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Loss of ASXL1 tumour suppressor promotes resistance to CDK4/6 inhibitors

Presented by
Dr Dhivya Sudhan, UT Southwestern Medical Center, TX, USA
SABCS 2021

The pathways leading to CDK4/6 inhibitor resistance are not yet clear. Using a technique called ‘accelerated mutagenesis,’ researchers unravelled the role of ASXL1 loss in CDK4/6 inhibitor resistance.

CDK4/6 inhibitors in combination with anti-oestrogens have prolonged survival of patients with oestrogen receptor (ER)-positive/HER2-negative metastatic breast cancer. However, this combination is not curative, mainly due to acquired drug resistance. Knowledge about mechanisms of such resistance remains incomplete. Dr Dhivya Sudhan (UT Southwestern Medical Center, TX, USA) reported results of new insights in the pathways leading to CDK4/6 inhibitor resistance [1].

Using CRISPR/Cas9 to delete the DNA mismatch repair gene MSH2 in MCF7 and T47D ER-positive breast cancer cells, Dr Sudhan and colleagues obtained cells with drug resistance-associated mutations. Clones resistant to CDK4/6 inhibitors were selected and subjected to whole exome sequencing. Of the 10 genes recurrently mutated in the CDK4/6 inhibitor resistant cells, loss of ASXL1 tumour suppressor was identified as top hit. Loss of ASXL1 has been implicated in myeloid transformation through epigenetic reprogramming. In line with these findings, among 1,769 tumours from patients treated with CDK4/6 inhibitor (TEMPUS database), 37 exhibited ASXL1 alterations. In addition, RNA sequencing of patient-derived organoids established from post-CDK4/6 inhibitor metastases, identified ASXL1 mutations in 2/7 organoids (29%). Functional studies showed that loss of ASXL1 was associated with maintenance of retinoblastoma phosphorylation in the presence of CDK4/6 inhibition, markedly higher levels of CDK2, CDK6, cyclins E and A, and downregulation of p21 and p27.

“We identified loss of ASXL1 as a novel mechanism of resistance to CDK4/6 inhibition,” concluded Dr Sudhan. “Knockdown of CDK2 and cyclin A restored sensitivity to CDK4/6 inhibitors and reduced viability of ASXL1 deficient cells, suggesting that CDK2 inhibitors are a treatment approach against these drug-resistant tumours.”

  1. Sudhan DR, et al. Loss of ASXL1 tumor suppressor promotes resistance to CDK4/6 inhibitors in ER+ breast cancer. GS3-09, SABCS 2021 Virtual Meeting, 7–10 December.

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