Identification of pathogenic CDK12 alterations in cell-free DNA (cfDNA) from patients with breast cancer.

1028 Background: Cyclin dependent kinase 12 ( CDK12) has both tumor suppressive and proto-oncogenic potential in metastatic breast cancers (MBC). CDK12 may be an important biomarker and target in MBC. However, a comprehensive genomic analysis of CDK12 alterations from cfDNA in MBC has not been investigated and the genomic impact of CDK12 alterations across the MBC spectrum is unknown. The purpose of this study was to identify the incidence of CDK12 genomic alterations occurring in cfDNA from patients with MBC and elucidate which CDK12 alterations may impact CDK12 kinase activity. Methods: We queried 13,070 MBC samples from the Guardant Health database between April 2019 – November 2020 to identify the incidence of CDK12 alterations detected in cfDNA. We classified each alteration type as: missense mutations, indels, or truncations. Amino acid changes occurring at conserved regions across multiple species were identified. Three-dimensional biochemical in silico analyses with ChimeraX were used to determine which CDK12 alterations may impact CDK12 kinase activity. To gain further biologic insights into CDK12 altered MBC we made associations with CDK12 alterations and co-occurring mutated genes. Results: Nonsynonymous CDK12 alterations from the Guardant Health database were found in 317 samples from a cohort of 13,070 patients indicating an overall incidence of 2.43%. Alterations included: 239 (75.4%) missense mutations; 26 (8.2%) indels; and 52 (16.4%) truncations. We identified 62 alterations within the kinase domain with all occurring at highly conserved regions across species. The most frequent hotspot mutation identified was I76M/T, occurring in 11 unique breast cancers. Three-dimensional analyses indicate that CDK12 alterations within the hinge, HRD, DFG, catalytic spine, and regulatory spine may impact CDK12 kinase activity. The significantly co-occurring mutations from the Guardant Health breast cancer database in samples with CDK12 alterations were ARID1A, APC, RB1, and PTEN. Conclusions: A modest incidence of CDK12 genomic alterations occur in cfDNA from patients with breast cancer. Novel somatic alterations in CDK12 were identified from Guardant Health that were not detected in the public domain. A portion of these occurred at highly conserved regions across species suggesting these specific CDK12 mutations may impact CDK12 kinase expression and be actionable therapeutic targets in breast cancers. Three dimensional analyses of the CDK12 gene further illustrate which specific alterations may induce CDK12 kinase expression or lead to inactivation. Co-occurring mutations reveal a unique genotype associated with CDK12 alterations that may play a biologic role in CDK12-mediated breast cancer pathogenesis. Preclinical studies to determine the prognostic and therapeutic implication of CDK12 alterations in MBC are warranted.