Study deepens understanding of molecular genetics in diabetes mellitus

AGÊNCIA FAPESP/DICYT The human immune system can fail to recognize organs and tissues as belonging to the individual concerned and instead attack them, causing autoimmune diseases. A study performed by researchers at the University of São Paulo's Ribeirão Preto School of Medicine (FMRP-USP) in Brazil shows how this happens at the molecular level.

The researchers detected microRNA (miRNA) molecules capable of altering the expression of certain defense genes in immune system cells so that T-lymphocytes inadvertently attack insulin-producing beta cells in the pancreas, causing type 1 diabetes mellitus.

The study resulted from the FAPESP-funded Thematic Project “Control of the transcriptome in diabetes mellitus”, supported by FAPESP and conducted at the Center for Research on Inflammatory Diseases (CRID), one of the Research, Innovation and Dissemination Centers (RIDCs) supported by FAPESP. A paper on the study was published in the journal PLoS One.

The study was also part of research performed by Thaís Arouca Fornari Tavares for her PhD in genetics at FMRP-USP. Her master’s research was supported by a scholarship from FAPESP.

“We showed the specific mechanism whereby miRNAs participate in alterations to the molecular genetic control of T-lymphocytes that make them attack insulin-producing beta cells in the pancreas,” said Geraldo Aleixo da Silva Passos Júnior, Associate Professor at FMRP-USP and principal investigator for the project.

Autoimmune type 1 diabetes accounts for between 5% and 10% of all diabetes cases in Brazil according to the Brazilian Diabetes Society.

How T-lymphocytes attack pancreatic beta cells was already known, Passos told Agência FAPESP. After developing in the thymus, a gland located in the chest, T-lymphocytes migrate to the spleen and later enter the blood stream to perform their immunological functions.

However, some clones of T-lymphocytes, such as CD4+ or CD8+ T cells, do not recognize pancreatic proteins as elements of the organism and infiltrate the pancreas, where they start to destroy insulin-producing beta cells.

This autoimmune reaction, known as insulitis, eventually causes type 1 diabetes. “Although this process has been fairly thoroughly investigated, until we performed our study, it was not known which elements were involved in altering control of the functional genome of these T-lymphocytes that attack insulin-producing beta cells,” Passos said.

The researchers set out to identify these elements by monitoring the development of T-lymphocytes, from maturation in the thymus and migration to the spleen to infiltration of the pancreas and destruction of insulin-producing cells.

For this purpose, they deployed an experimental model using non-obese diabetic (NOD) mice, which have a form of the disease comparable with type 1 diabetes in humans.

“When this type of mutant mouse reaches between five and eight months of age, the T-lymphocytes in its immune system start attacking the insulin-producing beta cells in its pancreas. In humans, this process also occurs, but it’s gradual and can take a few years, so that type 1 diabetes is diagnosed in infancy or early adolescence,” Passos said.

The T-lymphocytes produced by the immune system of the NOD mice were isolated in the different stages of development, first in the thymus, then in the spleen and finally in the pancreas.

Interaction identified

Using a functional genomics technique called a microarray, the researchers performed a complete study of T-lymphocyte gene expression in the NOD mice during each of these three stages.

In this study, the researchers identified messenger RNAs (mRNAs) and miRNAs in the animals’ T-lymphocytes. mRNAs convey DNA-encoded information from the cell nucleus to the cell cytoplasm, whereas miRNAs interact with mRNAs in the cytoplasm to prevent cells from producing proteins.

Using a bioinformatics tool to analyze the complete set of mRNAs and miRNAs, the researchers were able to identify which elements interacted during the evolution of T-lymphocytes.

“This is the first time a research group has demonstrated all the interactions between mRNAs and miRNAs in T-lymphocytes, from their development in the thymus through migration to the spleen to infiltration of the pancreas, in the context of type 1 diabetes,” Passos said.

The results of the analysis showed alteration of the expression of mRNAs that code for two proteins – Ccr7 and Cd247 (CD3 zeta) – in T-lymphocytes that attack the pancreas.

The researchers found that this alteration resulted from the action of a specific miRNA (miR-202-3p) and could lead to autoimmunity against the pancreas.

“The possibility that these two mRNAs could participate in the autoimmune process had already been hypothesized,” Passos said. “We’ve now discovered how this occurs by identifying a specific miRNA that disturbs molecular control in the T-lymphocytes that attack insulin-producing beta cells.”

In his opinion, the study opens up prospects for the design of oligonucleotides – fragments of DNA or RNA – that could help to control type 1 diabetes.

“We’ve identified possible targets,” he said. “We now plan to perform a new study to investigate how we could interfere in this inflammatory process, which is insulitis, by preventing the action of miR-202-3p and whether this alters the development of type 1 diabetes.”