By Dean Haycock
Special To BioWorld Today
Technological advances first put typesetters on the endangered list and in time, made them extinct. In this age of computers, their work now seems quaint. A typesetter like Ben Franklin picked up a small block engraved with an single letter and placed it next to others to form words, sentences and paragraphs one letter at a time. If he picked up the wrong letter and didn¿t notice it, the final product would have a misspelled word.
The same thing would happen if Franklin failed to notice that a letter broke as he picked it up. If the damaged letter looked like another letter, the result would also be a misspelled word.
An analogous process may produce aberrant proteins, even in cells that are not replicating their DNA and dividing. The phenomenon does not involve a mutation that is propagated from parent to daughter cell, although most people immediately think of that scheme when aberrant proteins are implicated in a disease process. It is true that DNA, the master genetic molecule, contains the genes that encode proteins. It is also true that mutations in the original blueprint are sure to lead to abnormal proteins. But the traditional link between mutant genes and mutant proteins can not account for the appearance of aberrant proteins in cells that that lack mutations in the gene encoding the affected proteins.
Increasing Scrutiny¿ Likely In Future
A process that generates mutant proteins without involving mutant genes inherited from parent cells is called ¿transcriptional bypass.¿ In one case, it involves a typesetting error similar to that Franklin would have made had he picked up a ¿C,¿ somehow mangled it into the shape of a ¿U¿ without noticing and inserted it where ¿C¿ belonged.
This substitution can be readily demonstrated in test-tube experiments that make use of the four-letter DNA alphabet that is used to encode entire genomes: T for thymine; A for adenine; G for guanine; and C for cytosine. The letter U stands for uracil, a building block found in RNA. The error in ¿transcriptional bypass¿ occurs when the DNA code is being transcribed into messenger RNA (mRNA), a step which must occur before it can be translated into a protein.
In biological systems, the ¿break¿ in the letter C is really the loss of one of its chemical components, which biochemists refer to as cytosine deamination. What is left is uracil, U. To the protein-synthesizing machinery, uracil looks more like thymine than cytosine. Where the C was meant to be, there is now, effectively, a T.
In many cases where a transcription error occurs, specialized molecules detect it and correct or eliminate it.
Since U, however, looks a lot like T to this repair system, the U often goes undetected and protein synthesis proceeds to produce multiple copies of an abnormal protein. Of course, the effects would be the same if the gene encoding the protein had a mutation common to its parent cell. The negative consequences of generating multiple copies of an abnormal protein might be just as harmful, and could be expected to occur even in cells that are not growing and dividing.
Because aberrant proteins have been detected in the brains of Alzheimer¿s disease patients in the absence of mutant genes encoding the proteins, transcriptional bypass has been suggested as a contributing factor in the pathophysiology of this dementia. That remains conjecture. The only data concerning the phenomenon came from in vitro systems. Transcriptional bypass hadn¿t been demonstrated in a living cell. Until now. And the clinical implications of this demonstration, according to Bryn Bridges of the University of Sussex, Falmer, in Brighton, U.K., ¿should ensure that this field comes under increasing scrutiny in the future.¿
The work is described in ¿Phenotypic Change Caused by Transcriptional Bypass of Uracil in Nondividing Cells,¿ an article in the April 2 issue of Science by Anand Viswanathan, Ho Jin You, and Paul Doetsch of the Emory University School of Medicine, in Atlanta. The authors demonstrated transcriptional bypass in non-dividing, but very much alive, E. coli bacteria.
¿This is basically a proof of principle¿ that a cell does not have to be dividing in order to manifest the deleterious physiological consequences of genetic damage,¿ Doetsch, a professor of biochemistry and radiation oncology, told BioWorld Today. ¿We show that it is possible for a non-dividing cell to suffer some genetic damage where previously it had no mutations in any gene, and express that damage as a mutant protein, as the cell¿s gene expression machinery attempts to simply make the proteins necessary for normal housekeeping functions.¿
Why Non-Dividers Have DNA Repair Enzymes
Doetsch and his collaborators showed that the uracil substitution could occur in vivo by using a ¿reporter¿ to indicate the effects of DNA damage on living cells. The reporter was luciferase, a protein that emits light under defined conditions.
¿If we can demonstrate that this happens in luciferase, it is certainly very likely that this goes on in non-dividing cells,¿ Doetsch said. ¿You pick the critical function that could go wrong, and you pick the deleterious biological consequences.¿
The senior author sees important implications for age-related diseases. ¿You could name a lot of them, particularly neurological diseases like Alzheimer¿s disease, where we know that neurons are not dividing,¿ Doetsch said. ¿As time goes on and perhaps they accumulate unrepaired DNA damage, for example oxidative DNA damage, it is certainly possible that these kind of damages could be expressed, and the result could be a mutant protein. The bottom line is the cell never has to divide to express the damage.¿
The results also offer an explanation for why non-dividing cells still produce DNA repair enzymes. ¿Why are they there?¿ Doetsch said. ¿Well, it is probably because they need to repair those damages, which can cause the mutations at the level of transcription.¿
The Emory University team now wants to see how widespread is this phenomenon, by studying different kinds of DNA damage in non-dividing cells.