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HnRNP D RNA Binding Protein Functions in Telomere Maintenance
Molecular Cell
Previews AUF1/HnRNP D RNA Binding Protein Functions in Telomere Maintenance Liuh-Yow Chen1 and Joachim Lingner1,* 1Swiss Institute for Experimental Cancer Research (ISREC), School of Life Sciences, Frontiers in Genetics National Center of Competence in Research, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Station 19, 1015 Lausanne, Switzerland *Correspondence: [email protected] http://dx.doi.org/10.1016/j.molcel.2012.06.031
In this issue of Molecular Cell, Pont et al. (2012) show that AUF1/hnRNP D promotes TERT transcription, which is required for telomere maintenance in mice. Telomeres are nucleoprotein complexes at the ends of linear chromosomes that protect DNA ends from DNA degradation, DNA repair activities, and DNA damage checkpoint activation (de Lange, 2009). Most of the telomeric DNA is replicated by semiconservative DNA replication, but the very last bit requires telomerase for stable maintenance. For this, the telomerase reverse transcriptase (TERT) reverse transcribes in an iterative fashion the template region of its tightly associated RNA moiety, adding short telomeric repeats to the ends of chromosomes (Blackburn et al., 2006). In humans and other long-lived animals, telomerase is expressed ubiquitously only early in embryogenesis and then is transcriptionally repressed later during development in many tissues for the rest of life, thus turning on the telomere clock (Artandi and DePinho, 2010; Shay and Wright, 2011). The telomere clock may on one side repress tumorigenesis by limiting the replicative potential of precancerous cells whose cell-cycle control has gone awry. On the other side, short telomeres promote genome instability, and they accumulate during aging. Short telomeres are also causative of telomere diseases in which patients suffer from tissue damage and accelerated aging and die prematurely, for example, due to bone marrow failure as seen in dyskeratosis congenita (Dokal, 2011). Correct regulation of telomerase at chromosome ends involves multiple telomeric proteins and possibly RNAs, topics that are subject to intensive investigations. In addition, in humans TERT but not other telomerase components is tightly regulated at the level of transcription. Despite the importance of the latter, surprisingly little is known
about the regulation of TERT expression. In this issue of Molecular Cell, Pont et al. (2012) report that mice with AUF1 deficiency suffer from telomere erosion and premature aging, which increases with successive generations, thus resembling the phenotype seen upon deletion of telomerase. The authors generated heterozygous Auf1+/ mice, which they intercrossed for seven generations. Interestingly, Auf1 is haploinsufficient. The viability of Auf1/ offspring from Auf1+/ parents decreased with successive generations, demonstrating genetic anticipation. The Auf1/ double mutant animals could not be intercrossed. Auf1/ animals had decreased telomere lengths in late generations and showed the phenotype typically seen for telomerase-deficient mice, such as accumulation of senescent cells, tissue atrophy, small testes and uterus, hunchback, and low body fat. Consistent with telomere loss, Auf1/ cells also showed DNA damage signaling from the telomeres. How does AUF1 promote telomere maintenance? Intriguingly, TERT mRNA levels were >20-fold reduced in the MEFs derived from the KO animals. Furthermore, AUF1 stimulated reporter gene expression driven from an mTert promoter construct. and AUF1 was detected by chromatin immunoprecipitation at the endogenous Tert promoter. This suggests that AUF1 counteracts telomere shortening by stimulating mTERT expression. Is the promotion of mTert expression the only mechanism by which AUF1 promotes telomere maintenance and function? In support of this notion, telomere shortening in Auf1/ MEFs was rescued upon ectopic expression of mTert. However, Auf1 deletion in the mouse gives
a more severe phenotype than what is seen upon telomerase loss only (Lu et al., 2006). The Auf1/ animals cannot be intercrossed (Pont et al., 2012), whereas telomerase KO mice can be intercrossed for approximately up to six generations (Blasco et al., 1997). Indeed, AUF1 is involved multiple biological processes (Figure 1). It is not unanticipated that AUF1 can function as a transcriptional activator. When assembling with the LR1 DNA-binding complex, AUF1 activates the Epstein-Barr virus promoter (Zucconi and Wilson, 2011). More established, however, are AUF1’s roles in promoting rapid decay of possibly hundreds of mRNAs through direct binding of AUrich elements in their 30 untranslated regions, upon which AUF1 nucleates assembly of RNA degrading enzymes whose identity is not fully revealed. Destabilized mRNAs include regulators of the cell cycle and proinflammatory cytokines (Figure 1). Such factors have been linked to telomere dysfunction, and their misregulation could contribute to the phenotype. Finally, AUF1 has been described to bind single-stranded telomeric DNA and telomeric repeat-containing RNA (TERRA), a long noncoding RNA expressed at telomeres in eukaryotes (Feuerhahn et al., 2010). Thus, AUF1 may play additional direct roles at telomeres, which might contribute to the described phenotype. The dissection of AUF1 functions will be challenging. Interestingly, the Auf1-locus specifies the synthesis of four protein isoforms, p37 AUF1, p40 AUF1, p42 AUF1, and p45 AUF1, due to alternative splicing of premRNAs. All four proteins have in common two RNA recognition motifs (RRM) followed by a Gln-rich domain, but they
Molecular Cell 47, July 13, 2012 ª2012 Elsevier Inc. 1
Molecular Cell
Previews
slightly differ outside these control of one genetic locus. domains (Zucconi and WilThis all could make sense. son, 2011). The low TERT Through linking telomerase mRNA levels in Auf1/ activation to suppressing MEFs were rescued upon coinflammatory response, AUF1 expression of p42 AUF1 and may hit two flies with one swat. p45 AUF1, while the other isoforms did not contribute. Thus, establishment of geREFERENCES netic mouse models that are Artandi, S.E., and DePinho, R.A. defective in expression of in(2010). Carcinogenesis 31, 9–18. dividual AUF1 isoforms may reduce the complexity of the Blackburn, E.H., Greider, C.W., and Szostak, J.W. (2006). Nat. Med. 12, phenotype and more clearly 1133–1138. delineate specific functions. It could also be tried to clearly Blasco, M.A., Lee, H.W., Hande, M.P., Samper, E., Lansdorp, P.M., separate the telomeraseDePinho, R.A., and Greider, C.W. independent dysfunctions in (1997). Cell 91, 25–34. / Figure 1. Multiple Biological Functions of AUF1/HnRNP D animals by forced Auf1 AUF1 promotes decay of multiple mRNAs involved in various biological de Lange, T. (2009). Science 326, expression of Tert. processes with links to cancer and aging. Pont et al. (2012) discover that 948–952. Another important question AUF1 is required for telomere maintenance in the mouse through stimulation of TERT transcription. Dokal, I. (2011). Hematology (Am Soc is whether AUF1 function for Hematol Educ Program) 2011, telomere maintenance is con480–486. served in humans. Telomere biology between these mammals has re- linked to malignancy in mice and humans Feuerhahn, S., Iglesias, N., Panza, A., Porro, A., markably diverged in evolution. In partic- (Zucconi and Wilson, 2011), and given and Lingner, J. (2010). FEBS Lett. 584, 3812– 3818. ular, it seems that the transcriptional re- the new findings, it will be important to pression of TERT in differentiated tissues check if tumorigenic effects of AUF1 are Lu, J.Y., Sadri, N., and Schneider, R.J. (2006). Genes Dev. 20, 3174–3184. is much tighter in humans than in mice, linked to telomerase activation. and laboratory mouse strains have much In summary, the paper discovers an Pont, A.R., Sadri, N., Hsiao, S.J., Smith, S., and longer telomeres. Indeed, when consid- important new direct role of AUF1 in telo- Schneider, R.J. (2012). Mol. Cell 47, this issue, ering life span, body size, and cancer fre- mere maintenance. Dissecting the many 5–15. quency, the selective pressure to evolve functions and regulation of AUF1 in RNA the telomere clock as tumor suppressor stability, transcriptional control, and telo- Shay, J.W., and Wright, W.E. (2011). Semin. Cancer Biol. 21, 349–353. may have been much stronger in humans mere biology in the future may help to (Shay and Wright, 2011). Interestingly, better understand the biological meaning Zucconi, B.E., and Wilson, G.M. (2011). Front. however, AUF1 overexpression has been of putting so many functions under the Biosci. 17, 2307–2325.
2 Molecular Cell 47, July 13, 2012 ª2012 Elsevier Inc.
Previews AUF1/HnRNP D RNA Binding Protein Functions in Telomere Maintenance Liuh-Yow Chen1 and Joachim Lingner1,* 1Swiss Institute for Experimental Cancer Research (ISREC), School of Life Sciences, Frontiers in Genetics National Center of Competence in Research, Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Station 19, 1015 Lausanne, Switzerland *Correspondence: [email protected] http://dx.doi.org/10.1016/j.molcel.2012.06.031
In this issue of Molecular Cell, Pont et al. (2012) show that AUF1/hnRNP D promotes TERT transcription, which is required for telomere maintenance in mice. Telomeres are nucleoprotein complexes at the ends of linear chromosomes that protect DNA ends from DNA degradation, DNA repair activities, and DNA damage checkpoint activation (de Lange, 2009). Most of the telomeric DNA is replicated by semiconservative DNA replication, but the very last bit requires telomerase for stable maintenance. For this, the telomerase reverse transcriptase (TERT) reverse transcribes in an iterative fashion the template region of its tightly associated RNA moiety, adding short telomeric repeats to the ends of chromosomes (Blackburn et al., 2006). In humans and other long-lived animals, telomerase is expressed ubiquitously only early in embryogenesis and then is transcriptionally repressed later during development in many tissues for the rest of life, thus turning on the telomere clock (Artandi and DePinho, 2010; Shay and Wright, 2011). The telomere clock may on one side repress tumorigenesis by limiting the replicative potential of precancerous cells whose cell-cycle control has gone awry. On the other side, short telomeres promote genome instability, and they accumulate during aging. Short telomeres are also causative of telomere diseases in which patients suffer from tissue damage and accelerated aging and die prematurely, for example, due to bone marrow failure as seen in dyskeratosis congenita (Dokal, 2011). Correct regulation of telomerase at chromosome ends involves multiple telomeric proteins and possibly RNAs, topics that are subject to intensive investigations. In addition, in humans TERT but not other telomerase components is tightly regulated at the level of transcription. Despite the importance of the latter, surprisingly little is known
about the regulation of TERT expression. In this issue of Molecular Cell, Pont et al. (2012) report that mice with AUF1 deficiency suffer from telomere erosion and premature aging, which increases with successive generations, thus resembling the phenotype seen upon deletion of telomerase. The authors generated heterozygous Auf1+/ mice, which they intercrossed for seven generations. Interestingly, Auf1 is haploinsufficient. The viability of Auf1/ offspring from Auf1+/ parents decreased with successive generations, demonstrating genetic anticipation. The Auf1/ double mutant animals could not be intercrossed. Auf1/ animals had decreased telomere lengths in late generations and showed the phenotype typically seen for telomerase-deficient mice, such as accumulation of senescent cells, tissue atrophy, small testes and uterus, hunchback, and low body fat. Consistent with telomere loss, Auf1/ cells also showed DNA damage signaling from the telomeres. How does AUF1 promote telomere maintenance? Intriguingly, TERT mRNA levels were >20-fold reduced in the MEFs derived from the KO animals. Furthermore, AUF1 stimulated reporter gene expression driven from an mTert promoter construct. and AUF1 was detected by chromatin immunoprecipitation at the endogenous Tert promoter. This suggests that AUF1 counteracts telomere shortening by stimulating mTERT expression. Is the promotion of mTert expression the only mechanism by which AUF1 promotes telomere maintenance and function? In support of this notion, telomere shortening in Auf1/ MEFs was rescued upon ectopic expression of mTert. However, Auf1 deletion in the mouse gives
a more severe phenotype than what is seen upon telomerase loss only (Lu et al., 2006). The Auf1/ animals cannot be intercrossed (Pont et al., 2012), whereas telomerase KO mice can be intercrossed for approximately up to six generations (Blasco et al., 1997). Indeed, AUF1 is involved multiple biological processes (Figure 1). It is not unanticipated that AUF1 can function as a transcriptional activator. When assembling with the LR1 DNA-binding complex, AUF1 activates the Epstein-Barr virus promoter (Zucconi and Wilson, 2011). More established, however, are AUF1’s roles in promoting rapid decay of possibly hundreds of mRNAs through direct binding of AUrich elements in their 30 untranslated regions, upon which AUF1 nucleates assembly of RNA degrading enzymes whose identity is not fully revealed. Destabilized mRNAs include regulators of the cell cycle and proinflammatory cytokines (Figure 1). Such factors have been linked to telomere dysfunction, and their misregulation could contribute to the phenotype. Finally, AUF1 has been described to bind single-stranded telomeric DNA and telomeric repeat-containing RNA (TERRA), a long noncoding RNA expressed at telomeres in eukaryotes (Feuerhahn et al., 2010). Thus, AUF1 may play additional direct roles at telomeres, which might contribute to the described phenotype. The dissection of AUF1 functions will be challenging. Interestingly, the Auf1-locus specifies the synthesis of four protein isoforms, p37 AUF1, p40 AUF1, p42 AUF1, and p45 AUF1, due to alternative splicing of premRNAs. All four proteins have in common two RNA recognition motifs (RRM) followed by a Gln-rich domain, but they
Molecular Cell 47, July 13, 2012 ª2012 Elsevier Inc. 1
Molecular Cell
Previews
slightly differ outside these control of one genetic locus. domains (Zucconi and WilThis all could make sense. son, 2011). The low TERT Through linking telomerase mRNA levels in Auf1/ activation to suppressing MEFs were rescued upon coinflammatory response, AUF1 expression of p42 AUF1 and may hit two flies with one swat. p45 AUF1, while the other isoforms did not contribute. Thus, establishment of geREFERENCES netic mouse models that are Artandi, S.E., and DePinho, R.A. defective in expression of in(2010). Carcinogenesis 31, 9–18. dividual AUF1 isoforms may reduce the complexity of the Blackburn, E.H., Greider, C.W., and Szostak, J.W. (2006). Nat. Med. 12, phenotype and more clearly 1133–1138. delineate specific functions. It could also be tried to clearly Blasco, M.A., Lee, H.W., Hande, M.P., Samper, E., Lansdorp, P.M., separate the telomeraseDePinho, R.A., and Greider, C.W. independent dysfunctions in (1997). Cell 91, 25–34. / Figure 1. Multiple Biological Functions of AUF1/HnRNP D animals by forced Auf1 AUF1 promotes decay of multiple mRNAs involved in various biological de Lange, T. (2009). Science 326, expression of Tert. processes with links to cancer and aging. Pont et al. (2012) discover that 948–952. Another important question AUF1 is required for telomere maintenance in the mouse through stimulation of TERT transcription. Dokal, I. (2011). Hematology (Am Soc is whether AUF1 function for Hematol Educ Program) 2011, telomere maintenance is con480–486. served in humans. Telomere biology between these mammals has re- linked to malignancy in mice and humans Feuerhahn, S., Iglesias, N., Panza, A., Porro, A., markably diverged in evolution. In partic- (Zucconi and Wilson, 2011), and given and Lingner, J. (2010). FEBS Lett. 584, 3812– 3818. ular, it seems that the transcriptional re- the new findings, it will be important to pression of TERT in differentiated tissues check if tumorigenic effects of AUF1 are Lu, J.Y., Sadri, N., and Schneider, R.J. (2006). Genes Dev. 20, 3174–3184. is much tighter in humans than in mice, linked to telomerase activation. and laboratory mouse strains have much In summary, the paper discovers an Pont, A.R., Sadri, N., Hsiao, S.J., Smith, S., and longer telomeres. Indeed, when consid- important new direct role of AUF1 in telo- Schneider, R.J. (2012). Mol. Cell 47, this issue, ering life span, body size, and cancer fre- mere maintenance. Dissecting the many 5–15. quency, the selective pressure to evolve functions and regulation of AUF1 in RNA the telomere clock as tumor suppressor stability, transcriptional control, and telo- Shay, J.W., and Wright, W.E. (2011). Semin. Cancer Biol. 21, 349–353. may have been much stronger in humans mere biology in the future may help to (Shay and Wright, 2011). Interestingly, better understand the biological meaning Zucconi, B.E., and Wilson, G.M. (2011). Front. however, AUF1 overexpression has been of putting so many functions under the Biosci. 17, 2307–2325.
2 Molecular Cell 47, July 13, 2012 ª2012 Elsevier Inc.