Family studies of type 1 diabetes reveal additive and epistatic effects between MGAT1 and three other polymorphisms.

In a recent study on multiple sclerosis (MS), we observed additive effects and epistatic interactions between variants of four genes that converge to induce T-cell hyperactivity by altering Asn-(N)-linked protein glycosylation: namely, the Golgi enzyme MGAT1, cytotoxic T-lymphocyte antigen 4 (CTLA-4), interleukin-2 receptor-α (IL2RA) and interleukin-7 receptor-α (IL7RA). As the CTLA-4, IL2RA and IL7RA variants are associated with type 1 diabetes (T1D), we examined for joint effects in T1D. Employing a novel conditional logistic regression for family-based data sets, epistatic and additive effects were observed using 1423 multiplex families from the Type 1 Diabetes Genetic Consortium data set. The IL2RA and IL7RA variants had univariate association in MS and T1D, whereas the MGAT1 and CTLA-4 variants associated with only MS or T1D, respectively. However, similar to MS, the MGAT1 variant haplotype interacted with CTLA4 (P=0.03), and a combination of IL2RA and IL7RA (P=0.01). The joint effects of MGAT1, CTLA4, IL2RA, IL7RA and the two interactions using a multiple conditional logistic regression were statistically highly significant (P<5 × 10(-10)). The MGAT1-CTLA-4 interaction was replicated (P=0.01) in 179 trio families from the Genetics of Kidneys in Diabetes study. These data are consistent with defective N-glycosylation of T cells contributing to T1D pathogenesis.

Characteristics of heterologous beta 2-m exchange into H-2Db at the cell surface.

W6/32 is a monomorphic anti-HLA class I Ab that cross-reacts with the product of the exchange of murine beta 2-microglobulin (beta 2-m) with human or bovine beta 2-m on H-2 Db. Using W6/32 we have developed a simple and rapid flow cytometric method to measure the beta 2-m exchange kinetics for Db at the surface of intact H-2b cell lines. We find that 10 to 25% of the Db heavy chains exchange bound beta 2-m for soluble beta 2-m with a t1/2 of 10 to 15 min. The kinetics of the exchange are consistent with a mechanism that includes a free heavy chain intermediate on the reaction coordinate. The remainder of Db molecules appear to be refractory to beta 2-m exchange even after long-term culture with an exogenous source of beta 2-m. The exchange process proceeds at a similar rate on wild-type cells that bear a diverse complement of autologous peptides on their class I molecules, and on mutant RMA-S cells having class I molecules primarily occupied with a single, defined, high affinity synthetic peptide Ag. The t1/2 for dissociation of a radiolabeled analogue of a high affinity naturally presented peptide Ag is 10 h or more, implying that the free heavy chain intermediate in the beta 2-m exchange mechanism retains bound peptides. Further, we find that synthetic peptide Ags added to non-mutant cells bind both to class I molecules that subsequently exchange bound beta 2-m, and to molecules that retain endogenous beta 2-m.