Multigenic basis for type I diabetes. Association of HRAS1 polymorphism with HLA-DR3, DQw2/DR4, DQw8

Transmission disequilibrium as a test of linkage and association between HLA alleles and pauciarticular-onset juvenile rheumatoid arthritis

OBJECTIVE

To determine if HLA class I and II alleles previously found to be associated with (or protective against) pauciarticular-onset juvenile rheumatoid arthritis (pauci-onset JRA) in population-association studies are transmitted from heterozygous parents to an extent different from the expected 50%.

METHODS

One hundred one Caucasian North American families that had a child with pauci-onset JRA and at least 1 parent who was heterozygous for the allele of interest were available for analysis. Both biologic parents and all children (affected and unaffected) were typed for HLA class I and II alleles. The transmission disequilibrium test (TDT) was used to determine if affected offspring received the disease-associated (or protective) allele more (or less) frequently than its alternate allele. In families in which an unaffected sibling was available, the unmatched chi-square test was used to determine if a meiotic segregation distortion bias existed.

RESULTS

HLA class I alleles A2, B27, and B35 showed a significantly higher than expected frequency of transmission to affected offspring, as did class II alleles DR5 and DR8. HLA-DR4 was found to be transmitted significantly less frequently than expected to affected, but not unaffected, offspring. All alleles that showed an excess transmission to affected offspring were transmitted to unaffected offspring at expected rates. When the data were stratified by age and sex, the likelihood of transmission of some of the alleles was strongly influenced by these variables. For example, excess transmission of HLA-DR5 was found exclusively in female patients who were younger at the time of disease onset.

CONCLUSION

Results from these family-based studies rule out the possibility that HLA disease associations found in earlier studies were a result of population stratification and establish linkage and association between the major histocompatibility complex and pauci-onset JRA.

A genome-wide search for human type 1 diabetes susceptibility genes

We have searched the human genome for genes that predispose to type 1 (insulin-dependent) diabetes mellitus using semi-automated fluorescence-based technology and linkage analysis. In addition to IDDM1 (in the major histocompatibility complex on chromosome 6p21) and IDDM2 (in the insulin gene region on chromosome 11p15), eighteen different chromosome regions showed some positive evidence of linkage to disease. Linkages to chromosomes 11q (IDDM4) and 6q (IDDM5) were confirmed by replication, and chromosome 18 may encode a fifth disease locus. There are probably no genes with large effects aside from IDDM1. Therefore polygenic inheritance is indicated, with a major locus at the major histocompatibility complex.

Insulin gene region-encoded susceptibility to type 1 diabetes is not restricted to HLA-DR4-positive individuals

Type 1 or insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease of the insulin-producing pancreatic beta-cells which is determined by both genetic and environmental factors. The major histocompatibility complex and the insulin gene region (INS) on human chromosomes 6p and 11p, respectively, contain susceptibility genes. Using a mostly French data set, evidence for linkage of INS to IDDM was recently obtained but only in male meioses (suggesting involvement of maternal imprinting) and only in HLA-DR4-positive diabetics. In contrast, we find evidence for linkage in both male and female meioses and that the effect of the susceptibility gene(s) in the INS region is not dependent on the presence of HLA-DR4.

Involvement of human muscle acetylcholine receptor alpha-subunit gene (CHRNA) in susceptibility to myasthenia gravis

The muscle acetylcholine receptor is the major target of the autoimmune response in generalized myasthenia gravis. To investigate the role of the gene encoding the alpha subunit of the receptor (CHRNA), two stable polymorphic d[(GT).(CA)]dinucleotide repeats, designated HB and BB, were characterized within the first intron of CHRNA. The HB*14 allele conferred a relative risk for myasthenia gravis of 2.5 in 81 unrelated patients compared with 100 control subjects. Very significantly, family analysis based on haplotype segregation data indicated that parental haplotypes associated with HB*14 always segregated to the child with myasthenia gravis (P < 0.0002 for the comparison with the transmission of haplotypes not bearing HB*14), whereas their transmission to unaffected siblings was equilibrated. Myasthenia gravis patients also showed a high frequency of microsatellite variants unseen in controls. These findings implicate the CHRNA in susceptibility to myasthenia gravis.

The immunogenetics of insulin-dependent diabetes

Insulin-dependent (type 1) diabetes mellitus (IDDM) is a multifactorial disease with a strong genetic component. The majority of the genetic component can be explained by associations between IDDM and genes in the major histocompatibility complex (MHC). The best single marker for IDDM is based on amino acid polymorphism of the HLA-DQ gene. Current evidence, however, indicates that the MHC susceptibility to IDDM is determined by a combination of HLA class I, II and III genes contained on HLA haplotypes. A non-MHC genetic component to IDDM also exists. To date, the most consistent association is between IDDM and markers of the insulin gene locus.

Insulin-IGF2 region on chromosome 11p encodes a gene implicated in HLA-DR4-dependent diabetes susceptibility

A class of alleles at the VNTR (variable number of tandem repeat) locus in the 5' region of the insulin gene (INS) on chromosome 11p is associated with increased risk of insulin-dependent diabetes mellitus (IDDM), but family studies have failed to demonstrate linkage. INS is thought to contribute to IDDM susceptibility but this view has been difficult to reconcile with the lack of linkage evidence. We thus investigated polymorphisms of INS and neighbouring loci in random diabetics, IDDM multiplex families and controls. HLA-DR4-positive diabetics showed an increased risk associated with common variants at polymorphic sites in a 19-kilobase segment spanned by the 5' INS VNTR and the third intron of the gene for insulin-like growth factor II (IGF2). As INS is the major candidate gene from this region, diabetic and control sequence were compared to identify all INS polymorphisms that could contribute to disease susceptibility. In multiplex families the IDDM-associated alleles were transmitted preferentially to HLA-DR4-positive diabetic offspring from heterozygous parents. The effect was strongest in paternal meioses, suggesting a possible role for maternal imprinting. Our results strongly support the existence of a gene or genes affecting HLA-DR4 IDDM susceptibility which is located in a 19-kilobase region of INS-IGF2. Our results also suggest new ways to map susceptibility loci in other common diseases.