Abstract LB-442: The nuclear matrix anchor domain of the DNA replication factor Ciz1 is commonly disrupted in tumours and is a circulating biomarker for lung cancer

Abstract

Abstract The DNA replication protein Ciz1 promotes initiation of mammalian DNA replication in cooperation with cyclin A-dependent kinase, most likely by delivering cyclin A to sites where cyclin E-dependent pre-replication complex assembly has taken place. Normally, Ciz1 is anchored within nuclear matrix-associated foci that co-localize with sites of DNA replication, but in the absence of anchor domain Ciz1 retains the ability to promote initiation of DNA replication in isolated nuclei so attachment to the nuclear matrix is not essential for function. Expression of DNA replication and nuclear matrix anchor domains of Ciz1 are uncoupled and uneven at the transcript level in a wide range of common solid tumours, including breast and lung. In cell-based assays, recombinant anchor domain protein interferes with attachment of endogenous Ciz1 to the nuclear matrix, revealing a dominant negative effect that also impacts on nuclear matrix-recruitment of key components of the pre-replication complex. This suggests that Ciz1 normally plays a role in localizing initiation of DNA replication to the nuclear matrix. These findings implicate spatially unconstrained DNA replication as a source of nuclear disorder in cancer cells. We identified a variant Ciz1 isoform with alterations in the nuclear matrix attachment domain and tumour-restricted expression. RNAi-mediated selective inhibition of variant Ciz1 expression is sufficient to restrain the growth of tumour cells that express it, identifying variant Ciz1 as a functionally relevant driver of cell proliferation. We also present evidence that this form of Ciz1 is expressed in 34/35 lung tumours but not adjacent tissue, giving rise to stable protein quantifiable in less than a microlitre of patient plasma by western blot. Using two independent sets, with 170 and 160 samples, variant Ciz1 correctly identified stage 1 lung cancer patients with clinically useful accuracy. For set 1, mean variant Ciz1 level (+SD) in individuals without diagnosed tumours established a threshold that correctly classified 94% of small cell lung cancers (SCLC) and non-SCLC patients. Within set 2, comparison of stage 1 non-SCLC with asymptomatic age-matched smokers correctly classified 85% of patients, while comparison with individuals with benign lung nodules correctly classified 83% of patients. The data show that this cancer-specific, single marker is capable of indentifying early stage lung cancer within at-risk groups, without resort to invasive procedures. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-442. doi:1538-7445.AM2012-LB-442