Germline CARD11 Mutation in a Patient with Severe Congenital B Cell Lymphocytosis

Abstract

Purpose Activating germline mutations in CARD11 have recently been linked to a rare genetic disorder associated with congenital B cell lymphocytosis. We describe a patient with a similar clinical phenotype who had a de novo germline G123D CARD11 mutation. Methods Whole exome sequencing was performed on DNA from the patient and his biological parents. Laboratory studies examined characteristics of the patient’s B and T lymphocytes. A CARD11 cDNA containing the mutation was transfected into a lymphocyte cell line to gain an understanding of its function. RNA sequencing was performed on samples from the patient and from patients with alternate germline CARD11 mutations and differential gene expression analysis was performed. Results The patient had a decade-long history of severe polyclonal B lymphocytosis in the 20,000–90,000 lymphocytes/mm3 range, which was markedly exacerbated by EBV infection and splenectomy at different times. He had a heterozygous germline CARD11 mutation causing a G123D amino acid substitution, which was demonstrated to induce NF-κB activation in unstimulated lymphocytes. In contrast to previous patients with CARD11 mutations, this patient’s B cells exhibited higher expression of several cell cycle progression genes, as well as enhanced proliferation and improved survival following B cell receptor stimulation. Conclusions This is the third reported germline and first de novo CARD11 mutation shown to cause congenital B cell lymphocytosis. The mutation was associated with a dramatically greater lymphocytosis than in previously described cases, disproportionate to the level of constitutive NF-κB activation. However, comparative review of the patient’s clinical history, combined with additional genomic and functional analyses, underscore other important variables that may affect pathophysiology or regulate mutant CARD11 function in B cell proliferation and disease. We now refer to these patients as having BENTA disease (B cell Expansion with NF-κB and T cell Anergy).

Mild B-cell lymphocytosis in patients with a CARD11 C49Y mutation

“…The congenital lymphoproliferative disorder known as BENTA (B cell Expansion with NF-κB and T cell Anergy) was recently described in five patients that all presented with the disease shortly after birth (1, 2). Salient clinical features of BENTA include splenomegaly and profound polyclonal B cell lymphocytosis (above 2200/μl, 60-80% CD19 + in peripheral blood), with elevated transitional and mature naïve B cells but few circulating class-switched/memory B cells.…”

“…Indeed, splenectomy appears to be associated with significantly higher peripheral blood B cell counts in patients with BENTA, and may be a counterproductive clinical measure that predisposes patients to certain infections (e.g. encapsulated bacteria) and/or greater risk of B cell malignancy (1, 2). We can only speculate that specific CARD11 mutations may be intrinsically linked to mild (C49Y) or severe (G123D) B cell lymphocytosis, although comparable gain-of-function activity was measured in various in vitro systems (1, 6).…”

Lymphocyte signaling and activation by the CARMA1-BCL10-MALT1 signalosome

“…Importantly, the effects of CARMA1 mutations in B cells provide a rational explanation why CBM alterations are predominately found in B cell lymphoid malignancies. BENTA patients may be predisposed to lymphomagenesis, but additional genomic alterations are required to develop B cell lymphomas (Brohl et al, 2014). Collectively, these data prove that CBM mutations alter lymphocyte activation, reflecting that CARMA1, BCL10 and MALT1 are essential for adaptive immune responses.…”

“…As discussed earlier (see section 'Physiological role of the CBM complex in lymphocytes'), activating CARMA1 germline mutations induce the BENTA phenotype in humans that is characterized by B cell expansion, but at the same time T cell hyporesponsiveness and anergy (Snow et al, 2012). Even though CARMA1 mutation alone may not be sufficient to cause lymphomas, the B lymphocytes carrying CARMA1 germline or somatic mutation are likely more prone to lymphomagenesis (Brohl et al, 2014). Interestingly, T cell unresponsiveness in BENTA patients suggests the existence of specific negative regulatory mechanisms in T cells that may prevent formation of T cell lymphomas by oncogenic CARMA1.…”

“…Besides immunodeficiency caused by loss-of-function mutations in CARMA1, germline CARMA1 gain-of-function mutations have been identified that trigger an immune disorder called BENTA (B cell expansion with NF-κB and T cell anergy) (Snow et al, 2012;Brohl et al, 2014;Buchbinder et al, 2015). BENTA patients are characterized by a B cell lymphoproliferative disorder while the T cells become unresponsive.…”