide variety of other neoplastic conditions. Tumors that can, on occasion, be difficult to distinguish from renal cell carcinomas because of their morphologic features include lung adenocarcinomas, adrenal cortical carcinomas, hepatocellular carcinomas, mesotheliomas, chordomas, and hemangioblastomas. In those instances in which the differential diagnosis of renal cell carcinoma is with any of the previously mentioned tumors, PAX 8 immunostaining can be very useful, especially when it is used in conjunction with other markers that are commonly expressed in the latter tumors, but not in renal cell carcinomas. The markers that can be used with PAX 8 to facilitate distinguishing between the above-mentioned neoplasms and renal cell carcinomas, include TTF-1 when the differential diagnosis is with lung adenocarcinoma, melan A with adrenal cortical carcinoma, α-fetoprotein and hepatocyte antigen (Hep Par 1) with hepatocellular carcinoma, calretinin and keratin 5/6 with mesothelioma, brachyury with chordoma, and inhibin A with hemangioblastoma.31,46,77
The ovary is a common site of involvement by metastatic breast carcinoma, which clinically and morphologically can mimic a primary ovarian tumor. Because PAX 8 is expressed in the vast majority of ovarian carcinomas, but it is invariably absent in breast carcinomas, immunostaining for this marker can greatly facilitate this differential diagnosis. Although WT1 is frequently used to discriminate between ovarian carcinomas and breast carcinomas, it is less specific than PAX 8 as it can, in some instances, be positive in breast carcinomas.60 PAX 8 has another advantage over WT1 in that it is frequently expressed in clear cell and endometrioid carcinomas, whereas WT1 is generally negative or only focally positive.60,78 Primary and metastatic serous carcinomas involving the peritoneum can be difficult to distinguish from peritoneal mesotheliomas. As serous carcinomas commonly express PAX 8, but peritoneal mesotheliomas are usually negative or rarely weakly positive for this marker, immunostaining for PAX 8 can assist in this differential diagnosis, especially when it is used in conjunction with other markers, such as estrogen receptor and CD15, which are frequently positive in serous carcinomas, but negative in peritoneal mesotheliomas.75,79,80
A well-known problem in the diagnosis of thymic carcinoma is the fact that a metastatic carcinoma to the mediastinum needs to be excluded before the diagnosis of thymic carcinoma can be made. Because these neoplasms frequently express PAX 8, immunostaining for this marker can be helpful in distinguishing between a thymic carcinoma and a tumor that does not express this marker. For example, when the differential diagnosis is between a thymic carcinoma and a lung carcinoma, PAX 8 can assist in discriminating between these malignancies; however, it has no use in distinguishing between a thymic carcinoma and a metastatic renal cell carcinoma as both of these malignancies can be PAX 8 positive.
Nephrogenic adenoma is an uncommon lesion that most frequently occurs in the lower urinary tract and that, because of its morphologic features, can potentially be confused with a prostatic adenocarcinoma. As it has been demonstrated that PAX 8 is consistently expressed in nephrogenic adenomas, but absent in prostatic adenocarcinomas, immunostaining for this marker can assist in the differential diagnosis in those instances in which the distinction between these 2 lesions is difficult.25,28
ANTI-PAX 8 ANTIBODIES
At present, a relatively large number of anti-PAX 8 antibodies that react with human PAX 8 that can be used on formalin-fixed, paraffin-embedded tissue specimens are commercially available. These include several rabbit and goat polyclonal antibodies, as well as the PAX8R1, MRQ-50, and BC12 mouse monoclonal antibodies. The vast majority of published studies have used polyclonal antibodies, more frequently a rabbit polyclonal antibody (catalog #10336-1-AP, Proteintech Group Inc., Chicago, IL) that was raised against a 212 amino acid peptide encompassing the N-terminal region of PAX 8. This region includes the highly conserved 128 amino acid-paired DNA-binding domain present in all members of the PAX family. Because of this, antibodies raised using this region as immunogen are most likely to cross-react with other PAX family members. Recent studies have reported that the frequently used anti-PAX 8 rabbit polyclonal antibody from Proteintech cross-reacts with PAX 5 and PAX 6.72,76 Among the previously mentioned monoclonal antibodies, the PAX8R1 clone is the one that has been more frequently used in the most recent studies.72,76 This antibody was generated using a recombinant peptide corresponding to amino acids 318 to 426 of the PAX 8 transcription factor. This region, which is located at the carboxy terminal end of the PAX 8 protein, is highly divergent among PAX members and, therefore, exhibits target specific-recognition.72
Because of the restricted expression in epithelial tumors of the thyroid, kidney, female genital tract, and thymus, PAX 8 has proved to be a useful immunohistochemical marker that can assist in the differential diagnosis of these tumors. Although some studies have also reported that PAX 8 is expressed in B-cell lymphomas and well-differentiated pancreatic endocrine tumors, recent investigations have shown that in neither B cells nor pancreatic endocrine cells or the tumors derived from these cells express PAX 8 and that the PAX 8 expression reported in these tumors is the result of cross-reactivity of the polyclonal antibody used with the N-terminal region of PAX 5 and PAX 6, respectively. Because of this, it is recommended that some of the information regarding PAX 8 expression reported in tumors be validated using monoclonal antibodies raised against the more divergent C-terminal region of the PAX 8 protein as they are less likely to cross-react with other PAX protein family members.
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