Necroptosis, a form of programmed cell death, can be induced directly by pathogens or by cytokines released as part of the innate immune response. The downstream effector protein Mixed Lineage Kinase-domain Like (MLKL) is phosphorylated by RIPK3, oligomerises, and associates with membrane phospholipids to promote the release of pro-inflammatory cytokines and the lytic destruction of cells. MLKL’s two-helix ‘brace’ forms an essential conduit between its regulatory pseudokinase domain and membrane-busting four-helix bundle (4HB) effector domain. Our team recently showed that a single substitution of a conserved residue in this two-helix brace, D139V, randomly introduced by ENU mutagenesis, causes RIPK3-independent hyper-activation of mouse MLKL. This resulted in a lethal, postnatal systemic inflammatory syndrome.
Paradoxically, the most commonly inherited human MLKL missense gene polymorphisms map to the vicinity of the very same two-helix brace as the deadly D139V mouse MLKL substitution. This clustering is apparent not only in the MLKL monomer but also following daisy chain oligomerisation. The summed allele frequency for polymorphisms mapping to this region is 0.05, or 5 % of all human MLKL alleles sequenced in hundreds of thousands of individuals of diverse ancestry. As carriers of two MLKL alleles, we each have a 10% chance of being heterozygous for one of these MLKL variants when averaged across the global population. Have these human MLKL gene variants achieved such high frequencies by chance population bottlenecks, or have they conferred a selective survival advantage to one or more pathogens at some point in human history? What are the consequences of carrying these MLKL gene variants for present-day humans? We have recently shown that combinations of these MLKL gene variants are found at up to 12-fold the expected frequency in patients that suffer from a pediatric auto-inflammatory disease, chronic recurrent multifocal osteomyelitis (CRMO), and that they confer a context specific 'gain' of cell death function in vitro. At a systems level, we have shown that genetically modified mice show signs of defective hematopoiesis and pathogen clearance, and we are actively investigating their potential influence on a wide array of complex polygenic human traits and diagnoses using targeted Phenome-Wide Association Studies (PheWAS).