Autoimmune Diseases of the Brain:

 

AGS

Innate immunity is the body's first line of defence against infection. Recognition of viral and bacterial nucleic acids activates innate immunity. Similar nucleic acids are also important triggers for autoimmunity in lupus and Rheumatoid Arthritis. Aicardi-Goutières syndrome (AGS) is a genetic disorder which provides an important model for understanding the role of nucleic acids in autoimmunity. Our research goal is to study the biochemistry, cell biology and immunology of this condition to understand how it is caused, and to apply these insights to common autoimmune diseases.

 

 

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Research Study Information
Laboratory Publications

 

 

How do Mutant Nucleases Mimic Viral Infection?

Viral nucleic acids are recognised by numerous germline encoded receptors of the innate immune system, including members of the Toll-like and RIG-I-like receptor families (TLRs and RLRs). This rapidly triggers a series of signalling cascades which induce a powerful inflammatory response, most notably characterised by the production of type I interferons, including interferon alpha.High levels of interferon alpha in the brain cause white matter destruction and calcification, as is seen in congenital CMV/Rubella/HIV infections.

 

We hypothesise that mutations in the AGS nucleases, RNase H2 and TREX1, result in accumulation of nucleic acids within the cell, which inappropriately trigger innate immune signalling, resulting in the same white matter destruction and calcification seen in viral infections. Using a Trex1 null mouse model, two other groups have shown that single stranded DNA accumulates in the cytoplasm. This DNA has been suggested to be either a by-product of newly replicated genomic DNA, or arise from increased endogenous retroelement activity. As yet, it is unclear if and if so, what nucleic acids accumulate in RNase H2-deficient cells.

 

 

Our working hypothesis: AGS mimics viral infection as a result of the accumulation of nucleic acids within the cell (1) that then trigger the same innate immune pathways as viral infection (2).

 

Current Work on RNaseH2

 

We want to establish why mutations in the AGS genes mimic CNS viral infections and autoimmunity. To do this, we plan to isolate the nucleic acids accumulating in patient cells (1), and show that these can trigger an innate immune response (2).

 

To date, we have established:

1. The molecular structure of the RNase H2 trimer and the location of 25 out of 29 residues affected in AGS
2. That all AGS mutations cause a decrease in RNase H2 enzyme activity in vivo, either by directly affecting enzyme function or by affecting complex stability
3. That RNase H2 is directed to replication forks through its interaction with PCNA
4. That PCNA promotes the activity of archaeal type 2 RNase H, both on substrates mimicking Okazaki fragments and DNA with incorporated ribonucleotides

Aims

1. Investigate the normal cellular functions of RNase H2.
2. Determine the cellular effects of RNase H2 dysfunction and identify accumulating RNA/DNA substrates that may be present in AGS.
3. Analyse mouse models with reduced RNase H2 activity.
4. Investigate the roles of RNase H2 and its nucleic acid substrates in innate and adaptive immunity.

 

 

Themes

• Disorders of Human Brain Size:
  Primary Microcephaly
  Primordial Dwarfism
  

• Autoimmune Disease of the Brain
  Aicardi Goutieres Syndrome
  

 

 

Aicardi Goutières Syndrome

Aicardi-Goutières syndrome (AGS) is a genetic disorder which resembles congenital infection of the brain.

 

It usually becomes apparent in the first few months of life. Children become irritable, feed poorly and then develop unusual (dystonic) postures, spasticity and fits. These neurological problems progress, resulting in profound physical and intellectual disability. Often there are other symptoms, including enlarged liver and spleen, and intermittent fevers that also falsely suggest viral infection.

 

 

 

 

 

 

 

 

 

 

Identical Neuroimaging findings in AGS and Viral Infection.

 

As can be seen in these CT scans, identical changes are observed in the brains of individuals with Aicardi-Goutières syndrome (left) and those with HIV infection acquired during pregnancy (right). The white areas in the centre of the brain are calcification of the basal ganglia. Similar changes can also occur after CMV and Rubella infections.

 

 

Autoimmunity and AGS

There are clinical and immunological similarities between AGS and Systemic Lupus Erythematosus (SLE). A particularly striking similarity is the vasculitic skin lesions seen in AGS (upper figure). As with SLE, immunoglobulins and complement are deposited at the dermal-epidermal junction (lower figure). Also, certain mutations in one of the AGS genes (TREX1) have been found to cause chilblain lupus, whereas other, mono-allelic mutations are associated with SLE.

 

 

 

 

 

 

 

 

 

 

 

Am J Hum Genet. 2007 80(4):811

 

 

 

 

 

 

 

 

 

 

 

 

 

Nat Genet 38:917

 

The AGS proteins

We have identified four genes for AGS (Nature Genetics 38:910 and Nature Genetics 38:917). These genes encode intracellular nucleases. The first three genes encode the subunits of the heterotrimeric Ribonuclease H2 enzyme, while the fourth encodes the TREX1 exonuclease. Recently a fifth gene, SAMHD1, has been identified by Yanick Crow and colleagues.

 

RNase H2

The RNase H2 complex consists of three subunits: the catalytic RNASEH2A subunit and two accessory subunits (RNASEH2B and C), which are thought to provide complex stability and allow interactions with additional proteins, such as PCNA. The enzyme hydrolyses the RNA strand of RNA:DNA hybrids and can recognise ribonucleotides embedded in DNA, cleaving the 5’ phosphodiester bond. The cellular functions of RNase H2 are not well defined, although its substrate specificity suggests it may have roles in DNA replication and repair. It may have a  function in the removal of RNA primers during lagging strand DNA synthesis (Okazaki fragment maturation): it efficiently processes Okazaki fragments together with FEN1 in vitro, and this activity can be promoted by PCNA. However, the physiological relevance of this has been questioned, as experiments in yeast show that RNase H2 is not essential for this process. More recently, it has been shown that RNase H2 plays a central role in the removal of misincorporated ribonucleotides from genomic DNA both in S. cerevisiae and S. pombe. A better understanding of the physiological functions and cellular substrates of RNase H2 is needed to understand how its dysfunction causes AGS.

 

TREX1

Trex1 is a 3'-5' DNA exonuclease acting on single-stranded DNA, and is involved in DNA degradation during Granzyme A-mediated apoptosis. In addition, Trex1 has been shown to promote HIV replication by inhibiting autointegration and suppressing an interferon response. Trex1 null cells were shown to have increased levels of endogenous retroelement DNA in the cytoplasm, suggesting that these may be a possible source of immunogenic nucleic acids in AGS.

 

 

 

 

 

 

 

The Ribonuclease H2 complex

 

 

SAMHD1

The SAMHD1 protein was recently shown to be a dNTP triphosphohydrolase. It acts as a HIV restriction factor, presumably by reducing the dNTP pools available to the virus for its replication. The viral auxiliary protein Vpx binds to SAMHD1 and targets it for proteosomal degradation. Mutations in SAMHD1 may therefore prevent cells from effectively counteracting (endogenous) retrovirus replication.