A homozygous mucosa-associated lymphoid tissue 1 (MALT1) mutation in a family with combined immunodeficiency

Abstract

Background: Combined immunodeficiency (CID) is characterized by severe recurrent infections with normal numbers of T and B lymphocytes but with deficient cellular and humoral immunity. Most cases are sporadic, but autosomal recessive inheritance has been described. In most cases, the cause of CID remains unknown.

Objective: We wanted to identify the genetic cause of CID in 2 siblings, the products of a first-cousin marriage, who experienced recurrent bacterial and candidal infections with bronchiectasis, growth delay, and early death.

Methods: We performed immunologic, genetic, and biochemical studies in the 2 siblings, their family members, and healthy controls. Reconstitution studies were performed with T cells from mucosa-associated lymphoid tissue lymphoma-translocation gene 1-deficient (Malt1(-/-)) mice.

Results: The numbers of circulating T and B lymphocytes were normal, but T-cell proliferation to antigens and antibody responses to vaccination were severely impaired in both patients. Whole genome sequencing of 1 patient and her parents, followed by DNA sequencing of family members and healthy controls, showed the presence in both patients of a homozygous missense mutation in MALT1 that resulted in loss of protein expression. Analysis of T cells that were available on one of the patients showed severely impaired IκBα degradation and IL-2 production after activation, 2 events that depend on MALT1. In contrast to wild-type human MALT1, the patients' MALT1 mutant failed to correct defective nuclear factor-κB activation and IL-2 production in MALT1-deficient mouse T cells.

Conclusions: An autosomal recessive form of CID is associated with homozygous mutations in MALT1. If future patients are found to be similarly affected, they should be considered as candidates for allogeneic hematopoietic cell transplantation.

Figure 1. Schematic representation of the NFkB activation following ligation of the TCR

TCR ligation causes SRC kinase and Zap-70 dependent activation of protein kinase C θ (PKCθ), which phosphorylates CARMA1/CARD11. This enables recruitment of BCL-10 and MALT1, followed by oligomerization of the CARMA1/BCL-10/MALT1 (CBM) complex. The CBM complex activates TRAF6, which results in the K63 ubiquitinylation of NEMO and activation and the IKK complex. Activated IKK2 phosphorylates the inhibitor IκBα, resulting in its K48 ubiquitination and degradation by the proteasome. This frees the p50 and p65/RelA subunits of NF-κB from IκBα, permitting NF-κB to undergo nuclear translocation and initiate gene transcription necessary for cellular activation, including IL-2.

Figure 2. Family pedigree of the patients and T cell proliferation to mitogens and antigens

A. Family pedigree. Numbers refer to members studied. B, C. Response of PBMCs from the patients (Pt) to mitogens (B), and antigens (C) expressed as percentage of the proliferation of PBMCs from two healthy adult controls (C). Values represent mean± SE of two independent determinations. * P<0.05, *** P<0.001.

Figure 3. MALT1 mutation in the patients

A. Electrophoregram depicting the c.266G>T mutation in Pt 1 and her father. B. Genomic organization of MALT1 (top) and protein structure (bottom) of MALT1. Blue boxes represent exons. The mutation in exon 2 is shown. C. Alignment of MALT1 homologues. D. RT-PCR analysis of MALT1 mRNA in PHA T blasts. E. Immunoblot analysis of MALT1 in lysates of PHA T cell blasts. Data are representative of two experiments.

Figure 4. Impaired IκBα degradation and IL-2 expression in the patient's T cells

Intracellular FACS analysis of IκBα degradation (left panels) and intracellular IL-2 expression (right panels) in gated CD3⁺ T cells following stimulation of PHA T cell blasts from Pt 1 and a control with PMA+IO. Data are representative of two experiments.

Figure 5. Failure of the MALT1 mutant to reconstitute IκBα degradation and IL-2 expression in T cells from Malt1^−/− mice

A. Intracellular FACS analysis of IκBα and IL-2 in PMA+IO stimulated CD4⁺ T cells from Malt1^−/− mice that were uninfected, or transduced with human WT or mutant MALT1 and Thy1.1. Gated CD4⁺Thy1.1^high and CD4⁺Thy1.1^low cells were analyzed. The fluorescence intensity of Thy1.1^high cells was comparable in cells transduced with mutant and WT MALT1. Data are representative of two experiments. B. Kinetic analysis of IκBα degradation post-stimulation with PMA+IO.