Rb Tumor Suppressor in Small Cell Lung Cancer: Combined Genomic and IHC Analysis with a Description of a Distinct Rb-Proficient Subset

Abstract

Purpose: RB1 mutations and loss of Rb expression represent consistent but not entirely invariable hallmarks of small cell lung cancer (SCLC). The prevalence and characteristics of SCLC retaining wild-type Rb are not well-established. Furthermore, the performance of targeted next-generation sequencing (NGS) vs immunohistochemistry for Rb assessment are not well-defined. Experimental design: 208 clinical SCLC samples were analyzed by comprehensive targeted NGS, covering all exons of RB1, and Rb immunohistochemistry. Based on established coordination of Rb/p16/cyclinD1 expression, p16-high/cyclinD1-low profile was used as marker of constitutive Rb deficiency. Results: Fourteen of 208 (6%) SCLC expressed wild-type Rb, accompanied by a unique p16-low/cyclinD1-high profile supporting Rb proficiency. Rb-proficient SCLC were associated with neuroendocrine-low phenotype, combined SCLC with non-SCLC (NSCLC) histology and aggressive behavior. These tumors exclusively harbored CCND1 amplifications (29%), and were markedly enriched in CDKN2A mutations (50%) and NSCLC-type alterations (KEAP1, STK11, FGFR1). The remaining 194/208 SCLC were Rb-deficient (p16-high/cyclinD1-low), including 184 cases with Rb loss (of which 29% lacked detectable RB1 alterations by clinical NGS pipeline), and 10 cases with mutated but expressed Rb. Conclusion: This is the largest study to-date to concurrently analyze Rb by NGS and immunohistochemistry in SCLC, identifying a 6% rate of Rb proficiency. Pathologic-genomic data implicate NSCLC-related progenitors as a putative source of Rb-proficient SCLC. Consistent upstream Rb inactivation via CDKN2A/p16↓ and CCND1/cyclinD1↑ suggests potential utility of CDK4/6 inhibitors in this aggressive SCLC subset. The study also clarifies technical aspects of Rb status determination in clinical practice, highlighting the limitations of exon-only sequencing for RB1 interrogation.

Biology and impact of lineage plasticity in ALK-positive NSCLC: a narrative review

“…function has been described as necessary but not sufficient for SCLC transformation in the context of EGFR-mutant NSCLC(101). This is consistent with the genomic landscape of de novo SCLC in which >95% of tumors are deficient in both p53/RB1(111)(112)(113).…”

Updates in pathology and molecular diagnostics to inform the evolving landscape of thoracic surgery and oncology

“…The pathogenesis of small cell carcinoma relies almost universally on biallelic loss of both TP53 and RB1, 90,91 key cell cycle regulators whose loss contributes to the uniquely high proliferative rate of these tumors.…”

“…The pathogenesis of small cell carcinoma relies almost universally on biallelic loss of both TP53 and RB1 , 90,91 key cell cycle regulators whose loss contributes to the uniquely high proliferative rate of these tumors. Small cell carcinoma transformation from adenocarcinoma, particularly as recurrence after oncogene‐targeted therapy, has been recognized as a resistance mechanism to tyrosine kinase inhibitor therapy 92 .…”

Small Biopsy and Cytology of Pulmonary Neuroendocrine Neoplasms: Brief Overview of Classification, Immunohistochemistry, Molecular Profiles, and World Health Organization Updates

“…In such instances, Rb loss and p16 overexpression can indirectly support the diagnosis, but these markers are not entirely sensitive or specific for SCLC. 35 Recently, transcriptional subtypes of SCLC (ASCL1, NEUROD1, YAP1, POU2F3) have been described. 36 Although ASCL1 and NEUROD1 are associated with neuroendocrine marker and TTF-1 expression, POU2F3 expression is described in SCLC lacking or having low levels of neuroendocrine marker expression by IHC.…”