PKMYT1 in Cell Cycle Regulation and Cancer Therapeutics

Membrane-associated tyrosine and threonine-specific cdc2-inhibitory kinase (PKMYT1 or MYT1) is a member of the WEE1 family of CDK1 (cdc2) phosphorylating inhibitors. Both WEE1 and PKMYT1 act on CDK1 to prevent premature entry into M-phase, if detectable levels of DNA damage exist. PKMYT1 plays a similar role in meiotic regulation. While WEE1 functions primarily in the nucleus, PKMYT1 acts in the cytosol, with its C-terminal domain anchored in the membranes of the Golgi apparatus and endoplasmic reticulum. Both WEE1 and PKMYT1 act to keep CDK1 inactive as it moves between the nucleus and cytoplasm. PKMYT1 can also sequester CDK1 outside the nucleus to restrict its nuclear abundance.¹

PKMYT1 phosphorylates CDK1 at Thr14 and Tyr15 in its ATP binding site, inactivating the Cyclin B1-CDK1 complex (M phase-promoting factor or MPF) at the G2/M cell cycle transition. If no DNA damage is detected in the genome, these phosphorylations are removed by CDC25C phosphatase. Then, subsequent activating kinases phosphorylate CDK1 at Thr161 and Cyclin B1 at four discrete serine residues, allowing stable MPF nuclear entry and mitosis to commence. In essence, PKMYT1 halts entry into mitosis until sufficient DNA repair can be accomplished, averting “mitotic catastrophe” and cell death. While this “checkpoint recovery” allows normal cells to maintain genomic integrity, it can also buy time for heavily damaged cancer cells to recover.¹

Outlining PKMYT1’s Structure and Function

PKMYT1 is 499 amino acids in length, weighing in at nearly 55 kDa. From front to back, it consists of a N-terminal regulatory domain, a central protein kinase domain, and C-terminal sites that mediate its MPF interaction and subsequent PIN1 modification once mitosis is underway.² The region upstream of the kinase domain possesses a PAYF motif and phosphorylation sites that negatively modulate PKMYT1’s activity. When MPF is fully active, CDK1 phosphorylates PKMYT1’s N-terminal Thr11 and Thr16 residues to decrease its potency. Phosphorylation by MPF and PLK1 at discrete sites downstream of the kinase domain provokes PKMYT1’s degradation. The post-translational modifications in each of its regulatory domains may even act cooperatively to modulate the activity of PKMYT1. This system of complex feedbacks is not yet completely understood.³

Too Much of a Good Thing

While preventing mitosis in normal cells to foster proper genomic repair and integrity is valuable, it has proven to be a potent mitigating factor in the treatment of cancer. Overexpression of PKMYT1 occurs and leads to a poor outcome in many cancers, especially lung, liver, and colorectal cancer. Cancers bearing ineffective and, even, oncogenic p53 mutations essentially lack the G1/S checkpoint for DNA repair. This places even more weight on the G2/M checkpoint to “buy time” for the repair of DNA lesions caused by chemotherapy or radiotherapy. While mutations of PKMYT1 are rarely found in tumors, upregulation is common. This overabundance shifts the cell cycle progression balance to inhibit mitotic entry, reducing mitotic catastrophe and diminishing the effectiveness of various cancer interventions.⁴

Recent research in lung cancer stem cells shows enhanced expression of a particular long, non-coding RNA (PKMYT1AR) sequesters the inhibitory microRNA, miR-485-5p, that binds PKMYT1 mRNA to regulate its translation. As a result, more PKMYT1 is produced, stabilizing beta catenin, increasing Wnt signaling, and accelerating cellular renewal. Enhanced Wnt signaling has also been observed in PKMYT1-overexpressing liver cancers. Overexpression of PKMYT1 also drives Notch and NF-kappa beta signaling in other cancers.⁵

Modulation of PKMYT1 activity could vastly increase the effectiveness of many cancer therapies. The Transcreener ADP² Kinase Assay provides a universal method to screen for kinase modulators in a high-throughput and easy-to-use format. The technology is based on direct detection of ADP produced by kinases. The direct detection format reduces the risk of false positives or missing a hit, giving researchers greater accuracy and confidence when pursuing lead molecules.

References

1. Ghelli Luserna di Rorà, A. et al. (2020) A WEE1 family business: regulation of mitosis, cancer progression, and therapeutic target. J Hematol Oncol, 13, 126. https://doi.org/10.1186/s13045-020-00959-2

2. Esposito F. et al. (2021) Wee1 Kinase: A Potential Target to Overcome Tumor Resistance to Therapy. International Journal of Molecular Sciences, 22(19):10689. https://doi.org/10.3390/ijms221910689

3. Aiba, Y. et al. (2022) Regulation of Myt1 kinase activity via its N-terminal region in Xenopus meiosis and mitosis. Cells & Development, 169:203754. https://doi.org/10.1016/j.cdev.2021.203754

4. Shao, C., et al. (2021). The DNA damage repair-related gene PKMYT1 is a potential biomarker in various malignancies. Translational Lung Cancer Research, 10(12), 4600-4616. https://doi.org/10.21037/tlcr-21-973

5. He, Y., et al. (2021) LncRNA PKMYT1AR promotes cancer stem cell maintenance in non-small cell lung cancer via activating Wnt signaling pathway. Mol Cancer, 20, 156. https://doi.org/10.1186/s12943-021-01469-6