CBL Syndrome: At the Intersection of Inborn Error of Immunity and Cancer Predisposition

Abstract

CBL syndrome is a form of RASopathies due to heterozygous germline mutations of the Casitas B-lineage lymphoma (CBL) gene. CBL encodes the cbl family of proteins which function as E3 ubiquitin ligases that negatively regulate intracellular signaling by various receptor tyrosine kinases including RAS-MAPK pathway. Individuals with CBL syndrome have variable clinical presentation but phenotypically resembles Noonan syndrome with predisposition to JMML. Patients affected by RASopathies are at risk to develop autoimmune disorders given the role of Ras-MAPK signaling pathway in peripheral tolerance. Prior mice studies suggested the role of cbl protein as negative regulator of TCR-mediated signaling. We describe a case of CBL syndrome presenting with multiple autoimmune conditions and reviewed reported cases in literature.A 14-year old male presented with pancytopenia, autoimmune thyroiditis and organomegaly. His past medical history was remarkable for ADHD, speech delay, short stature, idiopathic hepatosplenomegaly and history of acute disseminated encephalomyelitis at age 5. His bone marrow evaluation was normal. A somatic NGS panel identified VUS in CBL (c.1193A>G, p.H398R) with VAF of 53%. Autoimmune work up showed elevated ANA, DNA antibody, RNP antibody titer and mildly positive anti-C3 antibody. Immunophenotyping revealed normal Ig profile, NK cytopenia, T cell lymphopenia, decreased naïve T cells and elevated CD8+ effector memory T cell activation. ALPS genetic panel revealed paternally inherited VUS in CTLA4 which was later determined to be benign. Since his cytopenias did not respond to IVIG, he was treated with corticosteroid with improvement in counts although he remained thrombocytopenic. A skin biopsy was performed and sequencing of patient’s fibroblast DNA demonstrated a germline origin of CBL variant. Parental DNA sequencing confirmed a de novo event. Literature review revealed 28 germline CBL mutations (Figure 1), most of which were missense mutations (78%). At least nine of these mutations (32%) have been associated with immune dysregulation including autoimmune cytopenias, aHUS, RA, autoimmune thyroiditis, vasculitis and HLH. JMML has been reported in more than half of these germline mutations. [Display omitted] In conclusion, immune dysregulation is a common feature of CBL syndrome in addition to JMML. CBL syndrome should be considered in the differential diagnosis of individuals with ALPS-like phenotypes.