Abstract
PIDD1 encodes p53-Induced Death Domain protein 1, which acts as a sensor surveilling centrosome numbers and p53 activity in mammalian cells. Early results also suggest a role in DNA damage response where PIDD1 may act as a cell-fate switch, through interaction with RIP1 and NEMO/IKKg, activating NF-κB signaling for survival, or as an apoptosis-inducing protein by activating caspase-2. Biallelic truncating mutations in CRADD—the protein bridging PIDD1 and caspase-2—have been reported in intellectual disability (ID), and in a form of lissencephaly. Here, we identified five families with ID from Iran, Pakistan, and India, with four different biallelic mutations in PIDD1, all disrupting the Death Domain (DD), through which PIDD1 interacts with CRADD or RIP1. Nonsense mutations Gln863* and Arg637* directly disrupt the DD, as does a missense mutation, Arg815Trp. A homozygous splice mutation in the fifth family is predicted to disrupt splicing upstream of the DD, as confirmed using an exon trap. In HEK293 cells, we show that both Gln863* and Arg815Trp mutants fail to co-localize with CRADD, leading to its aggregation and mis-localization, and fail to co-precipitate CRADD. Using genome-edited cell lines, we show that these three PIDD1 mutations all cause loss of PIDDosome function. Pidd1 null mice show decreased anxiety, but no motor abnormalities. Together this indicates that PIDD1 mutations in humans may cause ID (and possibly lissencephaly) either through gain of function or secondarily, due to altered scaffolding properties, while complete loss of PIDD1, as modeled in mice, may be well tolerated or is compensated for.
Highlights
Biallelic missense mutations in the gene encoding caspase and RIP adaptor with death domain, CRADD, have been reported to cause intellectual disability (ID) with enlarged head circumference (MRT34; MIM 614499; ref. 1)
In our study we have shown that none of the genetic alterations reported here prevent self-processing of PIDD1, but differences in stability become apparent in PIDD1-CC variants with a truncated Death Domain (DD)
This suggests that the effect of the PIDD1 mutations that have been associated with ID19,21 are most likely due to the loss of function of PIDD1-CC, as PIDD1-C can be still be generated at rates comparable to WT (Fig. 4)
Summary
Biallelic missense mutations in the gene encoding caspase and RIP adaptor with death domain, CRADD ( known as RAIDD), have been reported to cause intellectual disability (ID) with enlarged head circumference (MRT34; MIM 614499; ref. 1). Several reports implicate caspase-2 in neuronal cell death caused by NGF or beta-amyloids[8,14]. Whether these effects of caspase-2 require PIDDosome formation remains less. Loss of caspase-2 associates with increased anxiety behavior, but not memory loss in mice[17]. Together, these studies indicate that caspase-2 activity controls neuronal stress responses and cognitive brain function. As PIDDosome-independent activation of caspase-2 has been reported in hematopoietic cells, fibroblasts, and neurons[14,18], it remains unclear if PIDD1 is needed for normal brain function
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