Hallmarks of Aging – Role of Glycation Part 2: Genomic Instability
by Nicholas Pokoluk
We have set the stage, in the first post of this series, for discussions of the role of glycation in the aging process by virtue of it being identified in each of the so called “hallmarks of aging” (1). The list of nine hallmarks are suggested to cover the ultimate pathological dysfunctions that cause the decline, and eventual death, of a human being. Here we will look at glycation’s role in genomic instability, hallmark number one.
Genomic stability, in an admittedly oversimplified definition, is the ability of cells to maintain the integrity of the DNA by various mechanisms of repair of structural defects and by ridding the cell of excessively damaged nuclear material. These mechanisms exist in all somatic cells. In the life of a eukaryote (an organism that has complex cells in which the genetic material is organized into a membrane-bound nucleus) DNA replication is meant to duplicate the genome. However, DNA damage is very common and can come from exogenous or endogenous forces. Every moment of our lives there is damage done to our DNA but when working properly it can be offset by the repair system correcting impacted DNA. When the DNA damage frequency is more then the repair mechanism can handle or the repair mechanism itself is compromised in some significant way this will lead to “genomic instability”. The mutations that ensue and go unchecked may cause dysfunction at the genetic, cellular, tissue, organ and systemic levels resulting in the manifestation of disease or what we see as physiological aging processes.
Genomic instability can manifest in several ways such as cell death (apoptosis), enhanced autophagy upsetting cellular homeostasis, or cellular senescence – the irreversible cessation of cell division.
So how is glycation involved in promoting geonmic instability? Firstly, the glycation process can inflict direct damage on DNA by reaction at its amine functionality. Although there may be several mechanisms that end in a glycated DNA the role of products of glycolysis are getting significant attention in underlying this process. During glycolysis (metabolism of carbohydrates) a major precursor of Advanced Glycation End Products (AGEs), methylglyoxal (MG), is formed. MG is a dicarbonyl compound that can inhibit mitochondrial respiration and other physiological pathways and induce cell death and produce an increase in reactive oxygen species. All reducing sugars can participate in the glycation reaction but MG is far more reactive than glucose or fructose and even greater than the highly reactive ribose in creating (AGEs). MG can react with free amino groups of proteins forming AGEs but also can react with amino groups of DNA to form DNA-AGEs. These DNA-AGEs have been shown to be genotoxic and also immuniogenetic.(2).
As mentioned, the type of DNA damage that gives rise to the genomic instability are many and can include replications errors, point mutations, chromosomal changes (gains and losses), telomere length shortening among others. Glycation of DNA can alter its structure by leading to partial unwinding and/or fragmentation of the double helix. This alone can cause significant pathological results. In addition, the oxidation caused during the formation of the DNA-AGE can also lead to reduced gene expression. Recently DNA-AGEs have been identified in human cancer tumors and also in Alzheimer’s disease. It is also know that these DNA-AGEs are involved in the natural history of many disease states such as diabetes. (3)
In addition to nuclear DNA, it is also found in some organelles including mitochondria. Mutations in mitochondria DNA (mtDNA) can also cause disease and associated aging phenomenon. The glycation of mitochondrial mtDNA can result in the same types of damage that nuclear DNA suffers. The extent of damage seems to be related to the glycating agent with methylglyoxal again being identified as the most potent culprit. In studies in our lab I have used primarily glucose as the in vitro glycating agent for several reasons. These includes easy of use, stability, and simply because a majority of research has used this molecule. However, MG is by far a more potent in vivo glycating agent and is estimated to be about 50,000 times more potent than glucose on mtDNA. (4) Not only can mtDNA glycation disrupt genome integrity but also it can alter gene expression. In states of hyperglycemia, the level of glucose derived MG increases significantly and can diffuse across organelle membranes such as those of mitochondria and damage mtDNA. MtDNA can become even more susceptible to damage than nuclear DNA since it does not have a degree of protection from histone proteins as does nuclear DNA(5) It is not only endogenously produced glycation that can cause mtDNA damage but also exogenously produced AGEs and their fragments may do the same. It has been shown that exogenous AGEs can also cause DNA damage since incubation of cells with these AGEs induces mitochondrial DNA malfunctioning. (6)
There is another phenomena that will come up again as we discuss the damage caused by glycation and that is the role of glycated protein aggregates within the cell causing cellular dysfunctions. In a study by Kahn (7) results indicated damaged glycated proteins induced intracellular aggregates that can affect the genome. The mechanisms within cells to rid the cell of damaged proteins can in itself be affected by glycation and give rise to cellular “garbage” being accumulated as seen in these protein aggregates. It is these glycated protein fragments and their aggregates that are now known to be genotoxic.
As one can clearly see there is a significant involvement of glycation with the genome and also mtDNA. This glycation damage comes from both endogenous glycation processes, led by the formation of such very reactive molecular species such as methylglyoxal, and also from exogenously derived AGEs coming from the diet. In addition, intercellular glycated protein fragments can aggregate to promote a genotoxic environment. All this would argue that within the hallmark of genomic instability glycation is a prominent factor.
In our next post we will look at the role of glycation in another hallmark of aging – telomere attrition.
- Lopez-Otin, C. Et al, The Hallmarks of Aging, 2013, Cell 152, 1194-1217
- Ahmad, S., et al, Genotocicity and immunogenicity of DNA-advanced glycation end products formed by methylglyoxal and lysine in the presence of Cu2+, 2011, Biochem Biophys Res Commun Apr 15;407(3) 568-74
- Ahmad, S. et al, Glycation of biological macromolecules: A critical approach to halt the menace of glycation. 2014, Glycobiology, 1-12.
- Rabbani, N., et al, Dicarbonyls linked to damage in the powerhouse: glycation of mitochondrial proteins and oxidative stress, 2008, Biochemical Society Transactions, vol 36 no , pp 1045-1050
- Li Boon, PB, Murphy, MP, Pathological Significance of Mitochondrial Glycation, 2012, International Journal of Cell Biology, ID 843505
- Lo, MC, et al, Glycoxidative stress-induced mitophagy mitigates mitochondria fates, 2010, Annals of New York Academy of Sciences vol 1201, pp 1-7
- Kahn, TA, et al, Glycation promotes the formation of genotoxic aggregates in glucose oxidase, 2011, Amino Acids. 3(3):1311-22.