Mouse models of sporadic thyroid cancer derived from BRAFV600E alone or in combination with PTEN haploinsufficiency under physiologic TSH levels

A computational approach to assessing the prognostic implications of BRAF and RAS mutations in patients with papillary thyroid carcinoma

Papillary thyroid carcinoma (PTC) is the most common thyroid cancer, posing a growing clinical challenge. PTC exhibits two age-related peaks, with established risk factors including family history and radiation exposure. Managing even low-risk, localized PTC cases remain complex, with growing interest in active surveillance as an alternative to immediate surgery. This study employed single-cell RNA sequencing (scRNA-Seq) to explore the predictive value of BRAF and RAS mutations in PTC, shedding light on their impact on disease progression and outcomes. The analyses emphasized the significance of BRAF and RAS mutations in tumor advancement, particularly the unique BRAF V600E mutation associated with aggressive features. The methodology involved scRNA-Seq analysis of PTC and normal samples, unveiling distinct cell clusters and indicating upregulated BRAF and RAS genes. Pathway enrichment analysis highlighted altered biological processes and immune-related pathways in PTC. The study consolidated previous research showing the prevalence of BRAF and RAS mutations in PTC, subtypes with distinct molecular profiles, and the impact of TERT promoter mutations on disease severity. In summary, this study unveils the complex interplay of genetic mutations and the cellular microenvironment in PTC through scRNA-Seq. The upregulated BRAF and RAS genes suggest their roles as PTC drivers, and pathway enrichment reveals alterations in immune-related processes. This synthesis of prior research enhances our understanding of PTC's molecular foundations, informing better prognosis and personalized treatment approaches. These insights advance the landscape of PTC management and provide directions for further research.

Genomic alterations in thyroid cancer: biological and clinical insights

Tumours can arise from thyroid follicular cells if they acquire driver mutations that constitutively activate the MAPK signalling pathway. In addition, a limited set of additional mutations in key genes drive tumour progression towards more aggressive and less differentiated disease. Unprecedented insights into thyroid tumour biology have come from the breadth of thyroid tumour sequencing data from patients and the wide range of mutation-specific mechanisms identified in experimental models, in combination with the genomic simplicity of thyroid cancers. This knowledge is gradually being translated into refined strategies to stratify, manage and treat patients with thyroid cancer. This Review summarizes the biological underpinnings of the genetic alterations involved in thyroid cancer initiation and progression. We also provide a rationale for and discuss specific examples of how to implement genomic information to inform both recommended and investigational approaches to improve thyroid cancer prognosis, redifferentiation strategies and targeted therapies.

Borealin/CDCA8 deficiency alters thyroid development and results in papillary tumor-like structures

Background

BOREALIN/CDCA8 mutations are associated with congenital hypothyroidism and thyroid dysgenesis. Borealin is involved in mitosis as part of the Chromosomal Passenger Complex. Although BOREALIN mutations decrease thyrocyte adhesion and migration, little is known about the specific role of Borealin in the thyroid.

Methods

We characterized thyroid development and function in Borealin-deficient (Borealin +/-) mice using histology, transcriptomic analysis, and quantitative PCR.

Results

Thyroid development was impaired with a hyperplastic anlage on embryonic day E9.5 followed by thyroid hypoplasia from E11.5 onward. Adult Borealin +/- mice exhibited euthyroid goiter and defect in thyroid hormone synthesis. Borealin +/- aged mice had disorganized follicles and papillary-like structures in thyroids due to ERK pathway activation and a strong increase of Braf-like genes described by The Cancer Genome Atlas (TCGA) network of papillary thyroid carcinoma. Moreover, Borealin +/- thyroids exhibited structural and transcriptomic similarities with papillary thyroid carcinoma tissue from a human patient harboring a BOREALIN mutation, suggesting a role in thyroid tumor susceptibility.

Conclusion

These findings demonstrate Borealin involvement in critical steps of thyroid structural development and function throughout life. They support a role for Borealin in thyroid dysgenesis with congenital hypothyroidism. Close monitoring for thyroid cancer seems warranted in patients carrying BOREALIN mutations.

Mechanistic Insights of Thyroid Cancer Progression

Differentiated thyroid cancers (DTCs) are primarily initiated by mutations that activate the MAPK signaling cascade, typically at BRAF or RAS oncoproteins. DTCs can evolve to more aggressive forms, specifically, poorly differentiated (PDTC) and anaplastic thyroid cancers (ATC), by acquiring additional genetic alterations which deregulate key pathways. In this review, we focused on bona fide mutations involved in thyroid cancer progression for which consistent mechanistic data exist. Here we summarized the relevant literature, spanning approximately 2 decades, highlighting genetic alterations that are unquestionably enriched in PDTC/ATC. We describe the relevant functional data obtained in multiple in vitro and in vivo thyroid cancer models employed to study genetic alterations in the following genes and functional groups: TP53, effectors of the PI3K/AKT pathway, TERT promoter, members of the SWI/SNF chromatin remodeling complex, NF2, and EIF1AX. In addition, we briefly discuss other genetic alterations that are selected in aggressive thyroid tumors but for which mechanistic data is still either limited or nonexistent. Overall, we argue for the importance conveyed by preclinical studies for the clinical translation of genomic knowledge of thyroid cancers.

MIEAP and ATG5 are tumor suppressors in a mouse model of BRAFV600E-positive thyroid cancer

Mitochondria-eating protein (MIEAP) is a molecule important for non-canonical mitophagy and thought to be a tumor suppressor. Our previous study found that MIEAP expression is defective in thyroid oncocytomas, irrespective of being benign or malignant, and also in non-oncocytic thyroid cancers. Thyroid oncocytomas are composed of large polygonal cells with eosinophilic cytoplasm that is rich in abnormal mitochondria. Thus, our data indicate that, together with increased mitochondrial biogenesis that compensates for the dysfunction of the mitochondria, MIEAP plays a critical role in the accumulation of mitochondria in thyroid oncocytic tumors, whereas a defective MIEAP expression alone is not sufficient for mitochondrial accumulation in non-oncocytic cancers with normal mitochondria. To clarify whether MIEAP is a tumor suppressor in the thyroids and whether MIEAP knockout (KO) alone is sufficient for the oncocytic phenotype and also to extend our effort toward canonical mitophagy (a selective autophagy), we here conducted mouse studies using genetically engineered mice. BrafCA/wt mice developed thyroid cancers 1 year after intrathyroidal injection of adenovirus expressing Cre, while cancer development was observed at 6 months in adenovirus-Cre-injected BrafCA/wt;MieapKO/KO and BrafCA/wt;Atg5flox/flox mice [where autophagy-related 5 (ATG5) is a component of autophagic machinery], although KO of either molecule alone was not sufficient for cancer development. These data demonstrate that MIEAP or ATG5 KO accelerated thyroid cancer development. However, cancers in adenovirus-Cre-injected BrafCA/wt ;MieapKO/KO and BrafCA/wt ;Atg5flox/flox mice were not oncocytic. In conclusion, we here show that MIEAP and ATG5 are both tumor suppressors in thyroid carcinogenesis, but as we have anticipated from our previous data, KO of either molecule does not confer the oncocytic phenotype to BRAFV600E-positive thyroid cancers. The combination of disruptive mitochondrial function and impaired mitochondrial quality control may be necessary to establish a mouse model of thyroid oncocytoma.

Tissue architecture delineates field cancerization in BRAFV600E-induced tumor development

Cancer cells hijack developmental growth mechanisms but whether tissue morphogenesis and architecture modify tumorigenesis is unknown. Here, we characterized a new mouse model of sporadic thyroid carcinogenesis based on inducible expression of BRAF carrying a Val600 Glu (V600E) point mutation (BRAFV600E) from the thyroglobulin promoter (TgCreERT2). Spontaneous activation of this Braf-mutant allele due to leaky activity of the Cre recombinase revealed that intrinsic properties of thyroid follicles determined BRAF-mutant cell fate. Papillary thyroid carcinomas developed multicentrically within a normal microenvironment. Each tumor originated from a single follicle that provided a confined space for growth of a distinct tumor phenotype. Lineage tracing revealed oligoclonal tumor development in infancy and early selection of BRAFV600E kinase inhibitor-resistant clones. Somatic mutations were few, non-recurrent and limited to advanced tumors. Female mice developed larger tumors than males, reproducing the gender difference of human thyroid cancer. These data indicate that BRAFV600E-induced tumorigenesis is spatiotemporally regulated depending on the maturity and heterogeneity of follicles. Moreover, thyroid tissue organization seems to determine whether a BRAF-mutant lineage becomes a cancerized lineage. The TgCreERT2;BrafCA/+ sporadic thyroid cancer mouse model provides a new tool to evaluate drug therapy at different stages of tumor evolution.

Sex Bias in Differentiated Thyroid Cancer

Differentiated thyroid cancers are more frequent in women than in men. These different frequencies may depend on differences in patient's behavior and in thyroid investigations. However, an impact on sexual hormones is likely, although this has been insufficiently elucidated. Estrogens may increase the production of mutagenic molecules in the thyroid cell and favor the proliferation and invasion of tumoral cells by regulating both the thyrocyte enzymatic machinery and the inflammatory process associated with tumor growth. On the other hand, the worse prognosis of thyroid cancer associated with the male gender is poorly explained.

PTEN Hamartoma Tumor Syndrome: Skin Manifestations and Insights Into Their Molecular Pathogenesis

Germline PTEN pathogenic variants cause a spectrum of disorders collectively labeled PTEN Hamartoma Tumor Syndrome (PHTS) and featured by hamartomas, developmental anomalies and increased cancer risk. Studies on experimental models provided evidence that PTEN is a “haploinsufficient” tumor-suppressor gene, however, mechanisms involved in the pathogenesis of clinical manifestations in PHTS patients remain elusive. Beyond analyzing clinical and molecular features of a series of 20 Italian PHTS patients, we performed molecular investigations to explore the mechanisms involved in the pathogenesis of PTEN-associated manifestations, with special focus on mucocutaneous manifestations. Typical mucocutaneous features were present in all patients assessed, confirming that these are the most important clue to the diagnosis. The most frequent were papules located in the trunk or extremities (73.7%), oral mucosa papules (68.4%), acral/palmoplantar keratosis and facial papules (both 57.9%), according with literature data. Molecular analyses on one trichilemmoma suggested that the wild-type PTEN allele was retained and expressed, reinforcing the evidence that PTEN does not require a second somatic hit to initiate pathogenic processes. Unexpectedly, one patient also displayed a cutaneous phenotype consistent with atypical mole/melanoma syndrome; no variants were detected in known melanoma genes, but Whole Exome Sequencing showed the rare truncating variant c.495G>A in the CDH13 gene that might have cooperated with PTEN-haploinsufficiency to generate such phenotype. Our findings confirm the reproducibility of known PHTS manifestations in real-world practice, highlighting the role of mucocutaneous manifestations in facilitating prompt diagnosis of the syndrome, and provide some insights into the pathogenic process induced by PTEN alterations, which may contribute to its understanding.

The Genomic Landscape of Thyroid Cancer Tumourigenesis and Implications for Immunotherapy

Thyroid cancer is the most prevalent endocrine malignancy that comprises mostly indolent differentiated cancers (DTCs) and less frequently aggressive poorly differentiated (PDTC) or anaplastic cancers (ATCs) with high mortality. Utilisation of next-generation sequencing (NGS) and advanced sequencing data analysis can aid in understanding the multi-step progression model in the development of thyroid cancers and their metastatic potential at a molecular level, promoting a targeted approach to further research and development of targeted treatment options including immunotherapy, especially for the aggressive variants. Tumour initiation and progression in thyroid cancer occurs through constitutional activation of the mitogen-activated protein kinase (MAPK) pathway through mutations in BRAF, RAS, mutations in the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) pathway and/or receptor tyrosine kinase fusions/translocations, and other genetic aberrations acquired in a stepwise manner. This review provides a summary of the recent genetic aberrations implicated in the development and progression of thyroid cancer and implications for immunotherapy.

Acceleration of BRAFV600E-induced thyroid carcinogenesis by TGFβ signal deficiency in mice

PURPOSE

Transforming growth factor-β (TGFβ) has pleiotropic actions, including both anti- and pro-tumorigenic abilities. We have previously shown no tumor development in the thyroid-specific TGFβ receptor type II knockout (Tgfβr2 KO) mice, indicating the insufficiency of defective TGFβ signal itself for thyroid cancer initiation. In the current study, we evaluated whether defective TGFβ signal accelerates BRAF^V600E-mediated thyroid carcinogenesis in our mouse model, in which intrathyroidal injection of adenovirus expressing Cre under thyroglobulin (TG) promoter (Ad-TgP-Cre) into thyroid lobes of conditional Braf^V600E knock-in mice (Braf^CA) induces thyroid cancers 12 months later.

METHODS

Braf^CA/wt;Tgfbr2^{floxE2/floxE2} mice were generated by crossing Tgfbr2^{floxE2/floxE2} and Braf^CA mice, and Ad-TgP-Cre was injected into the left lobes of 4-6-week-old mice. Mice were sacrificed at 6 and 12 months, and the thyroid tissues were subjected to H&E and immune-histochemistry and -fluorecence.

RESULTS

Thyroid tumors were observed in 8 of 10 mice at 6 months and 4 of 7 mice at 12 months. These tumors were judged to be malignant by H&E staining, because of the presence of papillary growth of atypical follicular cells, intranuclear cytoplasmic inclusions and so on. Immunohistochemical analyses using thyroid cancer tissues obtained at 6 months demonstrated variable levels of TG but steady levels of Paired Box-8 expression and higher Ki67 positivity. The degree of epithelial-to-mesenchymal transition could not be evaluated because normal thyroid tissues and thyroid cancers developed in Braf^CA and Braf^CA/wt;Tgfbr2^{floxE2/floxE2} mice were all E-cadherin⁺/vimentin^-, that is, epithelial type.

CONCLUSION

In a mouse model, defective TGFβ signaling pathway accelerates BRAF^V600E-induced thyroid cancer development, which is occasionally accompanied by reduced TG expression implying dedifferentiation. The former finding is consistent with anti-tumorigenic ability of TGFβ in early tumorigenic process, but the latter is contradictory to generally accepted concept for TGFβ-induction of dedifferentiation.

Mouse Models as a Tool for Understanding Progression in BrafV600E-Driven Thyroid Cancers

The development of next generation sequencing (NGS) has led to marked advancement of our understanding of genetic events mediating the initiation and progression of thyroid cancers. The NGS studies have confirmed the previously reported high frequency of mutually-exclusive oncogenic alterations affecting BRAF and RAS proto-oncogenes in all stages of thyroid cancer. Initially identified by traditional sequencing approaches, the NGS studies also confirmed the acquisition of alterations that inactivate tumor protein p53 (TP53) and activate phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) in advanced thyroid cancers. Novel alterations, such as those in telomerase reverse transcriptase (TERT) promoter and mating-type switching/sucrose non-fermenting (SWI/SNF) complex, are also likely to promote progression of the BRAF^V600E-driven thyroid cancers. A number of genetically engineered mouse models (GEMM) of BRAF^V600E-driven thyroid cancer have been developed to investigate thyroid tumorigenesis mediated by oncogenic BRAF and to explore the role of genetic alterations identified in the genomic analyses of advanced thyroid cancer to promote tumor progression. This review will discuss the various GEMMs that have been developed to investigate oncogenic BRAF^V600E-driven thyroid cancers.