Annals of Medicine

ISSN: 0785-3890 (Print) 1365-2060 (Online) Journal homepage: http://www.tandfonline.com/loi/iann20

The Role of Genetic Predisposition to Type I (Insulin-Dependent) Diabetes Mellitus Ingeborg Deschamps, Jean P. Beressi, Iman Khalil, Jean J. Robert & Jacques Hors To cite this article: Ingeborg Deschamps, Jean P. Beressi, Iman Khalil, Jean J. Robert & Jacques Hors (1991) The Role of Genetic Predisposition to Type I (Insulin-Dependent) Diabetes Mellitus, Annals of Medicine, 23:4, 427-435, DOI: 10.3109/07853899109148086 To link to this article: http://dx.doi.org/10.3109/07853899109148086

Published online: 08 Jul 2009.

Submit your article to this journal

Article views: 7

View related articles

Citing articles: 2 View citing articles

Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=iann20 Download by: [McMaster University]

Date: 23 April 2016, At: 03:23

Special Section: Insulin-Dependent Diabetes Mellitus - Epidemiology, Aetiology, Pathogenesis and Prevention

The Role of Genetic Predisposition to Type I (Insulin-Dependent) Diabetes Mellitus

Downloaded by [McMaster University] at 03:23 23 April 2016

lngeborg Deschamps, Jean P. Beressi, lman Khalil, Jean J. Robert and Jacques Hors

The aetiology of insulin-dependent diabetes (IDDM) involves genetic predisposition, a major component of which has been mapped in the HLA complex, near t o or identical with genes encoding class II molecules. In Caucasian populations IDDM is strongly associated with the serologically defined HLA-DR3 and DR4 antigens, which are widely recognised as markers of susceptibility. The particularly high risk of DR3/DR4 heterozygotes suggests that susceptibility is determined by two genes acting synergistically. The development of recombinant DNA technology has allowed a finer description of the class II region and provided evidence that DQ rather than DR determinants may primarily influence IDDM susceptibility. The search for specific structural changes of the DQA and DQB genes has shown that susceptibility correlates with the absence of aspartic acid at position 57 on the DQp chain (DQp 57 Asp-) and/or the presence of arginine at position 52 on the DQa chain (DQa 52 Arg+). In Caucasians the formation of a putative DQ susceptibility molecule (DQa 52 Arg', DQp 57 Asp-) accounts best for the disease associations when transcomplementation molecules consisting of DQa and p chains encoded by different haplotypes are postulated t o explain the excess risk of heterozygotes. The HLA-IDDM associations in the Japanese, however, are not explained by this model. These and other unresolved questions indicate that other residues of the DQa and p chains or other class II molecules (DRP chains), as well as non-MHC genes, may also contribute to the susceptibility. Key words: Type I (insulin-dependent) diabetes; genetics; HLA; DNA. (Annals of Medicine 23: 427-435,

1991)

Type I (insulin-dependent) diabetes mellitus (IDDM) is the result of a selective and irreversible destruction of the pancreatic B-cells due to autoimmune mechanisms. Its aetiology involves both genetic and environmental factors. The genetic contribution to the aetiology of IDDM is complex. The prevalence of the disease among first degree relatives of IDDM patients (5 to 6 %) (1-3) is much greater than in the general population (0.3 to 0.5 YO).Among identical twins the concordance rate for IDDM is only about 34 % (4). These data, as well as studies in animal models such as the Bio-Breeding (BB) rat and the non obese diabetic (NOD) mouse, indicate:

(1) several genes determine the genetic background of the disease; (2) non-genetic factors contribute equally to its development. The analyses during more than 15 years of major histocompatibility complex (MHC) antigens have accumulated evidence that major determinants of susceptibility map to the MHC (or HLA region), on chromosome 6 ( 5 , 6).

MHC: Markers of Susceptibility or Susceptibility Genes? HLA Genes and Products

From the Unite Endocrinologie et Diabete de I'Enfant and INSERM U30, Hbpital des Enfants-Malades, and INSERM U93, HBpital Saint-Louis, Paris, France. Address and reprint requests: I. Deschamps, M.D., Unite Endocrinologie et Diabete de I'Enfant, Hbpital des EnfantsMalades, 149, rue de Sevres, F-75015 Paris, France.

The MHC consists of many genes (some still without a known product) which have been classified into three families, classes I , II and I l l (Figure 1) ( 7 ) . Class I genes (HLA-A, B, C) encode a-chains which combine with P2-microglobulin to form transmembrane

Deschamps Beressi Khalil Robert Hors

428

MHC

/ I loookb

1

3500kb\

--

8

The class 111 region includes genes encoding complement components (Bf, C2, C4A, C4B), 21-hydroxylase, tumour necrosis factors and heat shock proteins. The class / I genes are subdivided into three main loci: DR, DQ and DP (Figure 2). They encode products which are expressed on the surfaces of particular cell types (macrophages, B lymphocytes, activated T lymphocytes, dendritic and endothelial cells). Foreign antigen processed by these cells is bound to class II molecules and presented to CD4+ T lymphocytes. The recognition by the latter of the HLA-peptide complex generates the signal(s) leading to activation of T-helper cells and initiation of the immune response. Class II molecules are heterodimers formed of two non-covalently associated chains, a and p, encoded by separate A and B genes (Figure 2). The extracellular domains of the two chains form the walls and floor of the antigen binding cleft. The HLA genes are highly polymorphic, i.e., each locus has many alleles resulting in variations of the molecules possessed by different individuals, The greatest polymorphism occurs in the first domain, which interacts with the antigenic peptide and the T-cell receptor and is thus critical to the function of the molecule. In addition to the considerable polymorphism generated by the genetic complexity of the different class II loci, somatic variants occur at the cell surface as the result of transcomplementation of Q and p chains encoded in trans-position by A and B alleles located on different haplotypes (8, 9).

-

I

,

d 5

B A 21-OH

I

TNF

Downloaded by [McMaster University] at 03:23 23 April 2016

HSP70

Figure 1. Schematic representation of the major histocompatibility complex (MHC) on the short arm of chromosme 6. It consists of three families of genes: Class I genes encode HLAA, B, C antigens; class II genes encode HLA-DR, DQ, DP antigens (for more details, see Fig. 2); class 111 genes encode complement factors (Bf,C2,C4A,C4B), 21 -hydroxylase (21-OH), tumour necrosis factors (TNF a, p) and heat shock proteins (HSF 70).

molecules (HLA-A, B and C antigens). These are expressed on the surfaces of most nucleated cells and are responsible for cytotoxic T-cell activity.

Detection of HLA Polymorphism The identification of the polymorphic alleles depends on a number of techniques which may detect either the

‘BABA

[genes]

2211 rl u

DR

DQ

DP

I

m

DNDO A B A B B B B B B A An 6 2 2 1 1 1 5 2 3 4 m

- n

U

\

IDRw1 52 53

DPw

Dchnr DR

1-6

1-9

=

DRw 15,16

1-18

P

Figure 2. Map of the MHC class II region. Black boxes represent expressed genes, white boxes pseudogenes or genes without known products. Some genes are expressed only on particular haplotypes: DRB3 (encoding DRw52 p chains) is expressed on DR3, 5 and 6 haplotypes, DRB4 (encoding DRw53 p chains) is expressed on DR4, 7 and 9 haplotypes, and DRB5 (encoding DRwl5 and 16 p chains) on DR2 haplotypes. All class II genes (except DRA) are polymorphic. The class I I products are heterodimeric trans-membrane glycoproteins consisting of non-covalently associated a and p chains encoded by the A and B genes.

Downloaded by [McMaster University] at 03:23 23 April 2016

Genetic Predisposition to IDDM variations of the molecules expressed or the variations of the genes. 1) The serological typing techniques (microlymphocytotoxicity testing) use the reaction between an epitope on the HLA molecule and a specific antiserum. They define class I antigens (A1-Aw69, B5-Bw65, Bw4-Bw6, C w l - C w l l ) and a rangeof class II antigens (DRl- DRwl8, DRw52, DRw53, DQwl-9,) (Figure 2). 2) Cellular typing by mixed lymphocyte culture (MLC), a functional test based on the proliferation of T-cells that bear distinct class It molecules, defines the specificities Dwl-26, which are subtypes of class I1 molecules, probably determined by variations at the DRB1 locus. 3) Biochemical techniques such as immunoprecipitation with chain specific monoclonal antibodies followed by two dimensional gel electrophoresis (2-D PAGE) allow detection of a and p chain variants and transcomplementation molecules. Important advances have been achieved in recent years by DNA technology which detects class II gene polymorphism at the DNA level and this has allowed detailed mapping of the HLA system. 4) Restriction fragment length polymorphism (RFLP) is analysed with the Southern Blot technique. After digestion of genomic DNA with endonucleases (or restriction enzymes) and electrophoretic separation, the fragments that correspond to the gene of interest are hybridised with a radiolabelled probe and visualised by autoradiography. 5) DNA sequencing has resulted in the most precise description of the polymorphic class II alleles. It has shown that each allele differs only by a few amino acids, which generally occur within short sequences, mostly in the first domain. 6) Dot blot analysis with allele specific oligonucleotide (ASO) probes allow the highly specific detection of these polymorphic residues. The possibility of producing a large number of copies of a gene by polymerase chain reaction (PCR) has revolutionised the concepts and methods of DNA sequence determinations. In addition, it has greatly reduced the volume of blood samples necessary for such investigations and allow typing in large groups of subjects. These methods have defined eight DQA1 and 13 DQB1 alleles (DQA1"0101-0601, DQB1"0201-0604), four DPAl and 19 DPB1 alleles (DPA1"0101-0201, DPBl"01011901), as well as new alleles of DRB1, DRB3, DRB4 and DRB5 loci (10).

HLA and IDDM Association Simultaneously with the development of techniques allowing a more detailed analysis of the HLA system, the markers associated with IDDM have been mapped more precisely (Table 1). In 1973, Singal and Blajchman (11) first reported a greater frequency of HLA-515 among IDDM patients compared with normal controls. Subsequent reports showed a positive association also for 8 8 and 518 and an inverse association for B7 (12-14). Genetic linkage between HLA and IDDM was confirmed by family studies (13, 14), suggesting that genes within or near the HLA region are involved in the susceptibility. The development later of class II serology and MLC (Table 1) showed much higher

429

Table 1. Historical evolution of HLA-IDDM associations.

Technique

Marker

Association with IDDM ~

1973-77

Serology

1980

Mixed lymphocyte culture, Class II serology

1983

1987

1990

Restriction fragment length polymorphism Two dimensional gel electrophoresis Sequence, Allele specific oligonucleotides

~~

+

88, 815, 818 87

-

Dw3, Dw4 DR3, DR4 Dw2, DR2

+

-

DQ,, BamHll2Kb, 4Kb 3.7Kb

-

+

+

DQ, 3.2 DQ, Asp57-

+

DQ, Asp57+

-

DO Arq52+/DQ.,As~57-

+

+

frequencies of the class II antigens DR3 (Dw3) and DR4 (Dw4) compared with the class I antigens in patients versus controls (5, 15-1 9), indicating that the DR region is more closely associated with IDDM than class I. The associations of the latter are, therefore, secondary to linkage disequilibrium of 8 8 and 518 with DR3 ( D w ~ ) , and of 515 with DR4 (Dw4) (18-20). The strongest inverse association was found with DR2 ( D w ~ )which , is in linkage disequilibrium with B7 (19-21). Population studies have shown that the frequencies of the different HLA antigens and their associations with disease vary according to ethnic and geographical origin (19, 22). In Caucasians over 90 Yo of IDDM patients are DR3 or DR4, compared with 40 to 50 o/o of controls (relative risk, RR, 3 to 6). Three principal extended haplotypes -two of which include complotypes -found in linkage disequilibrium with DR3 and DR4 i.e.: A7, Cw7, B8, BfS, C4AQ0, C4B7, DR3 (found mainly in Northern European populations), Aw30, Cw5, B18, BfF7, C4A3, C4BQ0, DR3 (more frequent in Mediterranean populations) and A2, Cw3, Bw62 (15), BfS, DR4, are found more frequently among IDDM patients (RR = 610) (20, 23, 24). In all Caucasian populations the greatest risk is found in DR3/DR4 heterozygous individuals, who make up 30 to 50 Yo of juvenile IDDM patients in different studies (17, 19, 23-26), but only 1 to 6 % of the controls (RR 15 to 50). This high risk contrasts with the low risk of subjects who possess only one of these antigens, either together with any other DR antigen or in the homozygous state. The excess of heterozygotes is strong evidence for two distinct susceptibility genes linked to DR3 and DR4 and against simple recessive, dominant or intermediate models of inheritance (27).

DR3, DR4 and Risk of Developing Diabetes The DR3, DR4 heterozygous combination has also received much attention as a marker of genetic susceptibilitywith a view to identifying subjects at risk of developing diabetes. In the general population the risk is usually evaluated by the Odd's ratio which estimates the relative risk of

Downloaded by [McMaster University] at 03:23 23 April 2016

430

Deschamps

Beressi

developing the disease when the marker is present in an individual relative to the risk when it is absent (5). The marker specific absolute risk for a given population can be estimated from the frequency of the marker in patients and controls and the overall risk for the population (25). Similarly, the risk can be estimated for first degree relatives of diabetic patients. Among siblings of diabetic children who have an overall life-long risk of 5-6 o/o (2, 3) and a risk of about 3 % by the age of 20 years (28) the risk differs according to both the number of HLA haplotypes they share with their diabetic sibling (10 %, 2 % and