Relevance of biallelic versus monoallelic TNFRSF13B mutations in distinguishing disease-causing from risk-increasing TNFRSF13B variants in antibody deficiency syndromes

Abstract

TNFRSF13B encodes transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), a B cell- specific tumor necrosis factor (TNF) receptor superfamily member. Both biallelic and monoallelic TNFRSF13B mutations were identified in patients with common variable immunodeficiency disorders. The genetic complexity and variable clinical presentation of TACI deficiency prompted us to evaluate the genetic, immunologic, and clinical condition in 50 individuals with TNFRSF13B alterations, following screening of 564 unrelated patients with hypogammaglobulinemia. We identified 13 new sequence variants. The most frequent TNFRSF13B variants (C104R and A181E; n=39; 6.9%) were also present in a heterozygous state in 2% of 675 controls. All patients with biallelic mutations had hypogammaglobulinemia and nearly all showed impaired binding to a proliferation-inducing ligand (APRIL). However, the majority (n=41; 82%) of the pa-tients carried monoallelic changes in TNFRSF13B. Presence of a heterozygous mutation was associated with antibody deficiency (P< .001, relative risk 3.6). Heterozygosity for the most common mutation, C104R, was associated with disease (P< .001, relative risk 4.2). Furthermore, heterozygosity for C104R was associated with low numbers of IgD(-)CD27(+) B cells (P= .019), benign lymphoproliferation (P< .001), and autoimmune complications (P= .001). These associations indicate that C104R heterozygosity increases the risk for common variable immunodeficiency disorders and influences clinical presentation.

Figure 1

Mutations of TNFRSF13B in 50 of 564 index patients with antibody deficiency. Distribution of the observed TNFRSF13B mutations across the TNFRSF13B/TACI protein. Each symbol represents one patient. ● indicates patients with homozygous TNFRSF13B mutations. ○ represents patients with heterozygous TNFRSF13B mutations. designates a patient with a double mutation on one allele of TNFRSF13B. Compound heterozygous mutated TACI-deficient patients are labeled as indicated in the figure. CRD indicates cysteine-rich domain; stalk, stalk region; and TM, transmembrane domain.

Figure 2

Pedigrees of familial TACI deficiency. Pedigrees of 7 families with TACI deficiency are shown. Symbols: circles indicate female; squares, male; black filled symbols, CVID; gray filled symbols, IgG subclass deficiency or dysgammaglobulinemia; hatched symbols, selective IgA deficiency; slash, deceased; and arrow, index patient. Mutations shown in brackets are inferred. §IDs refer to Voerechovsky et al.

Figure 3

TACI expression and APRIL binding on EBV-transformed cell lines. (A) The histograms show the mean fluorescence intensity (MFI) of stainings of EBV-transformed B-cell lines with a polyclonal antibody against TACI (left), a monoclonal antibody recognizing the TACI extracellular domain (clone 1A1; middle), and a Flag-tagged recombinant APRIL construct. ♦ indicates C104R homozygous (n = 2); ◇, C104R heterozygous (n = 12); , healthy donors (n = 5); and , CVID patients without TNFRSF13B mutation (n = 5). The thick gray line in each graph indicates the background staining of a previously described homozygous S144X cell line. Mean values and SDs are shown at the right side of the scatter diagram of each group. Testing for significance was performed with the Student t test. (B) TACI expression and APRIL binding on EBV-transformed cell lines with homozygous or compound heterozygous TNFRSF13B mutations, stained with a polyclonal antibody directed against the TACI extracellular domain (top panels), stained with a monoclonal antibody recognizing the TACI extracellular domain (middle panels), and stained with a Flag-tagged APRIL construct (bottom panels). Thick black lines indicate patients; thick dashed black lines, healthy donors; and gray filled histograms, patients with TNFRSF13B null mutation (S144X homozygous, negative control).

Figure 4

Immunologic phenotype of TACI-deficient patients. (A) CD19⁺ B-cell percentages of lymphocytes and IgD⁺IgM⁺CD27⁻, IgD⁻IgM⁺CD27⁺, and IgD⁻IgM⁻CD27⁺ B-cell subsets in patients with TACI deficiency. (B) Immunoglobulin levels at time of diagnosis in patients with TACI deficiency. Each circle represents a single value from one patient. ● indicates homozygous or compound heterozygous TNFRSF13B mutations; , heterozygous TNFRSF13B mutations other than C104R; ○, C104R heterozygous mutations; and , mean values for each patient group. Testing for significant differences between patient groups was performed with the Student t test.

Figure 5

Autoimmunity and lymphoproliferation in TACI deficiency. Histograms show the prevalence of autoimmune diseases (top panel) and lymphoproliferation (bottom panel) as percentages in CVID patients without TACI mutation, TACI-deficient patients, and subgroups of TACI-deficient patients with homozygous or compound heterozygous, C104R heterozygous, or other heterozygous mutations. Absolute numbers of patients are printed on the bars.