Molecular Autopsy by Exome Sequencing Identifies in Fraternal Twins a CARD11 p.Ser995Leu Variant Within GUK Domain

Background: We describe the post-mortem analysis of a CARD11 variant allele, p.Ser995Leu, identified in fraternal twins who died in early infancy with no identifiable cause of death. CARD11 variants through varied inheritance models can alter immune function through loss- or gain-of-function mechanisms, involving distinct protein domains; yet the significance of GUK domain variants remains poorly characterized. Twin autopsies showed non-specific findings, such as pulmonary macrophage accumulation and splenic white pulp expansion, but without infection or structural abnormalities. Methods: Whole-exome sequencing, performed as part of molecular autopsies, identified the shared CARD11 p.Ser995Leu variant, previously classified as a variant of uncertain significance (VUS). We assessed evolutionary conservation across CARD family proteins and species and predicted functional impact using in silico tools, which estimate the likelihood that a variant is deleterious. AlphaFold-based structural modeling emphasized qualitative biophysical assessment. Using epidemiological data, population allele frequency, and Bayesian ACMG variant classification, we assessed competing hypotheses under an autosomal dominant model. Results: The p.Ser995Leu substitution affects a conserved, surface-exposed β-sheet within the GUK domain. While CADD scores exceeded 20, other predictive algorithms offered only partial support of pathogenicity. Structural modeling suggested a potential GUK domain destabilization. Integrating genetic, pathologic, immunologic, and probabilistic modeling, we propose a biologically plausible model in which the variant, like other GUK variants, may alter NF-κB or other signaling pathways and is likely pathogenic. Conclusions: While the CARD11 p.Ser995Leu variant’s contribution to disease is uncertain without functional validation or parental testing, and phenotypic findings are non-specific, the presence of an ultra-rare GUK domain variant in both twins, combined with in silico and statistical modeling, supports its interpretation as likely pathogenic or high risk. The results highlight the challenges of data-limited post-mortem variant interpretation.