Introduction: Novel rearranged in transfection (RET)-specific tyrosine kinase inhibitors (TKIs) such as selpercatinib (LOXO-292) have shown unprecedented efficacy in tumors positive for RET fusions or mutations, notably RET fusionpositive NSCLC and RET-mutated medullary thyroid cancer (MTC). However, the mechanisms of resistance to these agents have not yet been described.

Telomerase activation is a common feature of most advanced human cancers and is postulated to restore genomic stability to a level permissive for cell viability and tumor progression. Here, we used genetically defined transformed mouse embryonic fibroblast (MEF) cultures derived from late generation mTerc −/− Ink4a/Arf −/− mice to explore more directly how telomere-based crisis relates to the evolution of cancer cell genomes and to tumor biology. An exhaustive serial analysis of cytogenetic profiles over extensive passage in culture revealed that the emergence of chromosomal fusions (including dicentrics) coincided with onset of deletions and complex nonreciprocal translocations (NRTs), whereas mTerc-transduced cultures maintained intact chromosomes and stable genomes. Despite a high degree of telomere dysfunction and genomic instability, transformed late passage mTerc −/− Ink4a/Arf −/− cultures retained the capacity to form subcutaneous tumors in immunocompromised mice. However, even moderate levels of telomere dysfunction completely abrogated the capacity of these cells to form lung metastases after tail-vein injection, whereas mTerc reconstitution alone conferred robust metastatic activity in these cells. Finally, serial subcutaneous tumor formation using late passage transformed mTerc −/− Ink4a/Arf −/− cultures revealed clear evidence of telomerase-independent alternative lengthening of telomeres (ALT). Significantly, despite a marked increase in telomere reserve, cells derived from the ALT+ subcutaneous tumors were unable to generate lung metastases, indicating in vivo functional differences in these principal mechanisms of telomere maintenance. Together, these results are consistent with the model that although telomere dysfunction provokes chromosomal aberrations that initiate carcinogenesis, telomerase-mediated telomere maintenance enables such initiated cells to efficiently achieve a fully malignant endpoint, including metastasis.[Keywords: Telomere; telomerase; metastasis; ALT (alternative lengthening of telomeres); chromosomal instability; SKY] Supplemental material is available at http://www.genesdev.org. Tumor formation represents the phenotypic endpoint of a mutational process that endows cells with a critical threshold of cancer-relevant genetic lesions. From a cytogenetic standpoint, epithelial neoplasms or carcinomas fall into one of two major classes-the first possessing microsatellite instability with retention of grossly normal karyotypes, and the second presenting with markedly altered chromosomal counts and structure. These two classes have been operationally defined as "microsatellite instability neoplasias" and "chromosomal instability neoplasias" (MIN and CIN), respectively (for review, see Lengauer et al. 1998). The molecular mechanism underlying MIN is clearly related to defective DNA mismatch repair. The basis for CIN is less well understood, although defects in mitotic checkpoint, nonhomologous end-joining, and DNA replication, as well as increased oxidative stress and telomere dysfu...

Reliable and reproducible methods for identifying PD-L1 expression on tumor cells are necessary to identify responders to anti-PD-1 therapy. We tested the reproducibility of the assessment of PD-L1 expression in non-small cell lung cancer (NSCLC) tissue samples by pathologists. NSCLC samples were stained with PD-L1 22C3 pharmDx kit using the Dako Autostainer Link 48 Platform. Two sample sets of 60 samples each were designed to assess inter- and intraobserver reproducibility considering two cut points for positivity: 1% or 50% of PD-L1 stained tumor cells. A randomization process was used to obtain equal distribution of PD-L1 positive and negative samples within each sample set. Ten pathologists were randomly assigned to two subgroups. Subgroup 1 analyzed all samples on two consecutive days. Subgroup 2 performed the same assessments, except they received a 1-hour training session prior to the second assessment. For intraobserver reproducibility, the overall percent agreement (OPA) was 89.7% [95% confidence interval (CI), 85.7-92.6] for the 1% cut point and 91.3% (95% CI, 87.6-94.0) for the 50% cut point. For interobserver reproducibility, OPA was 84.2% (95% CI, 82.8-85.5) for the 1% cut point and 81.9% (95% CI, 80.4-83.3) for the 50% cut point, and Cohen's κ coefficients were 0.68 (95% CI, 0.65-0.71) and 0.58 (95% CI, 0.55-0.62), respectively. The training was found to have no or very little impact on intra- or interobserver reproducibility. Pathologists reported good reproducibility at both 1% and 50% cut points. More adapted training could potentially increase reliability, in particular for samples with PD-L1 proportion, scores around 50%. .