Medical genetics was revolutionized during the 1980s by the application of genetic mapping to locate the genes responsible for simple Mendelian diseases. Most diseases and traits, however, do not follow simple inheritance patterns. Geneticists have thus begun taking up the even greater challenge of the genetic dissection of complex traits. Four major approaches have been developed: linkage analysis, allele-sharing methods, association studies, and polygenic analysis of experimental crosses. This article synthesizes the current state of the genetic dissection of complex traits—describing the methods, limitations, and recent applications to biological problems.

Genetic Dissection of Complex Traits

Recent progress in the understanding of autoimmune adrenal disease, including a detailed analysis of a group of patients with Addison’s disease (AD), has been reviewed. Criteria for defining an autoimmune disease and the main features of autoimmune AD (history, prevalence, etiology, histopathology, clinical and laboratory findings, cell-mediated and humoral immunity, autoantigens and their autoepitopes, genetics, animal models, associated autoimmune diseases, pathogenesis, natural history, therapy) have been described. Furthermore, the autoimmune polyglandular syndromes (APS) associated with AD (revised classification, animal models, genetics, natural history) have been discussed. Of Italian patients with primary AD (n 317), 83% had autoimmune AD. At the onset, all patients with autoimmune AD (100%) had detectable adrenal cortex and/or steroid 21-hydroxylase autoantibodies. In the course of natural history of autoimmune AD, the presence of adrenal cortex and/or steroid 21-hydroxylase autoantibodies identified patients at risk to develop AD. Different risks of progression to clinical AD were found in children and adults, and three stages of subclinical hypoadrenalism have been defined. Normal or atrophic adrenal glands have been demonstrated by imaging in patients with clinical or subclinical AD. Autoimmune AD presented in four forms: as APS type 1 (13% of the patients), APS type 2 (41%), APS type 4 (5%), and isolated AD (41%). There were differences in genetics, age at onset, prevalence of adrenal cortex/21-hydroxylase autoantibodies, and associated autoimmune diseases in these groups. “Incomplete” forms of APS have been identified demonstrating that APS are more prevalent than previously reported. A varied prevalence of hypergonadotropic hypogonadism in patients with AD and value of steroid-producing cells autoantibodies reactive with steroid 17-hydroxylase or P450 side-chain cleavage enzyme as markers of this disease has been discussed. In addition, the prevalence, characteristic autoantigens, and autoantibodies of minor autoimmune diseases associated with AD have been described. Imaging of adrenal glands, genetic tests, and biochemical analysis have been shown to contribute to early and correct diagnosis of primary non-autoimmune AD in the cases of hypoadrenalism with undetectable adrenal autoantibodies. An original flow chart for the diagnosis of AD has been proposed. (Endocrine Reviews 23: 327–364, 2002)

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Autoimmune Adrenal Insufficiency and Autoimmune Polyendocrine Syndromes: Autoantibodies, Autoantigens, and Their Applicability in Diagnosis and Disease Prediction

I. Introduction II. GHD in Children A. Clinical aspects B. Radiology C. GH provocative tests D. Physiological assessment of GH secretion E. IGFs F. IGFBPs G. Newer strategies for assessment of GH status H. Practical approach to the diagnosis of GHD III. GHD in Adults A. Clinical aspects B. GH provocative tests C. Physiological assessment of GH secretion D. IGF-I and IGFBP-3 E. Obesity F. Elderly G. Childhood onset H. Newer strategies for assessment of GH status I. Practical approach to the diagnosis of GHD

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The Diagnosis of Growth Hormone Deficiency in Children and Adults

The autoimmune thyroid diseases (AITD) are complex diseases that are caused by an interaction between susceptibility genes and environmental triggers. Genetic susceptibility, in combination with external factors (e.g., dietary iodine), is believed to initiate the autoimmune response to thyroid antigens. Abundant epidemiological data, including family and twin studies, point to a strong genetic influence on the development of AITD. Various techniques have been used to identify the genes contributing to the etiology of AITD, including candidate gene analysis and whole genome screening. These studies have enabled the identification of several loci (genetic regions) that are linked with AITD, and in some of these loci putative AITD susceptibility genes have been identified. Some of these genes/loci are unique to Graves' disease (GD) and Hashimoto's thyroiditis (HT), and some are common to both diseases, indicating that there is a shared genetic susceptibility to GD and HT. The putative GD and HT susceptibility genes include both immune modifying genes (e.g., human leukocyte antigen, cytotoxic T lymphocyte antigen-4) and thyroid-specific genes (e.g., TSH receptor, thyroglobulin). Most likely these loci interact, and their interactions may influence disease phenotype and severity. It is hoped that in the near future additional AITD susceptibility genes will be identified and the mechanisms by which they induce AITD will be unraveled.

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Searching for the autoimmune thyroid disease susceptibility genes: from gene mapping to gene function.

Pregnancy dramatically affects the hypothalamic-pituitary-adrenal axis leading to increased circulating cortisol and ACTH levels during gestation, reaching values in the range seen in Cushing's syndrome (CS). The cause(s) of increased ACTH may include placental synthesis and release of biologically active CRH and ACTH, pituitary desensitization to cortisol feedback, or enhanced pituitary responses to corticotropin-releasing factors. In this context, challenges in diagnosis and management of disorders of the hypothalamic-pituitary-adrenal axis in pregnancy are discussed. CS in pregnancy is uncommon and is associated with fetal morbidity and mortality. The diagnosis may be missed because of overlapping clinical and biochemical features in pregnancy. The proportion of patients with primary adrenal causes of CS is increased in pregnancy. CRH stimulation testing and inferior petrosal sinus sampling can identify patients with Cushing's disease. Surgery is a safe option for treatment in the second trimester; otherwise medical therapy may be used. Women with known adrenal insufficiency that is appropriately treated can expect to have uneventful pregnancies. Whereas a fetal/placental source of cortisol may mitigate crisis during gestation, unrecognized adrenal insufficiency may lead to maternal or fetal demise either during gestation or in the puerperium. Appropriate treatment and management of labor are reviewed.

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The Hypothalamic-Pituitary-Adrenal Axis in Pregnancy: Challenges in Disease Detection and Treatment

OBJECTIVE The best dynamic test for the assessment of the hypothalamic–pituitary–adrenal axis and the interpretation of the cortisol levels, remain a matter of controversy. We aimed to establish normal ranges with current assays, for both the short Synacthen (SST) and insulin stress tests (IST) and then to use these data to examine whether the SST can satisfactorily substitute for the IST in assessment of the hypothalamic–pituitary–adrenal axis.

DESIGN Thirty SSTs and 27 ISTs were performed on different healthy volunteers. The results of all paired tests performed on patients in the last three years are reviewed.

SETTING Programmed Investigation Unit.

SUBJECTS Fifty-seven healthy volunteers and 166 patients.

MAIN OUTCOME MEASURES Basal serum cortisol concentration and cortisol values obtained at 30 and 60 minutes during the SST compared to the maximum obtained with adequate hypoglycaemia (plasma glucose < 2mmol/l) during an IST.

RESULTS From normal data the mean-2SD 30-minute value during the SST was 392 nmol/l and 60-minute value was 497 nmol/l. The maximal cortisol response (mean − 2SD) during the IST was 519 nmol/l. Sixty patients failed the IST, none of whom had a basal cortisol > 450 nmol/l and only six (10%) had a 30-minute cortisol value > 600 nmol/l. The 30-minute value provided a better index than the 60-minute value. The basal, 30 and 60-minute values during the SST all correlated positively and significantly with the maximal cortisol on IST. The correlations persisted for all microadenomas and macroadenomas secreting prolactin, gonadotrophins or growth hormone, patients undergoing either pre or post-adenomectomy evaluation, and in those patients who had received long-term steroids provided that the medication had been reduced and stopped two days prior to admission.

CONCLUSIONS Using a 30-minute cortisol value > 600 nmol/l as a cut-off, the short Synacthen test provides a suitable substitute for the insulin stress test. Adopting this policy will decrease the number of insulin stress tests performed by one-quarter and thus provide a substantial saving without detriment to patient care.

The short Synacthen and insulin stress tests in the assessment of the hypothalamic-pituitary-adrenal axis

Although autoimmune Addison's disease (AAD) may occur as a component of the monogenic autoimmune polyendocrinopathy type 1 syndrome (APS1), it is most commonly found as an isolated disorder or associated with the autoimmune polyendocrinopathy type 2 syndrome (APS2) It is likely that sporadic (non-APS1) AAD is inherited as a complex trait; however, apart from the major histocompatibility complex, the susceptibility genes remain unknown We have examined polymorphisms at two non-major histocompatibility complex candidate susceptibility loci in sporadic (non-APS1) AAD: the cytotoxic T lymphocyte antigen-4 (CTLA-4) gene and the autoimmune regulator (AIRE-1) gene DNA samples from AAD subjects (n = 90) and local controls (n = 144 for CTLA-4; n = 576 for AIRE-1) were analyzed for the CTLA-4A/G polymorphism in exon 1 of the CTLA-4 gene and for the common mutant AIRE-1 allele (964de113) in United Kingdom subjects with APS1, by using the restriction enzymes Bst7II and BsrBI, respectively There was an association of the G allele at CTLA-4A/G in AAD subjects (P = 0008 vs controls), which was stronger in subjects with AAD as a component of APS2 than in subjects with isolated AAD In contrast, the mutant AIRE-1 964del13 allele was carried in one each of the 576 (02%) control subjects and the 90 (11%) AAD subjects as a heterozygote (P = 0254, not significant), suggesting that this common AIRE-1 gene abnormality does not have a major role in sporadic (non-APS1) AAD

Association analysis of the cytotoxic T lymphocyte antigen-4 (CTLA-4) and autoimmune regulator-1 (AIRE-1) genes in sporadic autoimmune Addison's disease.

S. J. Hurel

Papers

The short Synacthen and insulin stress tests in the assessment of the hypothalamic-pituitary-adrenal axis

Association analysis of the cytotoxic T lymphocyte antigen-4 (CTLA-4) and autoimmune regulator-1 (AIRE-1) genes in sporadic autoimmune Addison's disease.