Mutations Drive Unrestrained Secretion

The USP8 mutations identified in adenomas of the pituitary gland lead to overproduction of ACTH. Panel A: ACTH-producing cells in a normal gland. The other panels show cells non-mutant (B) or mutant (C) for USP8. Credit: S. Sbiera, Universität Würzburg

The USP8 mutations identified in adenomas of the pituitary gland lead to overproduction of ACTH. Panel A: ACTH-producing cells in a normal gland. The other panels show cells non-mutant (B) or mutant (C) for USP8. Credit: S. Sbiera, Universität Würzburg

Benign tumors in the pituitary gland lead to uncontrolled secretion of the stress hormone cortisol by the cells of the adrenal cortex. An international research effort has now characterized a new mechanism that triggers the syndrome.

Many individuals who suffer from Cushing syndrome are easy to recognize: They tend to be overweight particularly around the waist, and have round faces and bull necks. In addition to these obvious features, most of them have high blood pressure, develop muscle weakness, become diabetic and are extremely susceptible to infection. Cushing syndrome can often be treated effectively by surgical intervention, but patients succumb to infections or cardiovascular disease if the condition is left untreated.

In their efforts to understand how benign tumors in the pituitary provoke the development of Cushing’s disease, researchers based in Munich, Würzburg and Tokyo led by Professor Martin Reincke (Director of LMU’s Medical Clinic IV at Munich University Medical Center) have now pinpointed a novel molecular mechanism responsible for the condition. The results of the study have just appeared in Nature Genetics.

The perils of incessant secretion

All of the symptoms that typify Cushing syndrome are attributable to the unregulated secretion of the hormone cortisol – generally referred to as cortisone. Cortisol is normally released into the bloodstream only in stress situations, and helps the organism to cope with the challenge. However, when secreted in an uncontrolled fashion, the result is physiological havoc. Cortisol is synthesized in, and secreted by specialized cells in the adrenal cortex in response to the binding of a different hormone, the adrenocorticotropic hormone (ACTH). ACTH in turn is produced in the pituitary gland. Excessive cell proliferation in the pituitary can result in the formation of benign tumors (adenomas), which may lead to overproduction of ACTH and a corresponding increase in the level of circulating cortisol. However, the connecting links between the two processes are incompletely understood.

“We have now shown that tumor cells in more than one-third of patients with Cushing’s disease carry a mutation in one specific gene, which codes for an enzyme called ubiquitin-specific protease 8,” says Martin Fassnacht (Würzburg University Hospital), one of the authors of the publication. The mutation was discovered in the course of a detailed genetic characterization of benign tumors of the pituitary gland that overproduced ACTH.

Protease defect sets off a chain reaction

Ubiquitin-specific protease 8 (USP8) is one of a family of enzymes which play a key role in the destruction of proteins that are required only transiently by cells. One such protein is the receptor for epidermal growth factor (EGF), which is degraded and disposed of only when the USP8 gene is inactive, and no USP8 protein is present. The collaboration found that the effect of the mutations identified in pituitary tumor tissues is to keep the USP8 permanently active. As a consequence, the EGF receptor escapes demolition, and is instead recycled to its site of action on the cell membrane. The upshot of this is a life-threatening chain reaction, in which unrestrained synthesis of ACTH leads to uninhibited secretion of cortisol. “The identification of mutations in USP8 is a significant finding, because it opens up entirely new diagnostic and therapeutic approaches to the management of Cushing’s disease,” Martin Reincke adds.

Long-term focus on Cushing’s disease

Indeed, this is not the first time that the collaboration between the teams in Munich and Würzburg has shed light on the pathogenesis of Cushing’s disease. The two groups have previously identified mutations in a gene that is expressed in the adrenal cortex as a frequent cause of the pathological secretion of cortisol in a different patient population. The results of that study appeared in February 2014 in the “New England Journal of Medicine“. And only last week, a paper providing a detailed characterization of the molecular effects of the latter set of mutations was published in “Nature Communications“.

More information: “Mutations in the deubiquitinase gene USP8 cause Cushing’s disease.” Nature Genetics (2014) DOI: 10.1038/ng.3166

Genetics Research Demystifies Fatal Glandular Disease (Cushing’s)

Researchers at Tokyo Institute of Technology have identified genetic mutations responsible for Cushing’s disease, a potentially fatal glandular condition.

Symptoms of Cushing’s disease include weight gain, muscular weakness, mood and reproductive problems, and if untreated patients can die from the resulting infections and cardiovascular problems. Although first described by Harvey Cushing back in 1932, as Martin Reincke and colleagues in Germany and Japan point out in their latest Nature Genetics report, the mechanism causing the disease “has remained obscure since its first description”. Now by sequencing the tissues responsible the researchers have identified clusters of mutations that cause Cushing’s disease as well as how these mutations bring the disease into effect.

The disease arises from benign tumours on glandular pituitary tissue – corticotroph adenomas – which excessively secrete the hormone adrenocorticotropin (ACTH). Previous studies sought to identify mutations that might cause the disease through sequencing candidate genes and microarray studies, but these made little progress. Instead, the researchers applied a particular type of DNA sequencing known as ‘exome sequencing’ to the pituitary corticotroph adenoma.

The collaboration included researchers from Ludwig-Maximilians-Universität Munich, the University of Würzburg, the Max Planck Institute, the Helmholtz-Center Munich, Universität Hamburg , Universität Erlangen in Germany and Tokyo Institute of Technology in Japan. The research team exome-sequenced samples from 10 patients with Cushing’s disease and noticed a small number of protein altering mutations in the adenoma tissue. The researchers further identified the gene harbouring the mutations as ubiquitin-specific protease 8 (USP8), and were able to pinpoint the region of USP8 prone to mutation in Cushing’s disease.

Previous research observations of Cushing’s disease have highlighted strong expression of another gene, epidermal growth factor receptor (EGFR). By examining EGFR in HeLa cells expressing USP8, the researchers behind this latest research demonstrated that this was the result of USP8 mutations inhibiting downregulation of EGFR.

The researchers conclude that their results “not only identify the first of so far enigmatic driver mutations in corticotroph adenomas but also elucidate a novel mechanism by which the EGFR pathway is constitutively activated in human tumours.” Further research will be required for a more detailed understanding of genetic onset of the disease.

Reference

Martin Reincke etal, Nature Genetics, Advance Online Publication 9 December 2014

Background

Cushing’s disease adenomas

The adenomas that cause Cushing’s disease are benign tumours of epithelial tissue that grow on the pituitary gland. The tumours comprise corticotroph cells, a hormone producing cell that secretes asdrenocorticotropin (ACTH). While the pathological role of ACTH hypersecretion was already known, previous studies had been unable to identify the molecular mechanisms behind these hormone processes that lead to Cushing’s disease.

Exome sequencing

When RNA is processed by splicing, parts of the RNA – the introns – are removed. The remaining RNA, the exons, are collectively referred to as the exome.

While DNA sequencing finds the sequence of proteins for the whole DNA, by focusing on the exons, exome sequencing provides information specifically on the protein-coding genes. Changes to these genes are more likely to have significant ramifications on the organism.

Ubiquitination and USP8

Ubiquitination is a reversible protein modification process that occurs by means of a small protein called ubitquitin, which is found in all eukaryotic cells (cells containing a nucleus and other structures enclosed within a membrane). Ubiquitination regulates the fate and function of proteins.

USP8 is a ubiquitin-specific protease enzyme that can remove ubitquitin molecules from target proteins. The discovery of a high number of mutations in the USP8 gene in Cushing’s disease prompted the researchers to make further investigations on the mutant USP8 enzymes at biochemical and cellular levels. From these studies they could identify the mechanisms behind the mutations and the effect on epidermal growth factor receptor (EGFR), a gene that mediates the synthesis of an ACTH precursor.

Figure (click to view larger)

corticotroph

Figure caption: Schematic representation showing the proposed mechanisms how USP8 mutations lead to increased ACTH secretion and tumorigenesis in corticotroph.

Further information

Yukiko Tokida, Asuka Suzuki

Center for Public Affairs and Communications, Tokyo Institute of Technology

2-12-1, Ookayama, Meguro-ku, Tokyo 152-8550, Japan

E-mail: media@jim.titech.ac.jp

URL: http://www.titech.ac.jp/english/

Tel: +81-3-5734-2975     Fax: +81-3-5734-3661

About Tokyo Institute of Technology

As one of Japan’s top universities, Tokyo Institute of Technology seeks to contribute to civilization, peace and prosperity in the world, and aims at developing global human capabilities par excellence through pioneering research and education in science and technology, including industrial and social management. To achieve this mission, we have an eye on educating highly moral students to acquire not only scientific expertise but also expertise in the liberal arts, and a balanced knowledge of the social sciences and humanities, all while researching deeply from basics to practice with academic mastery. Through these activities, we wish to contribute to global sustainability of the natural world and the support of human life.

 

Source: Tokyo Institute of Technology, Center for Public Affairs and Communications: http://www.healthcanal.com/genetics-birth-defects/58155-tokyo-institute-of-technology-research-genetics-research-demystifies-fatal-glandular-disease.html

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