Blau syndrome, the prototypic auto-inflammatory granulomatous disease

Abstract

Blau syndrome is a monogenic disease resulting from mutations in the pattern recognition receptor NOD2, and is phenotypically characterized by the triad of granulomatous polyarthritis, dermatitis and uveitis. This paper reviews briefly the classical clinical features of the disease, as well as more recently described extra-triad symptoms. From an ongoing prospective multicenter study, we provide new data on the natural history of Blau syndrome, focusing on functional status and visual outcome. We also present an update of the range of different NOD2 mutations found in Blau syndrome as well as recent data on morphologic and immunohistochemical characteristics of the Blau granuloma. Finally, emerging insights into pathogenic mechanisms including activation of NOD2 signal transduction, and potential biomarkers of disease activity are discussed.

Keywords: Blau syndrome; Granulomatous diseases; NOD2; RIP2 kinase; Sarcoidosis; Uveitis.

Figure 1

Exuberant ‘boggy’ synovitis of wrist in a child with Blau syndrome.

Figure 2

Tenosynovial swelling affecting tibialis posterior and peroneal tendons in Blau syndrome.

Figure 3

Tan colored “dirty” mildly scaly (late) rash.

Figure 4

Granulomatous dermatitis. Inflammatory infiltrate of the subpapilar dermis with typical multinucleated giant cells within a non-caseating epithelioid cell Blau granuloma.

Figure 5

Distribution of visual acuity in a cohort of 22 patients with Blau syndrome associated uveitis after an average follow up of 12.5 years. Visual acuity was converted into logMAR units by calculating the –Log of the visual acuity in decimal Snellen scale in meters.

Figure 6

Distribution of NOD 2 mutations in 82 patients of the International Blau registry (6).

Figure 7

Morphological and immunohistochemical characteristics of Blau granulomas. Brown colour denotes positive cytokine staining. A (top left). H & E staining showing prominent lymphocyte corona, emperipoletic lymphocytes, and multinucleated giant cell death with fragmented cytoplasm and pycnotic nuclei. Using immunohistochmistry dense staining was observed for IFN-γ (B, top right, low magnification showing several granulomas), IL-6 (C, bottom left, single granuloma showing predominant staining in corona) and IL-17 (D, bottom right, single granuloma with giant cell in center showing strong Il-17 staining).

Figure 8

Schematic representation of the NOD2 signaling pathway. Upon binding of its natural ligand (MDP), the NOD2 protein unfolds and oligomerizes through the NACT domain. This is followed by recruitment of RIP2 kinase protein through CARD-CARD interactions and activation of downstream transcriptional factors (NF-κB and MAP kinase) and secretion of inflammatory cytokines.