RAC1 Alterations Induce Acquired Dabrafenib Resistance in Association with Anaplastic Transformation in a Papillary Thyroid Cancer Patient

Abstract

BRAF-activating mutations are the most frequent driver mutations in papillary thyroid cancer (PTC). Targeted inhibitors such as dabrafenib have been used in advanced BRAF-mutated PTC; however, acquired resistance to the drug is common and little is known about other effectors that may play integral roles in this resistance. In addition, the induction of PTC dedifferentiation into highly aggressive KRAS-driven anaplastic thyroid cancer (ATC) has been reported. We detected a novel RAC1 (P34R) mutation acquired during dabrafenib treatment in a progressive metastatic lesion with ATC phenotype. To identify a potential functional link between this novel mutation and tumor dedifferentiation, we developed a cell line derived from the metastatic lesion and compared its behavior to isogenic cell lines and primary tumor samples. Our data demonstrated that RAC1 mutations induce changes in cell morphology, reorganization of F-actin almost exclusively at the cell cortex, and changes in cell adhesion properties. We also established that RAC1 amplification, with or without mutation, is sufficient to drive cell proliferation and resistance to BRAF inhibition. Further, we identified polyploidy of chromosome 7, which harbors RAC1, in both the metastatic lesion and its derived cell line. Copy number amplification and overexpression of other genes located on this chromosome, such as TWIST1, EGFR, and MET were also detected, which might also lead to dabrafenib resistance. Our study suggests that polyploidy leading to increased expression of specific genes, particularly those located on chromosome 7, should be considered when analyzing aggressive thyroid tumor samples and in further treatments.

Target therapy forBRAFmutated anaplastic thyroid cancer: a clinical and molecular study

“…EGFR and RAC1 upregulation may represent off-target resistance mechanisms in melanoma and thyroid cancer under BRAF inhibitors. 23,34 Epigenetic mechanisms, that modulate DNA methylation, histone, and chromatin structure, may also have a role in DT resistance, because they can promote differential expression of genes, which could be associated with a decreased efficacy of BRAF inhibitors, as observed in melanoma and NSCLC. 31 This study has several strengths that support its results: the review by the same expert pathologist, the closely (monthly) radiological follow-up by the same expert radiologist, the long-term follow-up of ATC to assess OS and PFS, the PET-CT validation of CR, and two contemporary ATC groups that reflect the local practice.…”

“…In patient 3, a pathogenic NRAS p.Q61K mutation, which probably represents an acquired off-target resistance mechanism to DT, was detected. 25,34 In patient 4, no new alterations were observed in the surgical specimen compared with baseline, by NGS, or by Sanger sequencing analysis of RAC1 and PTEN genes. No gene fusions in BRAF, ALK, RET, and NTRK were detected.…”

“…23,31 Nevertheless, only approximately half of melanoma and NSCLC patients, studied by whole-genome sequencing, RNA sequencing and comparative genomic hybridization (CGH) arrays, had alterations detected at progression. 26,32 Recently, BRAF off-target resistance mechanisms in ATCs, such as RAS mutations, 21,33 RAC1 mutations, and copy number variations, 34 have been described. However, due to the recent introduction of DT in ATC, these resistance mechanisms are less well characterized.…”

Anti-EGFR/BRAF-Tyrosine Kinase Inhibitors in Thyroid Carcinoma

“…Referring to dabrafenib resistance levels, another study focused on an unexplored novel RAC1 (P34R) mutation acquired in BRAF. They reported an excessive EGFR /MET protein over-expression due to gene amplification in PTC cases that led to dedifferentiation enhancing their aggressive genotype/phenotype by increasing their metastatic potential (37). In contrast to successive responses in TKI-based therapeutic approaches, there are sub-groups of patients that lose clinical benefits after discontinuation of lenvatinib-based treatment (38).…”

Advances in Thyroid Carcinoma

“…Bagheri-Yarmand, Busaidy and colleagues characterized a metastatic PTC case that developed an acquired resistance to the BRAF inhibitor dabrafenib, demonstrating that this resistance is mediated, at least in part, by acquired alterations at RAC1 gene (encoding a GTPase), and perhaps the co-amplification of other genes within chromosome 7. The authors showed, by employing patient-derived cell lines and isogenic cell models, that RAC1 alterations enhance growth, invasion and signaling via phospho-PAK1, suggesting potential vulnerabilities for combinatorial therapies [ 12 ].…”