Scientists have discovered how mutations in DNA can cause neurodegenerative disease.
The discovery is an important step towards better treatment to slow the progression or delay onset in a range of incurable diseases such as Huntington’s and motor neurone disease – possibly through the use, in new ways, of existing anti-inflammatory drugs.
The team of scientists has shown experimentally, for the first time, how mutations ultimately set off an anti-viral like inflammatory response in cells that leads to cell death and, over time, progressive neurological damage.
Led by the University of Adelaide, the study published in Human Molecular Genetics is the culmination of over a decade of research with researchers at the Victor Chang Research Institute in Sydney, seeking to understand how DNA mutations result in neurological damage.
But it also may have implications for the progression of neurodegenerative diseases which aren’t necessarily inherited, such as Alzheimer’s and Parkinson’s, which evidence suggests are caused by a similar inflammatory response to environmental triggers.
“Together these conditions affect millions of families worldwide, and there are no cures or effective treatments,” says project leader Rob Richards, Professor of Genetics in the University of Adelaide’s School of Biological Sciences.
“If the new mechanism we have discovered proves to be correct, it will transform the field, providing a different way of thinking about these diseases and offering new opportunities for medical intervention.”
The so-called ‘DNA repeat diseases’ – named because of the repeat sequences found in the DNA of patients – share many common features in their symptoms, but the mechanisms by which symptoms arise have previously been thought to be different for each.
“We’ve known what mutations are involved for some years, and the set of outcomes that result, but, until now, we’ve not known how one leads to the other.
This new research shows us how each of these diseases can be caused by the same underlying cellular pathway.”
The study results centre around RNA, the molecule in our cells which is the intermediate step between the DNA in chromosomes and the proteins that are the cells’ main functional components.
The DNA provides a blueprint for producing RNA that is then normally ‘bar-coded’ to ensure cells recognise it as “self”, distinguishing it from the RNA of a foreign invader, such as viruses.
Using the experimental model fly Drosophila, Professor Richards and his team showed that the affected, ‘double-stranded RNA’ was instead recognised as foreign to the body, or “non-self”.
“This elicits an anti-viral like, auto-inflammatory response that leads to neuronal destruction and death, in time causing progressive neurological damage,” says Professor Richards.
“The abnormal RNA is made from regions of repeated DNA sequences that are found in greater numbers in people affected with Huntington’s and some other neurodegenerative diseases.”
Professor Richards says there are existing drugs for other types of auto-inflammatory disease, which may prove to be effective in treating the symptoms of these diseases, by inhibiting the anti-viral inflammatory response.
Neurodegeneration is the overarching term for medical conditions with progressive failure of neuronal networks and eventually the death of neurons participating in motor, sensory, and cognitive functions.
Neurodegenerative disorders (NDs), at least including Parkinson’s disease (PD)1, Huntington’s disease (HD)2, Alzheimer’s disease (AD)3, and so on, are complex and multifactorial diseases that threaten human health and have no specific diagnostic tests or effective therapies.
Since the number of cases is rapidly growing worldwide and the World Health Organization predicts that NDs will overtake cancer in the rank of top causes of death by 20504, the pressure on social–economic and the financial burden of medical care system propel governments to develop policies to counteract the impact. Pathophysiologically, there are several mechanisms underlying NDs, including an excessive abnormal structural aggregation-prone proteins accumulation5; impaired ubiquitin–proteasome and/or autophagy–lysosomal pathways6; apoptosis and autophagy7; glutamate transporters8; calcium, free radicals, and mitochondria9; and so on.
These versatile mechanisms suggest that NDs are caused by a complex interplay of many genetic factors, each of which acts individually or symphonically to lead to clinical features.
Disrupting gene expression is a common approach to investigating the functions of these genes. Classical strategies to assess the functions of these genes include RNA interference (RNAi) and homologous recombination (HR). RNAi is a rapid, inexpensive, and high-throughput method to knock down a specific gene10.
RNAi was the “gold standard” for gene silencing and studying gene function in vitro and in vivo in the past15.
However, several drawbacks regarding this technique include the following: (1) this technique is difficult to transfect multiple genes to a cell or animal in vitro or a gene to an adult animal in vivo; (2) effects of the mutant-selective RNAi targeting single nucleotide may be variable, incomplete, and temporary in different experiments and laboratories; (3) RNAi cannot generate stable gene knockouts or site-specific epigenetic modifications; and (4) this technique may produce unpredictable off-target effects16. These defects may restrict the use of RNAi in the clinical practice.
HR in mouse embryonic stem cells is a common and popular method of building up genetically modified animals for modeling human diseases.
However, the drawbacks of this technique include the following: (1) HR is time- and labor-consuming, (2) HR is of low efficiency, and (3) HR has the potential for unwanted mutagenic effects17.
Recent advances in gene-editing techniques including zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats–associated nucleases (CRISPR/CAS) can accelerate the pace of biological research, generate sets of gene-related disease models, and provide potential therapies targeting the incurable diseases. This review will focus on the clinical features of three distinct NDs: PD, HD, and AD, and the applications of gene-editing technology on these three debilitating disea
Principles of Gene-Editing Tools
Gene editing is a chimera of specific DNA-binding domains (DBDs) and nonspecific DNA cleavage domains (DCDs). DBDs enable efficient and precise-targeting sequence binding. DCDs, like genomic scissors, cleave the targeted DNA site to produce a double-strand break (DSB), which consequently stimulates the cellular DNA repair mechanisms including error-prone nonhomologous end joining (NHEJ) and homology-directed repair (HDR)36.
The HDR searches for homology between the damaged DNA sequence and the sister chromatids, homologous DNA strands, or other related DNA as templates and copies the sequence of the fragment between the 2 broken ends of the damaged sequence fragments to restore the original DNA sequence at DSB sites, regardless of whether the fragment contains the original sequence37.
Based on the machinery, the designed DNA can then be inserted into the targeted cleavage site and NHEJ directly connects the end of the broken strands.
The repair process can be error-prone, resulting in small insertions, deletions, and/or rearrangements41.
NHEJ can also cause frameshifts in the coding sequence of a gene to produce premature truncations, leading to an effective gene knockout.
The status of cycle phase in the target cells determines which DNA repair mechanisms will be initiated. HDR initiates in the synthesis (S) and the premitotic (G2) because sister chromatids are available at these phases42.
NHEJ activates in the growth 1 (G1) and the mitotic (M) phases43. DSB repair mechanism in mammals is mainly through NHEJ44. The repair systems are crucial in the maintenance of genomic integrity and the generation of genetic variability.
More information: Clare L Eyk et al. ‘Non-self’ Mutation: Double-stranded RNA elicits antiviral pathogenic response in a Drosophila model of expanded CAG repeat neurodegenerative diseases, Human Molecular Genetics (2019). DOI: 10.1093/hmg/ddz096
Journal information: Human Molecular Genetics
Provided by University of Adelaide