Mutant forms of tumour necrosis factor receptor I that occur in TNF‐receptor‐associated periodic syndrome retain signalling functions but show abnormal behaviour

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Abstract
Tumour necrosis factor (TNF)‐receptor‐associated periodic syndrome (TRAPS) is a hereditary autoinflammatory disorder involving autosomal‐dominant missense mutations in TNF receptor superfamily 1A (TNFRSF1A) ectodomains. To elucidate the molecular effects of TRAPS‐related mutations, we transfected HEK‐293 cells to produce lines stably expressing high levels of either wild‐type (WT) or single mutant recombinant forms of TNFRSF1A. Mutants with single amino acid substitutions in the first cysteine‐rich domain (CRD1) were produced both as full‐length receptor proteins and as truncated forms lacking the cytoplasmic signalling domain (Δsig). High‐level expression of either WT or mutant full‐length TNFRSF1A spontaneously induced apoptosis and interleukin‐8 production, indicating that the mutations in CRD1 did not abrogate signalling. Consistent with this, WT and mutant full‐length TNFRSF1A formed cytoplasmic aggregates that co‐localized with ubiquitin and chaperones, and with the signal transducer TRADD, but not with the inhibitor, silencer of death domain (SODD). Furthermore, as expected, WT and mutant Δsig forms of TNFRSF1A did not induce apoptosis or interleukin‐8 production. However, whereas the WT full‐length TNFRSF1A was expressed both in the cytoplasm and on the cell surface, the mutant receptors showed strong cytoplasmic expression but reduced cell‐surface expression. The WT and mutant Δsig forms of TNFRSF1A were all expressed at the cell surface, but a proportion of the mutant receptors were also retained in the cytoplasm and co‐localized with BiP. Furthermore, the mutant forms of surface‐expressed Δsig TNFRSF1A were defective in binding TNF‐α. We conclude that TRAPS‐related CRD1 mutants of TNFRSF1A possess signalling properties associated with the cytoplasmic death domain, but other behavioural features of the mutant receptors are abnormal, including intracellular trafficking and TNF binding.

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