New light on p27^kip1 in breast cancer

The CDKN1B gene, encoding for the cell cycle inhibitor p27^Kip1 is a tumor suppressor gene frequently inactivated in human cancer. CDKN1B inactivation mainly occurs at post-translational levels, via protein degradation, mis-localization and/or sequestration. Converging evidences suggest that p27^Kip1 plays important role(s) not only in the control of cell proliferation, but also in cell motility, autophagia and mitotic division. However, differently from canonical tumor suppressor genes, CDKN1B has been only rarely found mutated in human cancers.

A recent Nature report subverts this notion and identifies CDKN1B as one of the 18 most significantly mutated genes in luminal A breast cancer.¹ Interestingly, the vast majority of the observed mutations are located in the C-terminal portion of the protein, known to play a role in the control of protein stability and of cell motility. We discuss here the possible significance of these new data, shedding new light on the role of p27^Kip1 in human cancer.

Since the seminal studies of Loda² and Catzavelos,³ the CDK inhibitor p27^kip1 (hereafter p27) has been considered a tumor suppressor gene playing a pivotal role in both tumor onset and progression. Subsequent studies using mouse models further demonstrated that Cdkn1B, is a haplo-insufficient tumor suppressor gene.⁴ Accordingly, low levels of p27 protein have been observed in several types of tumors, often correlating with worse prognosis.⁵ Yet, up to now, CDKN1B gene mutation have been rarely found mutated in human cancers. General understanding suggested that the inhibition of cell cycle progression by p27 played a pivotal role in its function as tumor suppressor gene. The binding and inhibition of Cyclin/CDK complexes by p27 is located in its N-terminal portion while the regulation of protein stability, mainly due to posttranslational modification, is in the C-terminal portion of the protein.⁶ It is also clear that p27 has important CDK-independent functions that influence mainly, but not only, the ability of cancer cells to move and metastasize.⁵ These functions are usually associated with p27 cytoplasmic localization. Recent studies have hypothesized that p27 cytoplasmic displacement/retention could transform the protein from a substantial tumor suppressor (Jekyll) into a pro-metastatic oncogene (Hyde),⁷ thus rendering the thorough study of p27 functions even more compelling.

However, a recent report, looking for driver mutations in breast cancers,¹ subverts some of our previous notions and demonstrates that CDKN1B is among the 18 most significantly mutated genes. Interestingly, the vast majority of the described mutations reside in the C-terminal portion, thus strongly suggesting that the C terminus part of p27 is directly involved in its tumor-suppressive function (Fig. 1), at least in luminal A breast cancers.

Figure 1. Schematic representation of the mutations found in the p27^kip1 coding sequence, in breast cancer.¹ The mutations already functionally characterized are reported in red. The last 28 aminoacids of p27 are necessary for the binding to stathmin.^10,11 The T198A substitution may influence p27 binding to Rho⁸ or Stathmin,⁹ depending on the model system utilized. fs, frame shift; *, nonsense mutation; e2–1, altered exon junction.

Among the reported mutations the T198A and the E171* are of particular interest, since they have been already characterized for their significance in experimental studies. Phosphorylation of T198 residue is responsible for p27 increased proteasomal degradation via Skp2-independent mechanism, and it is also involved in the regulation of cell motility.^8,9 The E171* nonsense mutation leads to formation of a truncated protein lacking the last 28 amino acids. This deletion mutant has been thoroughly described in vitro; it is known to be more stable than the WT protein, it retains the ability to bind and inhibit the cyclin/CDK complexes; but it loses the ability to inhibit cell motility and metastasis formation.^9-12 Q163*, K134fs and P137fs mutations, which have not been studied yet, await for an equal characterization and will probably highlight important aspect of p27 functions, as well.

The fact that E171* and the T198A mutations, independently identified as important for the control of cell motility by p27 via the interaction with stathmin,^9-12 are also present in primary human breast cancer, strongly suggests that this interaction has clinical relevance and is critical for the tumor suppressor functions of p27, as we previously hypothesized.^10-12

Overall, we believe that the information arising from the study of this cohort of primary breast tumors will considerably improve our comprehension of p27 role in cancer and, eventually, of its potential use as biomarker and/or therapeutic target.