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Sensing DNA? Aim for the cytoplasm in Systemic Lupus
Erythematosus
Allyson Egan1, Jennifer Pacyna2, Margus Blagrove2, Michael Jones2 Department of
Medicine1, Genomics Laboratory, MRC Clinical Sciences Centre2 Hammersmith Hospital,
Imperial College London
Introduction
Aim2 is a novel cytoplasmic DNA sensor, which instigates the caspase pathway and formation
of the inflammasome leading to cell death via IL1β and IL181. The gene encoding the protein is
a member of the interferon response HIN200 family. The proteins contain a HIN-200 amino
acid region which binds to DNA and a pyrin domain which can homo/ oligodimerise 2.
Microarray data has reported over-expression of family member Ifi202 in the inbred Lupus
prone mouse model BXSB and New Zealand subcongenic B6.Nba22. The telomeric region of
chromosome 1 (C1) which contains the HIN200 locus in these two murine strains, along with a
syntenic region in humans, has been linked with the development of Systemic Lupus
Erythematosus (SLE) with phenotypic features of nephritis, autoantibody production and death2.
A bioinformatic approach that includes comparative sequence analysis has been employed to
investigate the homology and disparity in the genomic architecture at this locus.
Methods
C1 genomic DNA was isolated and fragmented from cells by the Wellcome Trust Sanger
Institute. The random fragments were sequenced using Illumina Next Generation Sequencing
technology to generate paired-end reads of 2 x 75 bases. The fragments were aligned to the
mouse reference genome and viewed using the Integrated Genome Viewer (IGV version 2.2.5).
Promoter, intronic and exonic sequences were scanned for single nucleotide polymporphisms
(SNPs). Ensemble/ncbi databases, blast searches and clustral sequence comparisons were
employed to investigate homology within the region. Real time PCR was performed to
investigate differential expression of Aim2 in autoimmune prone strain BXSB.
Results
The region is highly polymorphic. There are multiple SNPs identified in exonic, intronic and
promoter regions in genes Ifi202, Ifi203, Ifi205, Mnda and Aim2. A lysine to glutamine switch
occurs in Ifi202, Ifi203 and Ifi205. The start codon in Ifi203 contains a switch from methionine
to threonine. Ifi205 has altered charge due to glutamic acid replacement by glycine. In Mnda,
glycine is switched to arginine along with a leucine switch to proline, which may cause
alteration in protein structure. Exon 1 (E1) appears to be homologous in certain family
members. A common E1 appears in full length in Mnda. Truncated versions occur in Ifi202 and
Ifi204. The intact sequence demonstrates 98-100% homology with one another. Ifi205
demonstrates 81-83% homology with the other family members. AIM2 E1 shares only 17%
homology with Mnda E1. In real-time PCR differential down regulation of expression has been
identified in the autoimmune strains BXSB and subcongenic B10.Yaa.Bxs3 in comparison to the
non-autoimmune control strain B10.Yaa.
Conclusion
A bioinformatic approach that includes comparative sequence analysis has identified homology
between the gene families especially at E1, with the exception of Aim2. The lack of homology
may reflect their different location and function within the cell. Furthermore, decreased
expression of the cytosolic DNA sensor gene in the lupus prone mouse model BXSB and
subcongenic B10.Yaa.Bxs3 in comparison to control B10.Yaa has been identified. Extensive
polymorphic changes occurs in the region that result in amino acid substitutions which may
alter protein structure.
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References
1.An orthogonal proteomic-genomic screen identifies AIM2 as a cytosplasmic DNA sensor for the
inflammasome. Burckstummer et al Nature Immunology vol 10, Number 3 March 2009.
2.Dissection of BXSB Lupus Phenotype using mice congenic for Chromosome 1 demonstrates that
separate intervals direct different aspects of disease. Morley et al. JI 2004, 173:4277-4285