Relationship between thyroid carcinoma and PAX8-PPARγ gene rearrangement

In recent years, the incidence of thyroid cancer has been increasing rapidly in many countries and regions including China. In 2022, there were as many as 820,000 new cases worldwide, and the ratio of female to male patients was about 3:1[1].

Thyroid cancer is the most common malignant endocrine tumor. According to the difference in tumor origin and differentiation, it can be divided into differentiated thyroid cancer (DTC), medullary thyroid cancer (MTC), poorly differentiated thyroid cancer (PDTC) and undifferentiated thyroid cancer (ATC). DTC can be further divided into papillary thyroid cancer (PTC) and follicular thyroid cancer (FTC) (Figure 2)[2]. FTC is second only to PTC and accounts for about 10%–15% of DTC [2]. FTC differs from follicular cell-derived tumors (follicular adenoma (FA), follicular subtype papillary thyroid carcinoma (FVPTC)) in morphology, structure, and biological behavior. With the continuous research on the molecular mechanism of thyroid cancer, molecular detection has provided new ideas for auxiliary diagnosis, such as common RAS gene mutation of FTC (see "those things between thyroid cancer and RAS gene"). This phase will share with you another common mutant gene in FTC, the ——PAX8-PPARγ rearrangement.

PAX8-PPARγ rearrangement, as the name implies, is the combination of Pax8 gene and PPARγ gene into a new gene, which makes the original two genes abnormal function. Next, we will look at the PAX8 gene and the PPARγ gene separately.

The PAX8 gene, located on chromosomes 2q12~q14, is one of the important members of the PAX family. The PAX8 gene consists of 12 exons, of which exons 3, 4 and part of exon 5 jointly code for the DNA binding domain of PAX8 gene. Alternative splicing of exons 8–10 may result in the production of five different subtypes (designated as Pax-8a–Pax-8e), respectively (Figure 3)[3]. Studies have shown that PAX8 gene is an important transcription factor in the development of the thyroid gland, which drives the specific expression of genes such as thyroglobulin (Tg), thyroid peroxidase (TPO), and sodium iodide symporter (NIS/SLC5A5), thereby jointly regulating the differentiation and growth of thyroid cells (Figure 4)[4].

Peroxisome proliferator-activated receptor γ (PPARγ) gene, located on chromosome 3p25, is one of the members of the nuclear receptor family [5]. The PPARγ gene consists of nine exons, of which exon 1 and part of exon 2 jointly code for the amino-terminal regulatory AB domain. Exons 3 and part of exon 2 jointly code for a DNA binding domain; Exons 5, 6, and part of exon 4 jointly encode a ligand-binding domain (Figure 5)[6]. Since the PPARγ gene has two promoters, it can express two different protein subtypes, PPARγ1 and PPARγ2, both of which have the function of regulating adipocyte differentiation, fat and carbohydrate metabolism, cell proliferation and differentiation [5]. However, their expression patterns are not the same. PPARγ1 is widely expressed, while PPARγ2 is specifically expressed in adipose cells [6].

Therefore, PAX8-PPARγ rearrangement was generated by the translocation between PAX8 gene coding region 2q13 and PPARγ gene coding region 3q25, thereby expressing a fusion protein, PPFP, including the first nine exons of PAX8 and full-length PPARγ (Fig. 6)[6]. The fusion protein comprise a partial DNA binding domain and a partial activation domain of PAX-8A, a DNA bin domain and a ligand binding domain of PPARγ gamma, and has that effects of promoting cell proliferation and inhibit normal differentiation of cells, and inhibiting the expression of thyroid specific genes, thereby leading to the occurrence and development of FTC [7]. Studies have shown that the frequency of PAX8-PPARγ gene rearrangement in FTC is 30–35%, and most of them occur in younger patients [8]. Although PAX8-PPARγ gene rearrangement may also exist in follicular adenoma (FA) and follicular subtype papillary thyroid carcinoma (FVPTC), it occurs with extremely low frequency, FA less than 10%, and FVPTC less than 5%[8]. Most FTC have a lower malignancy and slow growth and distant metastasis, but FTC with PAX8-PPARγ gene rearrangement shows higher invasiveness and poor prognosis. The research by Nikiforova et al. supports this phenomenon. The research has found that the positive expression rate of Galectin-3 in FTC with PAX8-PPARγ gene rearrangement is 75%, while the negative expression rate of mesothelial antigen HBME-1 is 59%, with significant invasion. Therefore, the malignancy degree of PPFP-positive FTC is higher than that of PPFP-negative FTC [9].

01. The diagnosis of thyroid nodules benign and malignant

Thyroid nodules are common in the population. At present, the preferred method to identify the benign and malignant nodules is ultrasound-guided fine needle aspiration (US-FNA). However, FTC cannot be identified by US-FNA cytological analysis, because the diagnosis of FTC requires evidence of invasion [6]. This is also one of the main causes leading to the indeterminate cytological results of US-FNA in thyroid gland of about 30%, which brings certain difficulties and challenges to the judgment of benign and malignant nodules. With the advancement of technology, molecular detection has provided a new idea for the diagnosis of benign and malignant nodules. Evaluating the benign and malignant nodules from the gene level can avoid diagnostic surgery to exclude malignant tumors and avoid complications caused by excessive treatment. Previous studies have shown that PAX8-PPARγ gene rearrangement is closely related to the occurrence and development of FTC, which is second only to RAS gene mutation in frequency, and it occurs at a very low frequency in FA and FVPTC [8]. Similar to the RAS gene mutation in FTC, although PAX8-PPARγ gene rearrangement cannot 100% predict cancer, it certainly highly suggests FTC as an adjuvant for nodular type diagnosis of follicular lesions/suspicious follicular tumors.

02. Therapeutic targets

In view of more opinions that PPFP firstly acts on the transcription pathway of PPARγ, wild strains PPARγ may be a tumor suppressor gene. PPPFP leads to excessive proliferation of epithelial cells by down-regulating the expression of PPARγ, suggesting that intervention of PPARγ transcription pathway by highly selective targeted therapy may become a pathway for gene therapy of PPFP-positive FTC [5]. Copland et al. applied PPARγ agonist (RS5444) to interfere with PPARγ' s transcription pathway to achieve cancer inhibition, and targeted therapy of FTC with PPARγ agonist molecules is hopeful [10].

03. Summary

The PAX8-PPARγ gene rearrangement is mainly found in FTC, and more and more evidences show that the PAX8-PPARγ gene rearrangement detection has significant value in the diagnosis of FTC. The expert consensus also points out that for patients with thyroid nodules of Bethesda type III and type IV, genetic tests (such as BRAF mutation, RAS mutation, and RET/PTC rearrangement) are recommended to assist in the diagnosis of benign and malignant thyroid nodules.

04. Spacegen Productions of Thyroid Cancer Gene Detection

References

[1] CA Cancer J Clin. 2024 May-Jun; 74(3):229-263.

[2] Thyroid cancer diagnosis and treatment guidelines (2022)

[3] Foreign Medicine (Pediatric fascicle), 2000,(03):118-120.

[4] Endocrinology, 2006, 147(1):367- 376.

[5] Journal of Oncology, 2008,(09):763-766.

[6] Nature reviews.Endocrinology, 2014, 10(10):616-623.

[7] Science (New York, NY), 2000, 289(5483):1357-1360.

[8] Critical Reviews in OncologyHematology,2014,90(3):233-252.

[9] J Clin Endocrinol Metab,2003, 88(5):2318-2326.

[10] Oncogene, 2006,25(16):2304- 2317.

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