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Transient PLK4 Overexpression Accelerates Tumorigenesis in P53-Deficient Epidermis

2015-12-18 00:00:56

Nature Cell Biology; 18 (2016): 100–110; DOI: 10.1038/ncb3270


Özdemirhan Serçin, Jean-Christophe Larsimont, Andrea E. Karambelas, Veronique Marthiens, Virginie Moers, Bram Boeckx, Marie Le Mercier, Diether Lambrechts, Renata Basto, Cédric Blanpain



Abstract


Aneuploidy is found in most solid tumours, but it remains unclear whether it is the cause or the consequence of tumorigenesis. Using PLK4 overexpression (PLK4OE) during epidermal development, we assess the impact of centrosome amplification and aneuploidy on skin development and tumorigenesis. PLK4OE in the developing epidermis induced centrosome amplification and multipolar divisions, leading to p53 stabilization and apoptosis of epidermal progenitors. The resulting delayed epidermal stratification led to skin barrier defects. PLK4 transgene expression was shut down postnatally in the surviving mice and PLK4OE mice never developed skin tumours. Concomitant PLK4OE and p53 deletion (PLK4OE/p53cKO) rescued the differentiation defects, but did not prevent the apoptosis of PLK4OE cells. Remarkably, the short-term presence of cells with supernumerary centrosomes in PLK4OE/p53cKO mice was sufficient to generate aneuploidy in the adult epidermis and triggered spontaneous skin cancers with complete penetrance. These results reveal that aneuploidy induced by transient centrosome amplification can accelerate tumorigenesis in p53-deficient cells.



Introduction


Centrosomes are the microtubule-organizing centres of animal cells and participate in a variety of cellular processes such as cell division, and the establishment and maintenance of cell polarity. Centrosome number is tightly regulated by several key centrosome duplication factors. Abnormal centrosome number is associated with a variety of human diseases including cancer.



Since the pioneering observations of Boveri and Hansemann, centrosome amplification, defined by the presence of more than two centrosomes in a cell, has been associated with multipolar
spindle assembly and consequent abnormal cell division. Centrosome amplification has been reported during the early stage of tumorigenesis in different types of preneoplastic lesion such in situ breast ductal carcinomas and in situ carcinomas of the uterine cervix, prostate, and female breast suggesting that centrosome amplification could promote the early stage of tumorigenesis. Although complete inactivation of spindle assembly checkpoint components in mice is usually embryonic lethal, some hypomorphic alleles of these regulators lead to aneuploidy and, in some cases,
are associated with an increase of tumour incidence, but often with incomplete penetrance and very late appearance. However, several other mouse models leading to aneuploidy do not present increased tumour formation or even presented a decrease in tumour incidence, suggesting that either only some types of aneuploidy promote tumorigenesis or aneuploidy promotes tumour formation
only in a specific cellular context.



Centrosome amplification is a hallmark of human tumours, but its contribution to tumorigenesis still remains an open question. In flies, manipulation of the centrosome duplication machinery by overexpression of the master regulator of centriole biogenesis PLK4, also known as SAK, is a tumour-initiating event in transplantation assays. Interestingly, the aetiology of these tumours seems to be different according to the tissue that contains extra centrosomes. Centrosome amplification in developing fly neuronal progenitors results in spindle positioning defects that lead to the expansion of the progenitor pool at the expense of differentiating cells. However, these cells undergo bipolar division and maintain a highly stable diploid genome owing to the presence of highly efficient centrosome clustering mechanisms. In contrast, these clustering mechanisms were not efficient in fly epithelial cells with extra centrosomes, leading to the assembly and persistence of multipolar spindles that generated aneuploidy.



In mammals, the consequences of centrosome amplification have been evaluated only in the developing brain. Overexpression of PLK4 in embryonic neural progenitors resulted in abnormal mitotic divisions generating aneuploidy and apoptosis leading to microcephaly. Aneuploid cells in the developing brain are rapidly removed from the cycling population by p53-dependent apoptosis. The deletion of p53 in neural progenitors inhibits apoptosis of cells with extra centrosomes and aneuploidy but these cells did not give rise to brain tumours, leaving the question open of whether centrosome amplification and consequent aneuploidy can trigger tumorigenesis in mammals and if so in which context.



Non-melanoma skin cancers, basal cell carcinoma and squamous cell carcinoma represent the most frequent tumours in humans. Squamous skin carcinoma (SCC) is the second most frequent skin cancer and contains mutations in RAS, p53 and other oncogenes or tumour-suppressor genes but also presented recurrent gene amplification and deletion of key driver genes.Mouse models of skin SCCs recapitulate well the features of human skin SCCs. In mouse skin SCCs all tumour cells present some degree of aneuploidy. In human SCCs, high levels of aneuploidy are associatedwith poorer prognosis, suggesting that skin SCC would represent an excellent model to study the role of aneuploidy in tumour initiation and progression.



Here, we used a genetic mouse model to assess the impact of PLK4OE and centrosome amplification in the formation of skin cancers. We overexpressed PLK4 in the developing skin epidermis and
found that PLK4OE-mediated centrosome amplification led to abnormal mitosis and aneuploidy. The cells with centrosome supernumerary induced by PLK4OE were eliminated by apoptosis leading to a delay in skin stratification.However, simultaneous PLK4OE and p53 depletion in the epidermis allowed the survival of cells with extra centrosomes for a short period of time. Surprisingly, this transient presence of cells containing supernumerary centrosomes was sufficient to allow the accumulation of aneuploid cells in adult mice that invariably result in the formation of SCCs in 100% of the mice. Our data demonstrate that centrosome amplification followed by aneuploidy can accelerate tumorigenesis in mammals in the absence of p53.




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Key Words


Cancer | Centrosome | Embryogenesis | Skin stem cells