Telomeres distinguish the ends of linear chromosomes from DNA double strand breaks, which represent severe damage to the genetic material. In mammals telomeres are principally maintained by the enzyme telomerase. Insufficient telomerase can result in telomere erosion in aging human cells, eventually culminating in chromosome damage, cellular dysfunction and cell death. Hence, telomere maintenance is a critical component for cellular and organismal health and, conversely, for the unconstrained growth of cancer cells.
The genomic instability elicited by telomere erosion can result in the acquisition of carcinogenic genetic alterations. Thus, unraveling the concerted functions of telomerase, telomeres, and the cellular response to telomere dysfunction is crucial to illuminate the connections between genomic instability, cancer, degenerative disease and aging in humans. In this regard, a mouse model that is deficient in telomerase (mTerc) has provided significant insights into the cellular and physiological responses to telomere erosion, including its impact on cancer and aging. My research with this model system focuses on exploring these links.
Murine models of telomere dysfunction and Atm deficiency
The human disease ataxia-telangiectasia (A-T), caused by mutations in ATM, is characterized by cerebellar ataxia and childhood leukemia and lymphoma, among other phenotypes. The ATM protein kinase marshals an extensive DNA damage checkpoint pathway including p53, and its yeast homolog TEL1 functions in telomere maintenance. Although Atm deficient mice develop lymphoma, they fail to display several other features of A-T. Given ATM's role in the DSB response, we reasoned that it might play a similar role in signaling the presence of telomere dysfunction; conversely, the extensive telomere reserve of typical laboratory mice might block the expression of A-T phenotypes in mice.
To test these hypotheses, we interbred Atm and mTerc deficient mice, finding that the combined loss of Atm and telomerase accelerates telomere erosion and increases genomic instability. In turn, this led to tissue-specific stem cell deficits and several A-T phenotypes not previously observed in Atm mutant mice, including neurological deficits and accelerated aging. Rather than phenocopy p53 loss, which prevents telomere dysfunction-induced apoptosis and senescence, loss of Atm against the backdrop of telomere dysfunction caused a striking Atm-independent up-regulation of p53. Moreover, dysfunctional telomeres completely abrogated lymphomagenesis in Atm deficient mice. We then tested whether p53 was crucial for the prevention of lymphomagenesis in combined Atm mTerc mutant mice, finding that even p53 hemizyosity restores tumorigenesis in telomere dysfunctional Atm deficient mice, accompanied primarily by p53 loss of heterozygosity. These results indicate the importance of the signaling cascade responding to telomere dysfunction to in vivo phenotypes such as cancer and aging.
Checkpoint deficiency, telomere dysfunction, and cancer gene discovery
We reasoned that the cooperative effect of these three mutations (described above) would result in marked increase in genomic instability, potentially allowing the accumulation of tumor-promoting genetic alterations, and the presence of such instability was confirmed by spectral karyotyping. To narrow the search for such events, we employed array-based comparative genomic hybridization (aCGH), uncovering a large set of recurrent genetic aberrations that likely play important roles in tumorigenesis. We then reasoned that genetic lesions conserved in both this model system and in human cancers would highlight those regions most likely to harbor critical oncogenes and tumor suppressor genes. Thus we performed similar genomic profiling on human T cell tumors, revealing a substantial overlap in copy number alterations. We used this information to predict and test tumor sensitivity to several classes of therapeutic agents, including Akt and Notch pathway inhibitors. Lastly, sequencing of several candidate genes within conserved genomic regions uncovered a high frequency of somatic FBXW7 mutations in human T-ALL, a gene involved in regulating several important oncogenic proteins including Notch1, cyclin E, and c-myc. This work indicates that a concerted, unbiased effort to develop a complete picture of the cancer genome can yield insight into critical tumor-associated pathways and relevant therapeutic targets.
NHEJ deficiency and telomere dysfunction
The non-homologous end joining (NHEJ) DNA repair pathway facilitates rejoining of DNA DSBs, including the rearrangements critical for adaptive immune receptor diversity. NHEJ proteins have also been implicated in the seemingly opposing functions of telomere maintenance as well as the cellular response to dysfunctional telomeres. We sought to determine the physiological role of NHEJ in telomere homeostasis in the mouse by introducing a mutant allele of DNA-PKcs (the catalytic kinase subunit) into the mTerc mutant background. In contrast with other genetic models, loss of DNA-PKcs had little impact on apoptosis or chromosome fusions resulting from telomere erosion in the mouse, indicating that either NHEJ is dispensable or redundant in this setting. Nonetheless, DNA-PKcs-mTerc double mutant mice exhibited a significantly shorter lifespan than mice with either defect alone. Moreover, these mice were exquisitely sensitive to murine hepatitis virus compared to the single mutants, suggesting that telomere dysfunction and loss of DNA-PKcs combined resulted in aberrant stress response to this pathogen. We tested this hypothesis by challenging mice with a double-stranded RNA viral mimic, and found that the double mutants alone succumbed to a toxic-shock like syndrome characterized by hyper-inflammatory cytokine response and neutrophil infiltration into organ systems. This work illustrates the value of understanding the molecular networks that interact with telomeres on the cellular level and their differential impact on various organ systems and organismal viability.
Work on these themes is continuing in my laboratory, focusing on several questions, such as:
- How does telomere dysfunction impact organismal survival, and how are DNA damage checkpoint pathways engaged in this process?
- What is the mechanistic connection between telomere dysfunction, NHEJ deficiency and cytokine signaling?
- How does telomere dysfunction contribute to carcinogenesis in various tissues?
- What are the relevant target genes from the compendium of genetic alterations described by high-throughput genome analysis techniques, such as those found in our telomere dysfunctional mouse model?
Pre Doctorial Associate: Kyle Beauchemin
Research Administrative Assistant: Patricia Cherry
Inuzuka H, Shaik S, Onoyama I, Gao D, Tseng A, Maser RS, Zhai B, Wan L, Gutierrez A, Lau AW, Xiao Y, Christie AL, Aster J, Settleman J, Gygi SP, Kung AL, Look T, Nakayama KI, DePinho RA, Wei W. 2011. SCFFbw7 regulates cellular apoptosis by targeting Mcl-1 for ubiquitination and destruction. Nature 471(7336):104-109.
Sahin E, Colla S, Liesa M, Moslehi J, Muller F, Guo M, Kotton D, Fabian A, Walkley C, Maser RS, Tonon G, Foerster F, Xiong R, Wang Y, Shukla S, Jaskelioff M, Martin E, Heffernan T, Protopopov A, Ivanova E, Mahoney J, Kost-Alimova M, Perry S, Bronson R, Liao R, Mulligan R, Shirihai O, Chin L, DePinho RA. 2011. Telomere dysfunction induces metabolic and mitochondrial compromise. Nature 470(7334):359-365.
Simon NM, McNamara K, Chow CW, Maser RS, Papakostas GI, Pollack MH, Nierenberg AA, Fava M, Wong KK. 2008. A detailed examination of cytokine abnormalities in Major Depressive Disorder. Eur Neuropsychopharmacol 18:230-233. PMCID: PMC1004598
Perera SA, Maser RS, Xia H, McNamara K, Protopopov A, Chen L, Hezel AF, Kim CF, Bronson RT, Castrillon DH, Chin L, Bardeesy N, DePinho RA, Wong KK. 2008. Telomere dysfunction promotes genome instability and metastatic potential in a K-ras p53 mouse model of lung cancer. Carcinogenesis Apr;29(4):747-53.
Simon NM, McNamara K, Chow CW, Maser RS, Papakostas GI, Pollack MH, Nierenberg AA, Fava M, Wong KK. 2008. A detailed examination of cytokine abnormalities in Major Depressive Disorder. Eur Neuropsychopharmacol Mar;18(3):230-233. PMCID: PMC1004598
Maser RS, Choudhury B, Campbell PJ, Feng B, Wong KK, Protopopov A, O'Neil J, Gutierrez A, Ivanova E, Perna I, Lin E, Mani V, Jiang S, McNamara K, Zaghlul S, Edkins S, Stevens C, Brennan C, Martin ES, Wiedemeyer R, Kabbarah O, Nogueira C, Histen G, Aster J, Mansour M, Duke V, Foroni L, Fielding AK, Goldstone AH, Rowe JM, Wang YA, Look AT, Stratton MR, Chin L, Futreal PA, DePinho RA. 2007. Chromosomally unstable mouse tumours have genomic alterations similar to diverse human cancers. Nature Jun 21;447(7147):966-71. PMCID: PMC2714968
Maser RS, Wong KK, Sahin E, Xia H, Naylor M, Hedberg HM, Artandi SE, DePinho RA. 2007. DNA-dependent protein kinase catalytic subunit is not required for dysfunctional telomere fusion and checkpoint response in the telomerase-deficient mouse. Mol Cell Biol Mar;27(6):2253-65. PMCID: PMC1820500
O'Neil J, Tchinda J, Gutierrez A, Moreau L, Maser RS, Wong KK, Li W, McKenna K, Liu XS, Feng B, Neuberg D, Silverman L, DeAngelo DJ, Kutok JL, Rothstein R, DePinho RA, Chin L, Lee C, Look AT. 2007. Alu elements mediate MYB gene tandem duplication in human T-ALL. J Exp Med Dec 24;204(13):3059-66. PMCID: PMC2150982
Wong KK, Maser RS, Sahin E, Bailey ST, Xia H, Ji H, McNamara K, Naylor M, Bronson RT, Ghosh S, Welsh R, DePinho RA. 2007. Diminished lifespan and acute stress-induced death in DNA-PKcs-deficient mice with limiting telomeres. Oncogene May 3;26(20):2815-21.
Ji H, Houghton AM, Mariani TJ, Perera S, Kim CB, Padera R, Tonon G, McNamara K, Marconcini LA, Hezel A, El-Bardeesy N, Bronson RT, Sugarbaker D, Maser RS, Shapiro SD, Wong KK. 2006. K-ras activation generates an inflammatory response in lung tumors. Oncogene Mar 30;25(14):2105-12.
Simon NM, Smoller JW, McNamara KL, Maser RS, Zalta AK, Pollack MH, Nierenberg AA, Fava M, Wong KK. 2006. Telomere shortening and mood disorders: preliminary support for a chronic stress model of accelerated aging. Biol Psychiatry Sep 1;60(5):432-5.
Maser RS, DePinho RA. 2004. Telomeres and the DNA damage response: why the fox is guarding the henhouse. DNA Repair (Amst). Aug-Sep;3(8-9):979-88. Review.
Wong KK, Maser RS, Bachoo RM, Menon J, Carrasco DR, Gu Y, Alt FW, DePinho RA. 2003. Telomere dysfunction and Atm deficiency compromises organ homeostasis and accelerates ageing. Nature Feb 6;421(6923):643-8.
Maser RS, DePinho RA. 2002. Connecting chromosomes, crisis, and cancer. Science Jul 26;297(5581):565-9. Review.