Late-Onset Cryopyrin-Associated Periodic Syndromes Caused by Somatic NLRP3 Mosaicism-UK Single Center Experience

Abstract

Cryopyrin-associated periodic syndrome (CAPS) is caused by gain-of-function NLRP3 mutations. Recently, somatic NLRP3 mosaicism has been reported in some CAPS patients who were previously classified as "mutation-negative." We describe here the clinical and laboratory findings in eight British adult patients who presented with symptoms typical of CAPS other than an onset in mid-late adulthood. All patients underwent comprehensive clinical and laboratory investigations, including analysis of the NLRP3 gene using Sanger and amplicon-based deep sequencing (ADS) along with measurements of extracellular apoptosis-associated speck-like protein with CARD domain (ASC) aggregates. The clinical phenotype in all subjects was consistent with mid-spectrum CAPS, except a median age at disease onset of 50 years. Sanger sequencing of NLRP3 was non-diagnostic but ADS detected a somatic NLRP3 mutation in each case. In one patient, DNA isolated from blood demonstrated an increase in the mutant allele from 5 to 45% over 12 years. ASC aggregates in patients' serum measured during active disease were significantly higher than healthy controls. This series represents 8% of CAPS patients diagnosed in a single center, suggesting that acquired NLRP3 mutations may not be an uncommon cause of the syndrome and should be sought in all patients with late-onset symptoms otherwise compatible with CAPS. Steadily worsening CAPS symptoms in one patient were associated with clonal expansion of the mutant allele predominantly affecting myeloid cells. Two patients developed AA amyloidosis, which previously has only been reported in CAPS in association with life-long germline NLRP3 mutations.

Keywords: AA amyloidosis; ASC aggregates; IL-1β; NLRP3 somatic mutation; cryopyrin-associated periodic syndrome; mutant allele.

Figure 1

Somatic NLRP3 mutations identified in eight late-onset cryopyrin-associated periodic syndrome (CAPS) patients. (A) Sanger chromatograms showing somatic NLRP3 mutations and mutant allele frequency (MAF) in each case. (B) An increase in the MAF from 5.1 to 27 and 45% detected in blood samples collected 9 and 12 years apart in patient 1. (C) Schematic representation of the NLRP3 gene and the encoded cryopyrin protein showing the location of all mosaic mutations identified in this study (in black) and from previous publications (in green), mutations identified in all late-onset-mosaic CAPS patients are shown by *.

Figure 2

Response to IL-1 treatment in the eight patients with late-onset cryopyrin-associated periodic syndrome (CAPS) and somatic NLRP3 mosaicism. (A,B) Extracellular ASC protein aggregates measured in the serum of six patients collected before introduction of IL-1 blocking therapy (anakinra 100 mg/daily) and while on treatment (only serum samples with the SAA/CRP levels ≤10 mg/l were selected) compared to ASC specks measured in the serum obtained from healthy volunteers. (C) Response to IL-1 treatment in the eight patients with late-onset CAPS showing a reduction in the serum amyloid A protein (SAA) levels; in patient 1 anakinra was discontinued and 150 mg/8 weekly of canakinumab was introduced resulting in a massive disease flare-up. (D) Quality of life before and while on treatment with anti-IL-1 treatment. Left panel: data from patient 1 who had relentlessly worsening clinical CAPS and required increasing doses of IL-1 inhibitor. Right panel: data from the remaining seven subjects; a comparison of the mean scores in each domain before and on-treatment shows improvement in seven domains. PF, physical function; RP, role physical; BP, bodily pain; GH, general health; VT, vitality; SF, social function; RE, role emotional; MH, mental health. (E) Posterior images of the whole body radiolabeled serum amyloid P component (SAP) scintigraphies performed in patient 3 showing regression of amyloid deposits; on the left is a SAP scan taken during the first assessment and on the right a SAP scan obtained at a follow-up visit 5 years later.