Landon Wark

Tip60 is a haplo-insufficient tumour suppressor required for an oncogene-induced DNA damage response

The acetyl-transferase Tip60 might influence tumorigenesis in multiple ways. First, Tip60 is a co-regulator of transcription factors that either promote or suppress tumorigenesis, such as Myc and p53. Second, Tip60 modulates DNA-damage response (DDR) signalling, and a DDR triggered by oncogenes can counteract tumour progression. Using Eμ–myc transgenic mice that are heterozygous for a Tip60 gene (Htatip) knockout allele (hereafter denoted as Tip60+/– mice), we show that Tip60 counteracts Myc-induced lymphomagenesis in a haplo-insufficient manner and in a time window that is restricted to a pre- or early-tumoral stage. Tip60 heterozygosity severely impaired the Myc-induced DDR but caused no general DDR defect in B cells. Myc- and p53-dependent transcription were not affected, and neither were Myc-induced proliferation, activation of the ARF–p53 tumour suppressor pathway or the resulting apoptotic response. We found that the human TIP60 gene (HTATIP) is a frequent target for mono-allelic loss in human lymphomas and head-and-neck and mammary carcinomas, with concomitant reduction in mRNA levels. Immunohistochemical analysis also demonstrated loss of nuclear TIP60 staining in mammary carcinomas. These events correlated with disease grade and frequently concurred with mutation of p53. Thus, in both mouse and human, Tip60 has a haplo-insufficient tumour suppressor activity that is independent from—but not contradictory with—its role within the ARF–p53 pathway. We suggest that this is because critical levels of Tip60 are required for mounting an oncogene-induced DDR in incipient tumour cells, the failure of which might synergize with p53 mutation towards tumour progression.

Characterizing the Three-Dimensional Organization of Telomeres

Quantitative analysis can be used in combination with fluorescence microscopy. Although the human eye is able to obtain good qualitative results, when analyzing the spatial organization of telomeres in interphase nuclei, there is a need for quantitative results based on image analysis.

Homozygous BUB1B Mutation and Susceptibility to Gastrointestinal Neoplasia

A patient received a diagnosis of adenocarcinoma of the ampulla of Vater at 34 years of age. Two decades later, adenomatous polyps were found, followed by multiple primary invasive adenocarcinomas of both the colon and the stomach. Premature chromatid separation and mosaic variegated aneuploidy, combined with structural chromosomal abnormalities, were detected in his cells. We identified a germline homozygous intronic mutation, c.2386-11A→G, in the spindle-assembly checkpoint gene BUB1B, which creates a de novo splice site that is favored over the authentic (i.e., preferentially used) site. Our findings expand the phenotype associated with BUB1B mutations and the mosaic variegated aneuploidy syndrome to include common adult-onset cancers and provide evidence for the interdependency of the APC protein (encoded by the adenomatous polyposis coli gene) and the BUBR1 protein (encoded by BUB1B) in humans. (Funded by the Turner Family Cancer Research Fund and others.).

Nuclear remodeling as a mechanism for genomic instability in cancer.

This chapter focuses on the three-dimensional organization of the nucleus in normal, early genomically unstable, and tumor cells. A cause-consequence relationship is discussed between nuclear alterations and the resulting genomic rearrangements. Examples are presented from studies on conditional Myc deregulation, experimental tumorigenesis in mouse plasmacytoma, nuclear remodeling in Hodgkins lymphoma, and in adult glioblastoma. A model of nuclear remodeling is proposed for cancer progression in multiple myeloma. Current models of nuclear remodeling are described, including our model of altered nuclear architecture and the onset of genomic instability.

Telomeric aggregates and end-to-end chromosomal fusions require myc box II

Telomeres of tumor cells form telomeric aggregates (TAs) within the three-dimensional (3D) interphase nucleus. Some of these TAs represent end-to-end chromosomal fusions and may subsequently initiate breakage–bridge–fusion cycles. Wild-type (wt) and myc box II mutant (mt) Myc induce different types of genomic instability when conditionally expressed in mouse proB cells (Ba/F3). Only wt Myc overexpressing Ba/F3 cells are capable of tumor formation in severe combined immunodeficient mice. In this study, we investigated whether telomere dysfunction leading to TA formation is linked to the genetic changes that permit wt c-Myc-dependent transformation of Ba/F3 cells. To this end, we examined the 3D organization of telomeres after the deregulated expression of deletion myc boxII mutant (Δ106) or wt Myc. Δ106-Myc overexpression did not induce TAs, whereas wt-Myc deregulation did. Instead, Δ106-Myc remodelled the 3D telomeric organization such that telomeres aligned in the center of the 3D interphase nucleus forming a telomeric disk owing to a Δ106-induced G1/S cell cycle arrest. In contrast, wt-Myc overexpression led to distorted telomere distribution and TA formation. Analysis of chromosomal alterations using spectral karyotyping confirmed Δ106-Myc and wt-Myc-associated genomic instability. A significant number of chromosomal end-to-end fusions indicative of telomere dysfunction were noted in wt-Myc-expressing cells only. This study suggests that TAs may play a fundamental role in Myc-induced tumorigenesis and provides a novel way to dissect tumor initiation.

Increased genomic instability and altered chromosomal protein phosphorylation timing in HRAS‐transformed mouse fibroblasts

The RAS‐mitogen‐activated protein kinase signaling pathway is often deregulated in cancer cells. In metastatic HRAS‐transformed mouse fibroblasts (Ciras‐3), the RAS‐MAPK pathway is constitutively activated. We show here that Ciras‐3 cells exhibit a higher incidence of chromosomal instability than 10T1/2 cells, including higher levels of clonal and nonclonal chromosomal aberrations. Stimulation of serum starved 10T1/2 and Ciras‐3 cells with phorbol esters (TPA) results in the phosphorylation of histone H3 at serine 10 and serine 28. Regardless of the increased genomic instability in Ciras‐3 cells, TPA‐induced H3 phosphorylated at serine 10 and H3 phosphorylated at serine 28 partitioned into distinct nuclear subdomains as they did in the parental cells. However, the timing of the response of the H3 phosphorylation event to TPA induction was delayed in Ciras‐3 cells. Further Ciras‐3 cells, which have a more open chromatin structure, had increased steady state levels of phosphorylated H3 and HMGN1 relative to parental 10T1/2 cells. TPA‐induced H3 phosphorylated at serine 10 and 28 were colocalized with the transcriptionally initiated form of RNA polymerase II in 10T1/2 and Ciras‐3 cells. Chromatin immunoprecipitation assays demonstrated that TPA‐induced H3 phosphorylation at serine 28 was associated with the immediate early JUN promoter, providing direct evidence that this histone post‐translational modification is associated with transcriptionally active genes. Together our results demonstrate the increased genomic instability and alterations in the epigenetic program in HRAS‐transformed cells.

Nuclear imaging in three dimensions: A unique tool in cancer research

Tumorigenesis includes alterations in the three-dimensional (3D) nuclear organization of the genome. The combination of sensitive quantitative fluorescent in situ hybridization (Q-FISH) and three-dimensional (3D) microscopy have evolved as powerful tools in studying the dynamic 3D organization of telomeres and chromosomes in the interphase nucleus of individual normal and tumor cells. Tumor-specific alterations in 3D telomere architecture, particularly the appearance of telomeric aggregates, are early events in tumorigenesis and have diagnostic and prognostic value. Novel tools in the 3D nuclear imaging arsenal now include high-throughput scanning capabilities and new 3D nano-resolution microscopy of tissues and cells. In this review, we summarize our current understanding of the biology of telomeres in the context of tumorigenesis and elucidate the important integrating function of advanced 3D imaging technologies in translating new discoveries in basic cancer research into new diagnostic tools for clinical oncologists to improve patient care.

Nucleosomal response, immediate-early gene expression and cell transformation.

Multistep tumorigenesis is a progression of events resulting from alterations in the processing of the genetic information. These alterations result from stable genetic changes (mutations) in tumor suppressor genes and oncogenes (e.g. ras) and potentially reversible epigenetic changes (i.e. modifications in gene function without a change in DNA sequence) (Bird, 2007; Egger et al., 2004; Espino et al., 2005; Hake et al., 2004; Vogelstein and Kinzler, 2004). DNA methylation and protein modifications are two epigenetic mechanisms that are altered in cancer cells (Gal-Yam et al., 2008; Gronbaek et al., 2007). Chromatin modifying enzymes, catalyzing DNA methylation and protein modifications have a central role in the genesis of cancer (Ballestar and Esteller, 2008; Esteller, 2008; Gal-Yam et al., 2008; Gronbaek et al., 2007; Hake et al., 2004; Iacobuzio-Donahue, 2009; Medina and Cespedes, 2008; Momparler, 2003). In this review we will discuss how activation of signal transduction pathways, which are often deregulated in cancer cells, results in alterations in gene expression programming through histone modifications, with a focus on histone H3 phosphorylation.

High Mobility Group A2 protects cancer cells against telomere dysfunction

The non-histone chromatin binding protein High Mobility Group AT-hook protein 2 (HMGA2) plays important roles in the repair and protection of genomic DNA in embryonic stem cells and cancer cells. Here we show that HMGA2 localizes to mammalian telomeres and enhances telomere stability in cancer cells. We present a novel interaction of HMGA2 with the key shelterin protein TRF2. We found that the linker (L1) region of HMGA2 contributes to this interaction but the ATI-L1-ATII molecular region of HMGA2 is required for strong interaction with TRF2. This interaction was independent of HMGA2 DNA-binding and did not require the TRF2 interacting partner RAP1 but involved the homodimerization and hinge regions of TRF2. HMGA2 retained TRF2 at telomeres and reduced telomere-dysfunction despite induced telomere stress. Silencing of HMGA2 resulted in (i) reduced binding of TRF2 to telomere DNA as observed by ChIP, (ii) increased telomere instability and (iii) the formation of telomere dysfunction-induced foci (TIF). This resulted in increased telomere aggregation, anaphase bridges and micronuclei. HMGA2 prevented ATM-dependent pTRF2T188 phosphorylation and attenuated signaling via the telomere specific ATM-CHK2-CDC25C DNA damage signaling axis. In summary, our data demonstrate a unique and novel role of HMGA2 in telomere protection and promoting telomere stability in cancer cells. This identifies HMGA2 as a new therapeutic target for the destabilization of telomeres in HMGA2+ cancer cells.

Three-Dimensional Telomere Dynamics in Follicular Thyroid Cancer

BACKGROUND Over the last decade, annual incidence rates for thyroid cancer have been among the highest of all cancers in the Western world. However, the genomic mechanisms impacting thyroid carcinogenesis remain elusive. METHODS We employed an established mouse model of follicular thyroid cancer (FTC) with a homozygous proline to valine mutation (Thrb(PV/PV)) in the thyroid receptor β1 (TRβ1) and applied quantitative three-dimensional (3D) telomere analysis to determine 3D telomeric profiles in Thrb(PV)(/PV), Thrb(PV/)(+), and Thrb(+/+) mouse thyrocytes before and after histological presentation of FTC. RESULTS Using quantitative fluorescent in situ hybridization (Q-FISH) and TeloView™ image analysis, we found altered telomeric signatures specifically in mutant mouse thyrocytes. As early as 1 month of age, Thrb(PV/PV) mouse thyrocytes showed more telomeres than normal and heterozygous age-matched counterparts. Importantly, at the very early age of 1 month, 3D telomeric profiles of Thrb(PV/PV) thyrocyte nuclei reveal genetic heterogeneity with several nuclei populations exhibiting different telomere numbers, suggestive of various degrees of aneuploidy within the same animal. This was detected exclusively in Thrb(PV/PV) mice well before the presentation of histological signs of thyroid carcinoma. CONCLUSIONS We identified quantitative 3D telomere analysis as a novel tool for early detection and monitoring of thyrocyte chromosomal (in)stability. This technique has the potential to identify human patients at risk for developing thyroid carcinoma.