A novel HOIP frameshift variant alleviates NF-kappaB signalling and sensitizes cells to TNF-induced death.

ABSTRACT

BACKGROUND: HOIP is the catalytic subunit of the E3 ligase complex (linear ubiquitin chain assembly complex), which is able to generate linear ubiquitin chains. However, the role of rare HOIP functionally deficient variants remains unclear. The pathogenic mechanism and the relationship with immune deficiency phenotypes remain to be clarified. METHODS: Based on a next-generation sequencing panel of 270 genes, we identified a HOIP deletion variant that causes common variable immunodeficiency disease. Bioinformatics analysis and cell-based experiments were performed to study the molecular mechanism by which the variant causes immunodeficiency diseases. FINDINGS: A homozygous loss-of-function variant in HOIP was identified. The variant causes a frameshift and generates a premature termination codon in messenger RNA, resulting in a C-terminal truncated HOIP mutant, that is, the loss of the linear ubiquitin chain-specific catalytic domain. The truncated HOIP mutant has impaired E3 ligase function in linear ubiquitination, leading to the suppression of canonical NF-κB signalling and increased TNF-induced multiple forms of cell death. INTERPRETATION: The loss-of-function HOIP variant accounts for the immune deficiencies. The canonical NF-κB pathway and cell death are involved in the pathogenesis of the disease. FUNDING: This study was funded by the National Natural Science Foundation of China (No. 82270444 and 81501851). RESEARCH IN CONTEXT Evidence before this study LUBAC is the only known linear ubiquitin chain assembly complex for which HOIP is an essential catalytic subunit. Three HOIP variants have now been identified in two immunodeficient patients and functionally characterised. However, there have been no reports on the pathogenicity of only catalytic domain deletion variants in humans, or the pathogenic mechanisms of catalytic domain deletion variants. Added value of this study We report the first case of an autosomal recessive homozygous deletion variant that results in deletion of the HOIP catalytic structural domain. We demonstrate that this variant is a loss-of-function variant using a heterologous expression system. The variant has impaired E3 ligase function. It can still bind to other subunits of LUBAC, but it fails to generate linear ubiquitin chains. We also explored the underlying mechanisms by which this variant leads to immunodeficiency. The variant attenuates the canonical NF-κB and MAPK signalling cascades and increases the sensitivity of TNFα-induced diverse cell death and activation of mitochondrial apoptosis pathways. These findings provide support for the treatment and drug development of patients with inborn errors of immunity in HOIP and related signalling pathways. Implications of all the available evidence First, this study expands the HOIP pathogenic variant database and phenotypic spectrum. Furthermore, studies on the biological functions of pathogenic variants in relation to the NF-κB signalling pathway and cell death provided new understanding into the genetic basis and pathogenesis of HOIP-deficient immune disease, indicating the necessity of HOIP and related signalling pathway variants as diagnostic targets in patients with similar genetic deficiency phenotypes..