The DBAF is reaching out to our families and friends to help us to grow and to fulfill our mission. Your gifts of time, talents, and treasure are appreciated and necessary for our continued success.

Our Facebook page posts DBA facts written by DBA nurse, Ellen Muir, RN, MSN, CPON. We are pleased to share these facts with our patients and families. Thanks, Ellen! 

 
We have been working closely with an adult endocrinologist, Dr. Irwin Klein, at the Feinstein Institute for Medical Research, who has done a lot of research studying heart disease in relation to thyroid dysfunction. Being that DBA patients who are chronically transfused have thyroid issues due to iron overload, he has taken an interest in working with us to prevent thyroid disease as well as cardiac failure due to thyroid dysfunction. As we know, other endocrine organs are also affected - pancreas, gonads, pituitary, as well as linear height. Here is a list of recommended labs to monitor and prevent the devastating effects of iron overload in the thyroid, heart, and the effects of diabetes: 

total T3                                                                                                                                  

total T4                                                                                                                  

TSH                                                                                                                                       

T3 uptake (instead of free T4)                                                                         

IGF-1(monitors acute fluctuations in insulin action and determines inadequate insulin treatment or poor control of dietary intake)                                                                 NT-proBNP (aids in diagnosis of left ventricular dysfunction in heart failure)

Antithyroid Abs (Antithyroglobulin and AntiThyroperoxidase)   

Fructosamine (useful in situations where the A1C cannot be reliably measured - as with transfused persons)

Vitamin D             

Any questions, please feel free to e-mail me emuir@nshs.edu or call

1-877-DBA-NURSe (322-6877).   

Steven R. Ellis, PhD Research Director

This month's Journal Club is not a Journal Club per se, as no single manuscript prompted this discourse. Instead, I will use this space to expand on a subject that has been raised before on numerous occasions... p53. I've mentioned p53 in the context of how haploinsufficiency for ribosomal proteins leads to its activation, and how this could account, in part, for the premature death of erythroid progenitors in the marrow of DBA patients (DBAF newsletters; November 2010, April 2011).

What then is p53?

Let us begin with the name. p53... rather dull, don't you think? The p of p53 stands for protein; so alas, p53 is a protein. Proteins in turn are polymers of amino acids that fold into complex three dimensional structures that are capable of a wide range of functions. These functions include antibodies of the immune system, the light harvesting proteins of vision, proteins involved in muscle contraction, thousands of enzymes involved in intermediary metabolism, ribosomal proteins, and the list goes on and on. Proteins are capable of a wide range of functions because of the astronomically large number of structures they can assume. These structures are ultimately dictated by the order of 20 common amino acids along the length of a protein as they are polymerized during protein synthesis. The order of amino acids in a protein is dictated by the gene encoding that protein. The number of amino acids which comprise a protein brings me to the 53 in p53's name. The 53 refers to p53's size, or at least its apparent size, of 53,000 Daltons when analyzed by a routine laboratory gel electrophoresis technique. It is composed of 393 amino acids. This size is not unduly large or small for a protein, and so from a size perspective, p53 is fairly average. Despite its fairly average size there is really nothing average at all about what p53 does. To give you a feel for just how important p53 is, a search for p53 in the scientific literature compiled in PubMed gave 59,781 manuscripts that touched on this protein. For a frame of reference, a similar search for Diamond Blackfan anemia gave 578 hits.  

So, what is the function of p53 within a cell? One of the major functions for p53 is as a transcription factor. Transcription is the process by which a sequence of bases in DNA (genes) is converted to a sequence of bases in RNA. RNA then dictates the sequence of amino acids in a protein, through a process known as translation. Transcription factors play a central role in turning genes on and off in response to a vast array of physiological, developmental, and environmental signals ¹. The p53 protein gets activated through a variety of signals emanating from various cell stressors including DNA damage, oxidative stress, oncogenic stress, and yes, now even ribosome stress. In response to these stress signals, p53 stimulates the expression of a number of genes, some of which arrest cell division and others of which may function to resolve the stress imposed on the cell. If the stress is too great for the cell to resolve, p53 may also initiate a cell death program. In responding to oncogenic (or cancer promoting) stresses, one can think of the p53 response as putting the brakes on cancer. As such, p53 is classified as a tumor suppressor. It should come as no surprise therefore that p53 is frequently inactivated in human cancers.

So having p53 around is a good thing right? Well yes... and no. Activation of p53 in response to DNA damage or oncogenic stress protects against cancer. But, it has also been proposed that activation of p53 in response to oxidative stress plays a role in aging ², and as mentioned above, activation of p53 in response to ribosome stress likely contributes to the pathophysiology of  

DBA ³. Consequently, just as in the Goldilocks story, p53 has to be maintained at levels that are just right... responding to appropriate signals in its protective role while at the same time being held in check to temper its contribution to pathogenic states.

To maintain p53 at levels that are "just right" while still allowing p53 to fulfill its critical functions, this remarkable protein is shackled with numerous levels of control that often have layers of redundancy. One such level of control is mediated by the protein MDM2, which modifies p53 with a molecule known as ubiquitin. Ubiquitinated p53 is then directed to cellular garbage disposals where p53 is degraded ⁴. The action of MDM2 on p53 keeps p53 levels relatively low in normal cells that are not exposed to stress. While making and degrading p53 under non-stressful conditions may seem wasteful, this control strategy allows for a rapid response in times of stress. For example, signals resulting from DNA damage disrupt the MDM2/p53 interaction, so p53 is no longer labeled for degradation allowing p53 levels to quickly rise in response to this environmental insult.

The action of MDM2 however, represents only one level of control of p53 function. In addition to ubiquitination, p53 is subject to a large number of chemical modifications that also regulate its function. These modifications include phosphorylation, acetylation, methylation, sumoylation, and neddylation (yes, neddylation) ¹. A conservative estimate in a recent publication places the number of confirmed modifications of p53 at 52 ⁵. These various modifications superimpose additional, sometimes overlapping, layers of control on p53 and allow its function to be fine tuned to cellular demands. By dissecting the different signaling pathways that direct these various modifications it may be possible to develop drugs that target a specific pathway of p53 activation while leaving others essentially intact. This approach may allow the development of treatments for pathologies linked to inappropriate activation of p53 without having an unduly high risk of promoting cancer.

By targeting the enzymes that modify p53, the DBA community could tap into a very robust area of drug development. Drugs targeting enzymes that carry out protein phosphoryation have transformed certain deadly cancers to chronic diseases ⁶. Less specific drugs targeting protein acetylation and methylation have also shown promise in cancer clinical trials with the hope that these drugs will show improved success as they are refined to target more specific modifying enzymes ⁷.

CONCLUSION

By learning more about the signaling pathway from ribosome stress to p53 activation, it may be possible to specifically target this pathway, while leaving other pathways to p53 activation largely intact, thereby reducing the cancer risks associated with targeting p53 activation as a possible therapy for DBA.       

1.     Brooks CL, Gu W. New insights into p53 activation. Cell Res;20:614-621.

2.     Yi J, Luo J. SIRT1 and p53, effect on cancer, senescence and beyond. Biochim Biophys Acta;1804:1684-1689.

3.     Dutt S, Narla A, Lin K, et al. Haploinsufficiency for ribosomal protein genes causes selective activation of p53 in human erythroid progenitor cells. Blood;117:2567-2576.

4.     Vucic D, Dixit VM, Wertz IE. Ubiquitylation in apoptosis: a post-translational modification at the edge of life and death. Nat Rev Mol Cell Biol;12:439-452.

5.     Dai C, Gu W. p53 post-translational modification: deregulated in tumorigenesis. Trends Mol Med;16:528-536.

6.      Santos FP, Quintas-Cardama A. New drugs for chronic myelogenous leukemia. Curr Hematol Malig Rep;6:96-103.

7.     Wanczyk M, Roszczenko K, Marcinkiewicz K, Bojarczuk K, Kowara M, Winiarska M. HDACi--going through the mechanisms. Front Biosci;16:340-359.