The search for IDDM susceptibility genes: the next generation

Two human chromosomal regions, the HLA region on chromosome 6p2l and the insulin gene region on chromosome 11p15, have been investigated in detail for more than 10 years for the presence of IDDM susceptibility genes. Recent genome searches indicate the possible existence of many additional susceptibility genes in IDDM. The lengthy and protracted studies to prove the linkage and identity of the susceptibility genes in the HLA and insulin gene regions provide a perspective and background for understanding the complexities and time course for characterization of the putative additional IDDM susceptibility genes uncovered by genome searches.     

Electron tomography of large, multicomponent biological structures

Genome-wide scans for linkage of chromosome regions to type 1 diabetes in affected sib pair families have revealed that the major susceptibility locus resides within the major histocompatibility complex (MHC) on chromosome 6p21 (lambda s = 2.5). It is recognised that the MHC contains multiple susceptibility loci (referred to collectively as IDDM1), including the class II antigen receptor genes, which control the major pathological feature of the disease: T lymphocyte-mediated autoimmune destruction of the insulin-producing pancreatic beta cells. However, the MHC genes, and a second locus, the insulin gene minisatellite on chromosome 11p15 (IDDM2; lambda s = 1.25), cannot account for all of the observed clustering of disease in families (lambda s = 15), and the scans suggested the presence of other susceptibility loci scattered throughout the genome. There are four additional loci for which there is currently sufficient evidence from linkage and association studies to justify fine mapping experiments: IDDM4 (FGF3/11q13), IDDM5 (ESR/6q22), IDDM8 (D6S281/6q27) and IDDM12 (CTLA-4/2q33), IDDM4, 5 and 8 were detected by genome scanning, and IDDM12 by a candidate gene strategy. The results suggest that the clustering of type 1 diabetes in families is due to the sharing of alleles at multiple loci, and that the as yet unidentified environmental factors are not causing clustering, but instead appear to influence the overall penetrance of genetically programmed susceptibility. The data are consistent with a polygenic threshold model for the inheritance of type 1 diabetes.     

The genetics of human noninsulin-dependent (type 2) diabetes mellitus

Familial aggregation and concordance in monozygotic and dizygotic twins argue strongly for a genetic etiology to noninsulin-dependent diabetes (NIDDM). Nonetheless, studies of pathways implicated by the known physiology have failed to identify gene defects that can explain the genetic susceptibility. In contrast, studies of early onset dominant diabetes have revealed three major loci resulting in diminished insulin secretion. Recently, studies have taken a new approach to map the genes causing typical NIDDM using large numbers of families or sibling pairs. The first reports of these studies have suggested possible loci on chromosomes 1, 2 and 12, but no report has been confirmed. Other studies have examined the quantitative defects that may be precursors of clinical NIDDM such as hyperinsulinemia, hyperglycemia, insulin response to glucose and obesity. These studies have suggested additional loci that may contribute to NIDDM susceptibility, but the genes responsible for most of these loci remain unknown. Studies of NIDDM susceptibility and the role of obesity genes in that susceptibility have entered an exciting new phase, but the challenges of complex disease genetics in humans will have to be conquered to translate this research into preventive or therapeutic benefits.     

Linkage of genetic markers on human chromosomes 20 and 12 to NIDDM in Caucasian sib pairs with a history of diabetic nephropathy

The potential contribution of maturity-onset diabetes of the young (MODY) genes to NIDDM susceptibility in African-American and Caucasian NIDDM-affected sibling pairs with a history of adult-onset diabetic nephropathy has been evaluated. Evidence for linkage to NIDDM was found with polymorphic loci that map to the long arms of human chromosomes 20 and 12 in regions containing the MODY1 and MODY3 genes. Nonparametric analysis of chromosome 20 inheritance data collected with the MODY1-linked marker D20S197 provides evidence for linkage to NIDDM with a P value of 0.005 in Caucasian sib pairs using affected sibpair (ASP) analyses. Non-parametric analysis of chromosome 12 inheritance data collected with the MODY3-linked markers D12S349 and D12S86 provides evidence for linkage to NIDDM with P values of 0.04 and 0.006, respectively, in Caucasian sib pairs using similar analyses. No evidence for linkage of MODY1 and MODY3 markers to NIDDM in African-American sib pairs was observed. In addition, no evidence for linkage to MODY2 (glucokinase-associated MODY) was observed with either study population. Results of multipoint maximum logarithm of odds (LOD) score analysis were consistent with the ASP results. A maximum LOD score of 1.48 was calculated for linkage to MODY1-linked loci and 1.45 to MODY3-linked loci in Caucasian sib pairs. Tabulation of allele sharing in affected sib pairs with D20S197 and D12S349 suggests that affected sibling pairs may inherit susceptibility genes simultaneously from chromosome 20 and chromosome 12. The results suggest that genes contributing to NIDDM in the general Caucasian population are located in the regions containing the MODY1 and MODY3 genes.     

Role of myelin basic protein and proteolipid protein genes in multiple sclerosis: single strand conformation polymorphism analysis of the human sequences

Susceptibility to multiple sclerosis (MS) is widely held to have a strong genetic component. While the identities of genes conferring susceptibility are currently unknown, possible candidates include those genes coding for proteins which function in central nervous system (CNS) myelin. Two such genes are the human myelin basic protein (MBP) and proteolipid protein (PLP) genes, whose products make up approximately 80% of the total protein in CNS myelin. The association of a variable number tandem repeat (VNTR) 5' to the human MBP gene with MS has been the subject of conflicting reports. Here we test the hypothesis that mutations in the human MBP and PLP genes might be associated with MS by examining the entire expressed sequence of both genes by single strand conformation polymorphism (SSCP) analysis, using a panel of 71 MS patients and 71 controls. We have also re-examined the VNTR region in patients and controls. Three base changes were found in the human PLP gene and nine base changes in the human MBP gene; these were essentially equally distributed between patients and controls. No preferential distribution of various alleles of the VNTR between patients and controls was found. Although intronic and regulatory regions have not been examined, it would appear unlikely that mutations in these genes coding for the two major CNS myelin proteins contribute significantly to genetic susceptibility to MS.     

Lac repressor inducible gene expression in human breast cancer cells in vitro and in a xenograft tumor

As part of an ongoing search for susceptibility loci for NIDDM, we tested 19 genes whose products are implicated in insulin secretion or action for linkage with NIDDM. Loci included the G-protein-coupled inwardly rectifying potassium channels expressed in beta-cells (KCNJ3 and KCNJ7), glucagon (GCG), glucokinase regulatory protein (GCKR), glucagon-like peptide I receptor (GLP1R), LIM/homeodomain islet-1 (ISL1), caudal-type homeodomain 3 (CDX3), proprotein convertase 2 (PCSK2), cholecystokinin B receptor (CCKBR), hexokinase 1 (HK1), hexokinase 2 (HK2), mitochondrial FAD-glycerophosphate dehydrogenase (GPD2), liver and muscle forms of pyruvate kinase (PKL, PKM), fatty acid-binding protein 2 (FABP2), hepatic phosphofructokinase (PFKL), protein serine/threonine phosphatase 1 beta (PPP1CB), and low-density lipoprotein receptor (LDLR). Additionally, we tested the histidine-rich calcium locus (HRC) on chromosome 19q. All regions were tested for linkage with microsatellite markers in 751 individuals from 172 families with at least two patients with overt NIDDM (according to World Health Organization criteria) in the sibship, using nonparametric methods. These 172 families comprise 352 possible affected sib pairs with overt NIDDM or 621 possible affected sib pairs defined as having a fasting plasma glucose value of >6.1 mmol/l or a glucose value of >7.8 mmol/l 2 h after oral glucose load. No evidence for linkage was found with any of the 19 candidate genes and NIDDM in our population by nonparametric methods, suggesting that those genes are not major contributors to the pathogenesis of NIDDM. However, some evidence for suggestive linkage was found between a more severe form of NIDDM, defined as overt NIDDM diagnosed before 45 years of age, and the CCKBR locus (11p15.4; P = 0.004). Analyses of six additional markers spanning 27 cM on chromosome 11p confirmed the suggestive linkage in this region. Whether an NIDDM susceptibility gene lies on chromosome 11p in our population must be determined by further analyses.     

Mhc class I and non-class I gene organization in the proximal H2-M region of the mouse

A bacterial artificial chromosome (BAC) contig was constructed across the proximal part of the H2-M region from the major histocompatibility complex (Mhc) of mouse strain 129 (H2bc). The contig is composed of 28 clones that span approximately 1 megabasepair (Mb), from H2-T1 to Mog, and contains three H2-T genes and 18 H2-M genes. We report the fine mapping of the H2-M class I gene cluster, which includes the previously reported M4-M6, the M1 family, the M10 family, and four additional class I genes. All but two of the H2-M class I genes are conserved among haplotypes H2k, H2b, and H2bc, and only two genes are found in polymorphic HindIII fragments. Six evolutionarily conserved non-class I genes were mapped to a 180 kilobase interval in the distal part of the class I region in mouse, and their order Znf173-Rfb30-Tctex5-Tctex6- Tctex4-Mog was found conserved between human and mouse. In this Znf173-Mog interval, three mouse class I genes, M6, M4, and M5, which are conserved among haplotypes, occupy the same map position as the human HLA-A class I cluster, which varies among haplotypes and is diverged in sequence from the mouse genes. These results further support the view that class I gene diverge and evolve independently between species.     

Identification of microsatellite markers near the human ob gene and linkage studies in NIDDM-affected sib pairs

Non-insulin-dependent diabetes mellitus (NIDDM) is a complex metabolic disorder with a significant genetic component. Obesity is a frequent complicating factor for NIDDM. In the mouse, a number of single gene defects that result in obesity have been described. Mutations in one of these genes, the ob gene, results in both obesity and NIDDM. Recently, the cloning of the murine ob gene and its human homologue has been reported (Nature 372:425-432, 1994). In the present study, the contribution of genetic variation at the human ob locus to NIDDM susceptibility was assessed by analyzing allele sharing in NIDDM-affected sib pairs (ASPs) for markers located near the human ob gene. Four yeast artificial chromosome clones containing the human ob gene were isolated. These clones colocalized the ob gene and two microsatellite markers, D7S514 and D7S635, to a region of 280 kb on the long arm of human chromosome 7. The microsatellite markers were typed in 346 Mexican-American NIDDM-ASPs derived from 176 families and an additional 110 ethnically and geographically matched controls. No evidence of linkage or association between either microsatellite marker and NIDDM was observed in this population. These results suggest genetic variation in the human ob gene does not play a major role in susceptibility to NIDDM in Mexican-Americans.     

Human insulin receptor substrate-1 gene (IRS1): chromosomal localization to 2q35-q36.1 and identification of a simple tandem repeat DNA polymorphism

The protein designated as insulin receptor substrate-1 (IRS-1) is a major substrate for the insulin receptor tyrosine kinase. Since post-receptor defects in the insulin signalling pathway are a common feature of Type 2 (non-insulin-dependent) diabetes mellitus, we have cloned the human IRS-1 gene in order to study the role of genetic variation in this gene in the pathogenesis of diabetes mellitus. As a first step in these studies, we have mapped the IRS-1 gene to chromosome 2, bands q35-q36.1 and identified a simple tandem repeat DNA polymorphism in this gene that will be useful for genetic studies.     

Mutation of a mutL homolog in hereditary colon cancer

Some cases of hereditary nonpolyposis colorectal cancer (HNPCC) are due to alterations in a mutS-related mismatch repair gene. A search of a large database of expressed sequence tags derived from random complementary DNA clones revealed three additional human mismatch repair genes, all related to the bacterial mutL gene. One of these genes (hMLH1) resides on chromosome 3p21, within 1 centimorgan of markers previously linked to cancer susceptibility in HNPCC kindreds. Mutations of hMLH1 that would disrupt the gene product were identified in such kindreds, demonstrating that this gene is responsible for the disease. These results suggest that defects in any of several mismatch repair genes can cause HNPCC.     

Mutations in the hepatocyte nuclear factor-1alpha gene in Caucasian families originally classified as having Type I diabetes

Mutations in the hepatocyte nuclear factor-1alpha (HNF-1alpha) gene are the cause of maturity-onset diabetes of the young type 3 (MODY3), which is characterised by a severe impairment of insulin secretion and an early onset of the disease. Also at onset of diabetes some MODY patients show similar clinical symptoms and signs as patients with Type I (insulin-dependent) diabetes mellitus. The objective of this study was to estimate the prevalence of MODY3 patients misclassified as Type I diabetic patients. From a large population-based sample of unrelated Danish Caucasian Type I diabetic patients with an affected first degree relative, 39 patients (6.7%) who did not carry any high-risk HLA-haplotypes, i.e. DR3 or DR4 or both were examined by single-strand conformational polymorphism scanning and direct sequencing of the coding region and the minimal promoter of the HNF-1alpha gene. Four of the 39 Type I diabetic patients (10%) were identified as carrying mutations in the HNF-1alpha gene. One patient carried a missense mutation (Glu48Lys) in exon 1, two patients carried a missense mutation (Cys241Gly) in exon 4 and one patient carried a frameshift mutation (Pro291fsdelA) in exon 4. The mutations were all identified in heterozygous form, segregated with diabetes, and were not identified in 84 unrelated, healthy subjects. Furthermore, family history in three of the four families showed diabetes in four consecutive generations, suggestive of an autosomal dominant inheritance. In conclusion, about 10% of Danish diabetic patients without a high-risk HLA-haplotype, originally classified as having Type I diabetes could have diabetes caused by mutations in the HNF-1alpha gene. Clinical awareness of family history of diabetes and mode of inheritance might help to identify and reclassify these diabetic subjects as MODY3 patients.     

Molecular genetics of Huntington's disease]

Huntington's disease (HD) is a progressive neurodegenerative disorder which is clinically characterized by chorea, cognitive decline, and emotional disturbance; it is inherited in an autosomal dominant manner. The HD gene maps to chromosome 4p16.3. Our linkage analysis demonstrated a significant genetic linkage between Japanese HD families and the flanking markers, D4S127, D4S43. The molecular basis of the disease is an expansion of CAG repeat in the huntingtin gene. We performed molecular analysis of the repeat in Japanese HD patients and normal controls. The size of the CAG repeat ranged from 37 to 95 repeats in affected subjects and from seven to 29 in normal controls. A significant correlation was found between the age of onset and the CAG expansion. The length of the expanded repeat is unstable in meiotic transmission and large increases occur in paternal transmission. At the same time the CCG repeat polymorphism adjacent to the CAG repeat was analysed and haplotypes of HD chromosomes were identified. Striking linkage disequilibrium was found between the CAG repeat expansion and an allele of (CCG)10 in Japanese HD chromosome. It is distinct from that described previously in western populations. Western HD chromosomes strongly associate with an allele of (CCG)7.     

Quantitative trait locus mapping of human blood pressure to a genetic region at or near the lipoprotein lipase gene locus on chromosome 8p22

Resistance to insulin-mediated glucose disposal is a common finding in patients with non-insulin-dependent diabetes mellitus (NIDDM), as well as in nondiabetic individuals with hypertension. In an effort to identify the generic loci responsible for variations in blood pressure in individuals at increased risk of insulin resistance, we studied the distribution of blood pressure in 48 Taiwanese families with NIDDM and conducted quantitative sib-pair linkage analysis with candidate loci for insulin resistance, lipid metabolism, and blood pressure control. We found no evidence for linkage of the angiotensin converting enzyme locus on chromosome 17, nor the angiotensinogen and renin loci on chromosome 1, with either systolic or diastolic blood pressures. In contrast, we obtained significant evidence for linkage or systolic blood pressure, but not diastolic blood pressure, to a genetic region at or near the lipoprotein lipase (LPL) locus on the short arm of chromosome 8 (P = 0.002, n = 125 sib-pairs, for the haplotype generated from two simple sequence repeat markers within the LPL gene). Further strengthening this linkage observation, two flanking marker loci for LPL locus, D8S261 (9 cM telomeric to LPL locus) and D8S282 (3 cM centromeric to LPL locus), also showed evidence for linkage with systolic blood pressure (P = 0.02 and 0.0002 for D8S261 and D8S282, respectively). Two additional centromeric markers (D8S133, 5 cM from LPL locus, and NEFL, 11 cM from LPL locus) yielded significant P values of 0.01 and 0.001, respectively. Allelic variation around the LPL gene locus accounted for as much as 52-73% of the total interindividual variation in systolic blood pressure levels in this data set. Thus, we have identified a genetic locus at or near the LPL gene locus which contributes to the variation of systolic blood pressure levels in nondiabetic family members at high risk for insulin resistance and NIDDM.     

Nausea and vomiting following thyroid and parathyroid surgery

The human serotonin transporter gene (hSERT) is a strong candidate for involvement in the pathogenesis of mood disorder and, using a UK Caucasian case-control sample, Collier et al found a significant association between bipolar disorder and the 12 allele of the VNTR polymorphism in intron 2 of this gene. In a European collaborative sample, Collier et al found a significant association between affective disorder and a functional deletion polymorphism in the promoter of hSERT. We have undertaken association studies using these polymorphisms in a British Caucasian sample comprising 171 DSM-IV bipolar probands, 80 DSM-IV major depression probands and 121 unrelated controls matched to bipolar probands for age, sex and ethnicity. We found no association between the promoter deletion and affective disorder but our findings with the VNTR polymorphism are similar to those of Collier and colleagues: we found a significant excess of the 12 repeat allele in bipolar probands (P = 0.031, one-tall) with a suggestion of a gene dosage effect (using genotypes bearing no 12 repeat allele as baseline, the increased risks conferred by genotypes bearing 12 repeat alleles were: heterozygote, OR = 1.24; homozygote, OR = 1.76). Our findings add to the evidence that variation at or near hSERT influences susceptibility to bipolar disorder in the British Caucasian population.     

Taking the pith out of reality: a reflexive methodology for psychiatric nursing research

HLA-DQ genes are the main inherited factors predisposing to IDDM. This gene region harbors long terminal repeat (DQ LTR) elements of the human endogenous retrovirus HER V-K, which we analyzed for a possible association with disease. We first investigated whether LTR segregate with DQ alleles in families. Members (n = 110) of 29 families with at least one diabetic child, unrelated patients with IDDM (n = 159), and healthy controls (n = 173) were analyzed. Genomic DNA was amplified for DQ LTR3 by a nested primer approach as well as for DQA1 and DQB1 second exons, to assign DQA1 and DQB1 alleles. DQ LTR segregated in 24 families along with DQ alleles. Of the 29 families, 20 index patients were positive for DQ LTR. The DQ LTR was in all patients on the haplotype carrying the DQA1 *0301 and DQB1 *0302 alleles. A majority of patients had DQ LTR (62%) compared with controls (38%) (p < 1.3 x 10(-5)), even after matching for the high-risk alleles DQA1 *0501, DQB1 *0201-DQA1 *0301, and DQB1 *0302 (79% of patients and 48% of controls; p < 0.02). Subtyping for DRB1 *04 alleles in all DQB1 *0302+ individuals showed 56% DRB1 *0401, DQB1 *0302 [LTR' patients vs. 29% controls with the same haplotype (p < 0.002)]. In conclusion, these data demonstrate the segregation of DQ LTR with DQA1, DQB1 alleles on HLA haplotypes. Furthermore their presence on DRB1 *0401-, DQA1 *0301-, and DQB1 *0302-positive haplotypes suggest that they contribute to DQ-related susceptibility for IDDM.