Inherited interleukin-12 deficiency: IL12B genotype and clinical phenotype of 13 patients from six kindreds

Abstract

Interleukin-12 (IL12) is a cytokine that is secreted by activated phagocytes and dendritic cells and that induces interferon-gamma production by natural-killer and T lymphocytes. It consists of two subunits, p35 and p40, which are encoded by IL12A and IL12B, respectively. The first reported patient with a genetic cytokine disorder was a Pakistani child, who was homozygous for a large loss-of-function deletion (g.482+82_856-854del) in IL12B. This IL12-deficient child suffered from infections caused by bacille Calmette-Guérin (BCG) and Salmonella enteritidis. We herein report 12 additional patients from five other kindreds. In one kindred from India, the same large deletion that was described elsewhere (g.482+82_856-854del) was identified. In four kindreds from Saudi Arabia, a recessive loss-of-function frameshift insertion (g.315_316insA) was found. A conserved haplotype encompassing the IL12B gene suggested that a founder effect accounted for the recurrence of each mutation. The two founder mutational events-g.482+82_856-854del and g.315_316insA-were estimated to have occurred approximately 700 and approximately 1,100 years ago, respectively. Among a total of 13 patients with IL12 deficiency, 1 child had salmonellosis only and 12 suffered from clinical disease due to BCG or environmental nontuberculous mycobacteria. One patient also had clinical disease caused by virulent Mycobacterium tuberculosis, five patients had clinical disease caused by Salmonella serotypes, and one patient had clinical disease caused by Nocardia asteroides. The clinical outcome varies from case to case, since five patients (aged 2-11 years) died of overwhelming infection, whereas eight patients (aged 3-12 years) are still in good health and are not currently taking antibiotics. In conclusion, IL12 deficiency is not limited to a single kindred, shows significant variability of outcome, and should be considered in the genetic diagnosis of patients with mycobacteriosis and/or salmonellosis. To date, two founder IL12B mutations have been identified, accounting for the recurrence of a large deletion and a small insertion within populations from the Indian subcontinent and from the Arabian Peninsula, respectively.

Figure 1

Pedigrees of six families with IL12 deficiency caused by IL12B mutations. Each kindred is designated by a capital letter (A–F), each generation is designated by a roman numeral (I or II), and each individual is designated by an arabic numeral (1–8). Patients with mycobacteriosis and/or salmonellosis are represented by black symbols. The probands are indicated by arrows. “E?” indicates individuals for whom genetic analysis was not possible (e.g., B.II.1, who died of disseminated BCG infection at age 5 years).

Figure 2

Impaired production of IL12. A, Production of IL12p70, IL12p40, and TNFα, by PDBu-stimulated EBV-B cells from a healthy positive control (C+) and a negative control (C−; A.II.1) (both of whom had previously been reported by Altare et al. [1998c]) and from patients (B.II.3, C.II.3, and D.II.4). Supernatants were harvested after 18 h of activation, for cytokine quantification by ELISA. The results are the mean ± SDs of two experiments. B, Production of IL12p70, IL12p40, and TNFα, by whole-blood cells from a healthy positive control (C+) and from patient D.II.4, either unstimulated (Medium) or stimulated with BCG plus IFNγ. Supernatants were harvested after 12 h of activation. C, Production of IFNγ by whole-blood cells from healthy positive control (C+) and from patient D.II.4, either unstimulated (Medium) or stimulated with BCG alone or with BCG plus recombinant IL12p70. The supernatants were harvested after 72 h of activation, for cytokine quantification by ELISA.

Figure 3

IL12B mutant alleles. A, Schematic representation of structural organization of coding region of wild-type (WT) IL12B gene. Exons I and VIII are not translated. Intrachain disulfite bounds and amino acids essential for dimerization with IL12p35 are indicated by the single-letter amino acid code. The positions of the two mutations found in IL12-deficient patients are also indicated. B, Schematic representation of coding region of two mutant IL12B alleles. Gray regions correspond to stretches of new amino acids resulting from the frameshift mutation.

Figure 4

High-resolution map of IL12B locus. A, Physical map of IL12B region on chromosome 5q33. The physical distances (in Mb) between IL12B and microsatellite markers were obtained from BLAST. B, Genomic organization of human IL12B gene and location of intragenic SNPs. White boxes indicate coding exons; gray boxes indicate untranslated exons. Translation initiation and termination codons are indicated. Arrows indicate positions of intragenic SNPs (−1356, 364+92, and 1146).

Figure 5

Haplotypes encompassing IL12B in patients with IL12 deficiency. Markers encompassing IL12B, with physical distance (Mb) and recombination rate (θ), are shown. Physical distance was obtained from BLAST; the recombination rate was estimated by use of the correspondence of .672 cM for 1 Mb, over the whole region, and use of the Kosambi mapping function (see the “Dating of Mutations” subsection). The ancestral haplotypes shared by kindreds A and B, from the Indian subcontinent, is indicated by dark-gray shading; the ancestral haplotype shared by kindreds C–F, from Saudi Arabia, is indicated by intermediate-gray shading; and the ancestral haplotype shared by a subset of kindreds is indicated by light-gray shading.