Owing to the loss of heterochromatin integrity that occurs during thyroid tumorigenesis, the expression of Heterochromatin Protein 1 isoforms HP1a and HP1b was assessed by immunohistochemistry in 189 thyroid tumors and non-neoplastic tissues. Expression of HP1b was significantly decreased in all thyroid lesions, except in follicular adenomas, when compared with matched adjacent normal tissue. This loss of HP1b expression may in part be caused by microRNA dysregulation. An example is miR-205, a microRNA that is abundantly upregulated in thyroid carcinomas and shown to reduce the expression of HP1b. In contrast to HP1b, HP1a expression was only reduced in metastatic carcinomas and poorly differentiated lesions. These results suggest the reduction of HP1b followed by a decrease in HP1a contributes to the pathogenesis of thyroid carcinomas, and their loss is a potential marker of thyroid malignancy and metastatic potential, respectively.

Malignant cell invasion is accompanied by complex changes in nuclear shape and organisation. The onset of invasion correlates with the loss of HP1a in many solid tumours including those of the thyroid, kidney, colon and breast, while in cancer cell lines HP1a suppresses invasive potential. In normal cells HP1a maintains the condensation of transcriptionally silent heterochromatin and sequesters it at the inner periphery of the nuclear envelope; an event critical for the maintenance of nuclear stability and envelope rigidity. This role for HP1a leads us to hypothesise that the un-tethering of heterochromatin from the nuclear periphery that would occur after loss of HP1 causes a remodelling of nuclear envelope that enhances cellular invasion. If true this would explain how the nucleus, the largest and most rigid cellular organelles, becomes malleable enough to squeeze through interstitial spaces in the surrounding tissue during metastatic invasion. In support of this hypothesis we observed a 44% increase in the invasive potential of the poorly invasive MCF7 breast cells when HP1a is knocked down. Lamin A staining and electron microscopy also confirmed that these HP1a knockdown cells had the expected alterations in nuclear morphology and loss of heterochromatin at the nuclear periphery. Furthermore the loss of HP1 was correlated with a decrease in the mechanical stability of the nuclei. The significant enrichment (p-val 6.7x10-8) of nuclear envelope proteins shown by RNAseq analysis of transcripts that are differentially expressed in the MCF7 versus HP1a knock-down cells, suggests this loss of nuclear envelope integrity is associated with alterations to its composition. The most striking change identified in the nuclear envelope of cells lacking HP1a was the loss of two Nesprin1 isoforms. While reintroduction of HP1a in highly invasive MDA-MB-231 cells shows the return of these isoforms to the nuclear envelope. As the Nesprin1 isoforms act as signalling scaffolds connecting to the cytoskeleton or reaching into the nucleus, they represent a means of modulating nuclear malleability. Together these findings suggest that when loss of HP1a increases invasive potential there is a corresponding disruption to the nuclear periphery that is a weakness of these cells that could be exploited. Citation Format: Tracy K. Hale, Sarah Bond, David Wheeler. Untethering heterochromatin: how loss of HP1a enhances cell invasion by altering nuclear envelope integrity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1887. doi:10.1158/1538-7445.AM2017-1887