Type 1 Diabetes Mellitus (DM1): A genetic predisposition

Type 1 diabetes mellitus (DM1) is the most aggressive form and is considered the result of a specific autoimmune process against pancreatic beta cells , that is, the formation of antibodies that attack beta cells, leading to insulin deficiency.

The immune system is trained not to attack its own cells. Eventually, this immune “tolerance” fails and the result is an autoimmune disease, where the immune system starts to attack and damage healthy tissue.

UNDERSTANDING HLA

The major histocompatibility complex (MHC – Major Histocompatibility Complex ) is a large gene complex with multiple loci whose molecules present protein antigens to cells of the immune system, thus participating in the process of rejection of foreign or own tissues. This usually happens in coordination with the immune system that triggers an immediate response against these foreign bodies. Namely, in humans, this gene pool is called HLA ( Human Leukocyte Antigens ).

The main characteristic of the HLA complex is its polymorphism, that is, multiple stable forms of a gene in a population. Thus, different individuals within a species have slightly different forms of each HLA gene called alleles.

It is important to know that HLA is inherited, that is, we have one part of the mother and another part of the father. The HLA system is located on the short arm of human chromosome six. In short, HLA gene loci are grouped into three classes; I, II and III, according to their structure, function and location. The class I region is composed of the HLA-A, B, C loci , which encode the molecules present in virtually all nucleated cells. The class II region includes the loci HLA-DR, -DP and –DQ, which encode molecules present mainly on the surface of immunocompetent cells, including macrophages, dendritic cells, monocytes, activated T lymphocytes and B lymphocytes. Finally, the class III region has genes that encode components complement, such as C4A, C4B and factor B, in addition to containing genes for the 21-hydroxylase enzymes (CYP21), heat shock protein (Hsp.70) and tumor necrosis factors

The understanding of this polymorphism is substantial for understanding how HLA is related to autoimmune diseases, with the evolution of species, and also for the selection of donors in organ transplants.

DIABETES MELLITUS AND HLA

Two types of studies have been used to address the association between histocompatibility markers with diseases such as Diabetes Mellitus (DM): population and family studies. In population studies, the frequencies of HLA antigens or alleles observed in a group of unrelated patients are compared with those observed in healthy control subjects. The occurrence of association is assessed by comparing the frequencies of histocompatibility markers in patients and controls.

Diabetes Melito (DM) has been considered a disease with complex polygenic inheritance. Approximately 20 genes may be associated with susceptibility to the disease, but only 13 have statistically significant evidence of association. The greatest contribution comes from the region where the HLA genes are located, located on chromosome 6p21, since it contributes about 40% to the susceptibility to the disease (IDDM1 genes).

The DR and DQ loci are responsible for 40% to 50% of the genetic risk of developing DM1. As the HLA region exhibits a high degree of linkage imbalance (that is, DR and DQ alleles are not randomly associated with each other), HLA associations with the disease are better defined by haplotypes than by alleles. Thus, the greatest risk is conferred by the predisposition haplotypes HLA-DQA1 * 05: 01-DQB1 * 02: 01 (called DQ2), generally inherited with DRB1 * 03: 01 (DR3) and the HLA-DQA1 * -DQB1 * haplotype 03:02 (DQ8), inherited with DRB1 * 04: 01 or DRB1 * 04: 05 (DR4).

On the other hand, some haplotypes are associated with protection, particularly HLA-DRB1 * 15: 01 / DQA1 * 01: 02-DQB1 * 06: 02 (DR2-DQ6), with the DQB1 * 06: 02 allele being primarily responsible for protection from the organism to DM1.

However, the known genetic factors, so far, can be responsible for a maximum of 65% to 70% of DM1 cases. The incidence of the disease is increasing, so other factors and their relationship with our organism are being studied by specialists. Mainly, the hormonal influence, the influence of microorganisms present in our body and environmental factors, such as prolonged exposure to chemicals, are some of the examples.

METHODS OF DETECTING HLA ALLELES

Primarily the polymorphism of HLA specificities began to be discovered using cellular methods. However, advances in molecular genetics, especially with the discovery of the polymerase chain reaction (PCR – Polymerase Chain Reaction ), have enabled more detailed typification, with the identification of several alleles. Therefore, the most widely used molecular methods for this purpose are SSP ( Sequence-Specific primers ), SSOP ( Sequence-Specific Oligonucleotides Probes ) and SBT (S equence-Based Typing ).

The future of molecular genetics: New Generation Sequencing (NGS)

The Genome Project, through DNA sequencing or DNA mapping, opened several boundaries and, above all, provided the means to identify specific genetic mutations and diseases in more detail. In this sense, it allows to determine which stretches of DNA contain genes and which stretches carry regulatory instructions, turning genes on or off. It also paved the way for more personalized medicine, as it allows scientists to examine the extent to which a patient’s response to a drug is determined by their genetic profile.

As a result, these achievements required the development and improvement of new molecular technologies. The most recent is the new generation sequencing, or second generation sequencing or Next-Generation Sequencing (NGS).

Through NGS it is possible to perform the sequencing of an entire genome in a matter of days. Its speed, ability to generate data on a large scale and precision, at a cost lower than traditional methods, gains more space every day. The basic principle of NGS platforms is to sequence several different molecules (or several different fragments from the same genome) in parallel. In each reaction, a single DNA sequence is obtained, from a specific stretch. This is possible thanks to the integration of nanotechnology, robotics and computer tools in high performance devices.