Wednesday, 15 July 2009

Non-B DNA structures and disease

People seem to have started contributing actively to the blog again so I thought it was my turn to try again. And this is an achievement, as facing any science, in particular anything written by me about it, would have been impossible a month ago.

I decided to keep things simple for this one and just give a short introduction on the theme of non-B DNA structures and how they influence disease, the topic of my open essay.

When we think about DNA, we think about its B-DNA structure, the one that Watson and Crick described in that famous Nature paper more than 50 years ago, i.e. a right-handed double helix, 20Å diameter, in which the base pairs are almost perpendicular to the axis of the helix, 10.5bps per turn. Although most DNA will be in this structure, it can also form other structures, the so called non-B DNA structures. Some of these are shown in the figure. These alternative structures have a few common characteristics: firstly, whether or not a structure is formed, and which specific structure, seems to be sequence dependent, as it often involves formation of new pairings of bases. Secondly, most of these structures are in higher energetic state than normal B-DNA, for example, because they require the separation and reformation of hydrogen bonds. Overall, therefore, DNA with a favourable sequence tends to remain in its B-DNA form, requiring events such as DNA replication, transcription or protein binding to be transiently converted to these unusual structures. Sequences with propensity to form unusual structures are quite common in the human genome, and it seems that in some cases they are necessary for the normal functioning of the cell. However, they have also been implicated in disease, and this is what I’ll be focusing till the end of this post.

There is a considerable list of diseases thought to involve, as part of their pathology, the formation of non-B DNA structures, but the mechanisms by which this is though to happen can vary. For example, certain sequences/structures have been thought to cause genomic instability, e.g. by causing chromosomal rearrangements due to the propensity of these structures to promote double strand breaks. Another interesting form of genomic instability associated with these unusual structures is repeat expansion, implicated in motor diseases such as Huntington’s disease. Non-B DNA structures, namely Z-DNA, have also been associated with viral infections.

Now, there isn’t really enough space here to talk about everything, so I’m going to give one examples of a situation in which a non-B DNA structure is though to be involved in disease. Friedreich ataxia (FRDA) is a disease caused by the expansion of GAA•TTC tracts in intron 1 of a gene encoding the protein frataxin, essential for mitochondrial function. In this disease, repeat expansion is associated with loss of protein expression. One of the current models by which frataxin production is thought to be reduced in expanded GAA•TTC repeats is based on their ability to form triplexes (an alternative model suggests that epigenetic changes may also be important). Duplex opening within the repeated region, due to the passage of RNA polymerase, is thought to allow one of the separated single strands to form Hoogsteen hydrogen bonds with the purine strand of a B-DNA duplex within the same repeated sequence. This leads to the formation of a 3-stranded helix. Its formation on the non-template GAA strand in frataxin probably interferes with RNA polymerase progression. The free template strand is then thought to base pair with the newly synthesised RNA transcript, forming stable RNA/DNA dimers and preventing further transcription.

Now, this is only a model, for a specific type of structure within the context of a specific disease. And although different sequences have been shown to form these unusual structures, and these structures to be somehow associated with specific diseases, to neatly demonstrate how exactly one influences the other seems to be quite hard. This is particularly difficult because different groups often use different model organisms or protocols, leading to sometimes contradictory results. Overall, although I liked writing about this topic, since I had never considered how the structure of DNA could have an impact on disease, I got the feeling that a lot of work still needs to be done before convincing evidence is given for how exactly these structures impact on our wellbeing, and what sort of therapeutics can be developed from this knowledge.

Red, GAA repeat strand; blue, GTT complementary strand; orange, RNA transcript; yellow, RNA polymerase. 1, Intramolecular triplex; 2, Stalled RNA polymerase; 3, DNA/RNA hybrid

Hebert, M. (2008). Targeting the gene in Friedreich ataxia Biochimie, 90 (8), 1131-1139 DOI: 10.1016/j.biochi.2007.12.005

Bacolla, A. (2004). Non-B DNA Conformations, Genomic Rearrangements, and Human Disease Journal of Biological Chemistry, 279 (46), 47411-47414 DOI: 10.1074/jbc.R400028200

WELLS, R. (2007). Non-B DNA conformations, mutagenesis and disease Trends in Biochemical Sciences, 32 (6), 271-278 DOI: 10.1016/j.tibs.2007.04.003

Wang, G., & Vasquez, K. (2009). Models for chromosomal replication-independent non-B DNA structure-induced genetic instability Molecular Carcinogenesis, 48 (4), 286-298 DOI: 10.1002/mc.20508