DNA shape-change causes ALS

A mutation linked to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) changes the shape of DNA, making cells more vulnerable to stress and likely to die.

An alteration on the gene C9orf72 – located on the ninth human chromosome – is thought to cause the build up toxic RNA and stunt protein manufacture, now researchers from Johns Hopkins University (JHU) say the root of the problem is defective genetic material at the mutation site.

The mutation is a hexanucleotide repeat expansion – a six-letter ‘word’ of DNA repeated over and over in part of the gene not containing instructions for making proteins. Although anywhere up to 20 repeats is normal, some people with ALS or FTD can have hundreds.

Using sophisticated molecular techniques, Dr Jiou Wang and colleagues JHU studied the structure of the DNA making up the gene and the RNA that carries its instructions. Instead of forming a helix, the mutation causes the repeated sequences to fold into G-quadruplexes, stacks of square shaped molecules known as G-quartets.

This has a profound effect on RNA, which forms hairpins and bulges in the repeating sections. These R-loops and G-quadruplexes produce shorter transcription products compared with controls, which result in abnormal functioning of the cell and can lead to cell death.

“Unfortunately, these alternative DNA arrangements impede normal processing, much like a car encountering a series of speed bumps or the occasional roadblock while travelling to its destination,” said Wang.

The C9orf72 mutation has an effect in the nucleolus, which plays a key role in directing the cell’s response to stress. Binding of short transcripts to nucleolin – a protein involved in making ribosomes – has a toxic effect on the cell.

Researchers used induced pluripotent stem cells (iPSC) containing the C9orf72 mutation derived from the skin of ALS patients, which they turned into motor neurons. In healthy cells, nucleolin is present only in a certain area of the nucleus but is scattered throughout the nucleus in cells from ALS patients. Researchers predict this abnormal distribution caused cells to become stressed and more likely to die, resulting in the pathology associated with ALS and FTD.

“Understanding the biology of C9orf72 is fundamentally important as we work with pharmaceutical companies to develop therapies for these highly devastating diseases,” said Jeffrey D Rothstein, co-author of the paper published in Nature.

C9orf72 nucleotide repeat structures initiate molecular cascades of disease