Overview and Principles of Pediatric Endocrinology

Historical background

Endocrinology is a discipline of science that seeks to understand how chemical signals secreted by cells regulate the function of distant (endocrine) or local (paracrine) tissues, or even their own function (autocrine), to integrate vital processes of life, such as growth, reproduction, and metabolism ( Fig. 1.1 ). Classical endocrinology derived from careful clinical observation such as, for example, the gigantism associated with pituitary tumors, or the characteristic bodily changes now known as Cushing disease , which is also associated with pituitary tumors; histology indicated the former was likely the result of a product made by “acidophilic cells,” whereas the latter was associated with the expansion of “basophilic cells.” The products of these acidophilic or basophilic cells had to traverse the bloodstream to reach their distant and often multiple targets. Hence these were internal (“endocrine”) secretions, unlike the chemical substances secreted into ducts leading to a target tissue, for example, salivary glands into the cheeks; pancreatic enzymes into the duodenum (“exocrine”). Cushing disease was associated with hypertrophy of the adrenal cortex and certain tumors of adrenocortical tissues mimicked the features of Cushing disease. Therefore it was readily postulated that the pituitary secretes a substance that affects the adrenal glands and function; this substance was named adrenocorticotrophic hormone ( ACTH ) and it was deduced that the features of Cushing disease/syndrome were the result of a product or products from the adrenal gland. The destruction of adrenal tissue by tuberculosis or tumor was identified by Thomas Addison in 1855 and treatment of this entity with adrenal extracts, resulting in marked improvement, was first undertaken by William Osler in 1896. However, the purification of these “internal secretions” began in earnest only at the turn of the 20th century with spectacular success, as reviewed by Dr. Delbert Fisher. In the first quarter of the 20th century, epinephrine, thyroxin, insulin, and parathyroid hormone were purified, followed by the purification of the sex steroids from the ovary and testis, as well as the pituitary and placental gonadotropins that stimulated the gonads to secrete sex steroids. These purifications required laborious chemical methods and the elucidation or measurement of their function required costly and cumbersome bioassays. For example, the assay of the potency of insulin, discovered in 1921, required the use of rabbits; the definition of 1 international unit (IU) of insulin was assigned to be the amount of insulin that lowers the blood glucose of a healthy 2-kg rabbit, fasted for 24 hours, to 45 mg/dL or less, within 5 hours of injection. Clearly, such potency estimates reflected the relative crudeness of the purification; today’s pure recombinant human insulin possesses approximately 29 IU/mg, whereas the potency of porcine insulin in the early 1980s was around 23 IU/mg and likely less at the dawn of insulin therapy for diabetes mellitus. The bioassay of pure human insulin preparations has been abandoned; the purity of human insulin permits insulin to be now standardized as “one international unit of insulin (1 IU) is defined as the biological equivalent of 34.7 μg pure crystalline insulin.” It should be noted that 1 IU for insulin is not part of the International System of Units of the modern metric system, but is the pharmacologic International Unit as defined by the World Health Organization (WHO) Expert Committee on Biological Standardization. In addition to their cumbersome nature, the lack of sensitivity in the early bioassays prohibited the ability to measure insulin in normal blood or other biologic fluids; refinements, such as measuring the incorporation of labeled glucose into the fat pad or diaphragm of a rat, represented only an incremental improvement. Similarly, growth hormone (GH), isolated in 1944, was initially bioassayed by its ability to increase the width of the tibia growth plate in rats, after a defined period of injections, and by comparing the unknown relative to a dose response of known concentrations administered in vivo. Attempts to improve sensitivity and specificity led to the “sulfation factor-somatomedin hypothesis” ( Fig. 1.2 ), in which it was postulated that GH leads to the generation of a second substance, derived from the liver, which mediates the growth-promoting (somatotropic) effects and hence was named somatomedin . Subsequent studies demonstrated that this somatomedin substance was identical to a factor in serum which had insulin-like properties in vitro, and this insulin-like effect was retained even after all insulin was “quenched” by an excess of antibodies specific for insulin. The convergence of these two pathways eventually led to the discovery of the factor now known as insulin-like growth factor ( IGF )-1 and later as well as IGF-2 and a family of binding proteins that acted as carriers of the hormones in serum but also possessed biologic properties of their own, by which the actions of the IGFs were mediated and modulated. Despite these early bioassay limitations, the scientific curiosity of these chemical substances that regulated functions as diverse as blood pressure (epinephrine, cortisol), water metabolism (arginine vasopressin, cortisol), growth (GH, sex steroids, thyroid hormone), glucose (insulin, cortisol), and reproduction (sex steroids, follicle-stimulating hormone [FSH], luteinizing hormone [LH]), spurred the formation of medical societies focused on endocrine diseases. As detailed in the article by Fisher from which the following historical aspects are quoted, the Association for the Study of Internal Secretions was established in 1918 in the United States and renamed the Endocrine Society in 1952.

Pediatric endocrinology began as a subspecialty only in the 1940s with the establishment of endocrine clinics at the Massachusetts General Hospital and at Johns Hopkins. These programs attracted postdoctoral trainees who then established their own pediatric endocrine units in the burgeoning growth of academic medical centers in the 1950s and 1960s. In the United States, the Pediatric Endocrine Society, initially named the Lawson Wilkins Pediatric Endocrine Society ( LWPES ), was formed in 1972 and established as a subspecialty by the American Board of Pediatrics, with its first certification examination in 1978; there are more than 1000 board-certified pediatric endocrinologists today in the United States. The European Society for Pediatric Endocrinology was formed in 1966, followed by the Japanese Society for Pediatric Endocrinology in 1967 and the British Pediatric Endocrine Group in 1972, all preceding the LWPES in the United States. Several other regional pediatric endocrine groups were formed, including the Australian Pediatric Endocrine Group, the Sociedad Latino Americana de Endocrinologia Pediatrica, and the Asia Pacific Pediatric Endocrine Society. All of these groups now meet jointly every 4 years at an International Pediatric Endocrine Congress.

Impact of hormonal assays and molecular biology

Two discoveries revolutionized the field of endocrinology and led to an explosion of basic, as well as clinically relevant, therapeutic knowledge in the second half of the 20th century. The first was the development of radioimmunoassay by Yalow and Berson, reported for insulin in 1960. Here was a method for measuring the low concentrations of a hormone, using as little as 10 to 50 μL of a biological fluid in an accurate, reproducible way, with precision and sensitivity adequate for in vivo studies in humans or other species, as well as in vitro studies, such as the regulation of insulin secretion by nutrients, hormones, ions, and pharmaceutical agents in whole animals, including humans, in vivo, or in isolated perfused pancreas, or in isolated islets. This was followed by the rapid development of assays for various hormones and an explosion of discovery, including the distinction between absolute and relative insulin deficiency as the difference between “juvenile” and “maturity onset” diabetes now known respectively as Type 1 DM and Type 2 DM , the regulation of GH secretion in normal individuals at different ages and in clinical disorders of growth, the changes in thyroid function at birth and the possibility of screening for neonatal hypothyroidism, and the changes in gonadotropins and sex hormones during the process of normal and abnormal puberty and later in aging. The discovery and purification of the hypothalamic-releasing hormones for thyroid-stimulating hormone (TSH), FSH/LH, GH, and ACTH were made possible by these precise assays using (rat) pituitary cells, perfused by protein fractions derived from the hypothalami of animals. The discovery that a hormone produced in a cell could affect the function of its neighboring cell(s), without traveling through the bloodstream (paracrine action) or even its own function (autocrine), was also enabled by the use of these sensitive and precise tools, expanding our concepts of a hormone as a chemical messenger that influences, directs, and coordinates cellular functions throughout the body (see Fig. 1.1 ). Similar principles enabled the identification of the receptor molecules at the cell surface or in its cytoplasm that permit the hormone signal to be transduced to a message for turning biological processes on, or off, in specific tissues. Refinements using the principles of radioimmunoassay (RIA), but without radioactivity, as well as newer techniques of liquid chromatography coupled with mass spectrometry, are the bases of modern laboratory methodologies for hormone measurement, as well as for other chemical substances, such as drugs; examples of modern application of these methods, as well as the pitfalls in performance and interpretation, are reviewed in Chapter 4 . How these hormonal signals elicit a specific response after recognition and binding to a specific cell surface or cytoplasmic receptor, and the subsequent cascade of events known as the signal transduction pathways and their relevance to pediatric endocrinology are discussed in Chapter 3 . The notion that a hormone may not be capable of eliciting a response, despite high concentrations, was implicit in the entity labeled pseudohypoparathyroidism by Dr. Fuller Albright, but receptors and their signal transduction pathways were only systematically investigated beginning in the 1970s. These systematic studies, still ongoing, continue to identify the mechanistic pathways by which a hormone, after binding to its receptor, may elicit a response in one tissue but not in another. There may be other reasons why a hormone does not elicit an appropriate tissue response despite apparent high concentrations in the circulation. For example, an abnormal sequence in a hormone may prevent its full action at the receptor, and feedback control increases the hormone’s secretion, leading to high concentrations of partially functioning hormone with only minor or moderate impairment of function. Examples of such abnormalities include disorders of proinsulin conversion to insulin. ^{11 and OMIM #616214} Depending on the site of the abnormality in cleavage of proinsulin, both the measurement of insulin and proinsulin or its cleavage product C-peptide, may demonstrate higher than normal values that could be interpreted as “insulin resistance.” These studies also recognized that an activating mutation in a receptor will mimic the action(s) of a hormone, although the hormone concentration may be barely detectable, as exemplified by the precocious puberty in McCune-Albright syndrome or by an activating mutation of the LH receptor in males, which results in precocious puberty with high testosterone, but paradoxically very low concentrations of LH, an entity named testotoxicosis (see Chapter 18 ). In contrast, loss of function mutations cause the same clinical syndrome as hormone deficiency, even though the hormone concentration is markedly increased compared with normal, as exemplified in Laron syndrome, with poor growth and low IGF-1 concentrations, despite high circulating growth hormone concentrations (see Chapter 11 ). Thus the concentration of a hormone may not be directly related to its action. In summary, it is the ability to measure hormones, as well as intermediary molecules and metabolites, such as cyclic adenosine monophosphate at low concentrations (e.g., picomolar to nanomolar), in small volumes of biological fluids that enabled the rapid proliferation and understanding of endocrine regulation and function. It may be difficult for any reader who was not brought up in the era of bioassays to fully appreciate the impact of the application of the tool of RIA and its modifications, as various competitive protein binding assays without radioactive markers, to modern endocrine concepts and practice. The theoretical basis for the competitive binding assays, pitfalls in their interpretation, and their application to studies of receptor-ligand interactions are detailed in Chapter 4 .

The second revolution built on the discovery of the double helix by Crick, Watson, and Wilkins, a discovery for which they received the Nobel Prize in 1962. This discovery and its ongoing offspring propelled the ability to identify the molecular basis of cell function, the genes that regulate these processes, and the genetic mutations that underlie congenital or acquired disorders, including those of the endocrine system. Pediatric medicine is a particular beneficiary of these techniques because congenital malformations of the endocrine glands, the abnormalities of hormone signaling, as a result of defective hormone synthesis, hormone processing or receptor function to recognize and act on the hormonal signal are at the core of pediatric endocrinology, as reflected in this book. Chapters 2 and 3 provide an overview of the molecular and genetic methodologies applied in practice and research and also discusses the pitfalls in interpreting the results of genetic mutational analyses. This is a rapidly evolving field powered by the newer sequencing technologies (collectively termed next-generation sequencing [ NGS ]). These technologies enable the sequencing of deoxyribonucleic acid (DNA) and ribonucleic acid much more quickly and cheaply than the previously used Sanger sequencing, and in concert with the ability to store and analyze huge data sets via computers have revolutionized the study of genomics and molecular biology and the direct clinical application of this knowledge. For example, whole exome sequencing and whole genome sequencing is increasingly being applied and is becoming part of established medical diagnostic practice in several situations, for example, neonates with multiple congenital malformations or metabolic disturbances in the newborn intensive care unit. Use of fetal DNA in maternal cell free plasma is becoming the standard for antenatal diagnosis of aneuploidy, without the need for invasive screening procedures, such as amniocentesis or chorionic villous sampling, and has been used to diagnose congenital adrenal hyperplasia in at-risk pregnancy of heterozygote carriers of this disease. NGS allows prompt genetic diagnosis of different mutations responsible for a specific disease or syndrome complex, for example, Neonatal Diabetes Mellitus, so that remedial treatment can be expeditiously provided, thereby altering what has hitherto been considered the natural history of the particular disorder. Direct therapeutic applications, such as, for example, choice of drug for maximum effectiveness, while avoiding drug interactions or excluding drug sensitivity, are also being explored. Gene therapy and gene editing for pediatric endocrine disorders are on the horizon and some show promise, but with many practical and ethical issues still to be resolved, as discussed in Chapters 2 and 3.

Unique aspects of pediatric endocrinology