Uncovering a Novel Pathway for Autoinflammation : With a Little Help from a Wrinkled Friend

Abstract: A major challenge in medical genetics is to identify the mutations underlying heritable diseases. Dogs are excellent genetic models in the search for causative mutations, as they constitute a large library of naturally occurring heritable diseases many of which are analogous to those suffered by man. In addition, these animals have a genome structure well suited to gene mapping. The Shar-Pei dog has two breed-specific features; a strongly selected for wrinkled skin and a high predisposition to an autoinflammatory disease (AID). Abnormalities in the innate immune system cause this type of disease, presenting as spontaneous attacks of inflammation. Persistent inflammation puts an affected Shar-Pei at risk of amyloidosis, organ failure and premature death. In humans, similar AIDs occur and for a majority of cases, no underlying genetic cause has yet been identified. The aim of this thesis was to use the Shar-Pei as a genetic model for autoinflammation in order to find new genes and signalling pathways involved in disease. In paper I, a pleiotropic mutation was identified that could explain both the wrinkled skin and autoinflammation in Shar-Pei. The mutation is associated with an up-regulation of Hyaluronic Acid Synthase 2 (HAS2). Increased expression of HAS2 leads to abnormal depositions of hyaluronic acid (HA) in the skin, resulting in the wrinkled appearance. When fragmented, HA also function as a damage signal sensed by the innate immune system which then responds with inflammation. By selecting for the wrinkled skin, the autoinflammatory disease has inadvertently been enriched in the breed. In paper II, five different inflammatory signs could be associated with the same genetic risk factor, allowing the introduction of a new terminology: Shar-Pei autoinflammatory disease (SPAID) to describe the whole disease complex. In addition, a modifying locus containing several biologically attractive genes was suggested to contribute to varying incidence of amyloidosis in Shar-Pei. In paper III, signs of pathological changes in HA metabolism were investigated in human AID. HA concentration was found to be both higher in subjects with no molecular diagnosis and also associated to disease activity and severity. Taken together, this suggests HA is also involved in human AID.