DNA METHYLATION IN MEDICAL GENETICS

DNA Methylation Status in Medical Genetics Molecular Diagnostics

DNA Methylation Status in Medical Genetics Molecular Diagnostics

DNA methylation is an epigenetic process involved in the regulation of gene expression in two ways, directly by preventing the binding of transcription factors, and indirectly promoting the structure "closed" chromatin (Hirschhorn, 2002, pp 45-61). The DNA of 1000-1500 bp regions rich in CpG dinucleotides ("CpG islands"), which are recognized by DNA methyltransferase enzymes, which, during DNA replication methylate cytosine carbon 5 of the newly synthesized chain , maintaining the methylated state memory of the daughter molecule of DNA. It is generally considered that methylation is a unidirectional process, so, when a sequence acquires CpG methylation de novo, this modification is stable and is inherited as a clonal methylation pattern. Moreover, the loss of genomic methylation (hypomethylation) as primary event, is often associated with the neoplastic process and is proportional to the severity of the disease.

The genomes of preneoplastic cells, cancer and aging share three important changes in methylation levels as early events in the development of some tumors. First, the hypomethylation of heterochromatin leading to genomic instability and increased mitotic recombination events, and second, hypermethylation of individual genes, and finally hypermethylation of CpG islands of housekeeping genes and tumor suppressor genes(Risch, 2000, pp 847-856). The two levels of methylation may occur individually or simultaneously, in general, hypermethylation is involved in gene silencing and hypomethylation with the over-expression of certain proteins involved in the process of invasion and metastasis.

The methodological strategies for the analysis of the methylated CpG islands have been constantly evolving and currently has a number of techniques that share universal standards, optimum sensitivity and reproducibility. The success of most methods depends on the chemical conversion of unmethylated cytosines to uracils, by treatment with sodium bisulfite, which does not affect the 5-methylcytosine, reliably and marks the state and individual methylated or unmethylated of CpG dinucleotides(Cooper, 2004, pp 267-309). The modification of DNA, its amplification chain reaction (PCR) and / or automated sequencing techniques are most used for this purpose.

In recent years the technology based on the analysis of methylated DNA is considered a powerful tool for the diagnosis, therapy and prognosis of disease, as well as in the field of forensic medicine, pharmacogenetics, and epidemiology studies. The association between DNA hypomethylated status and cancer, and subsequently its relationship to hypermethylation, known since 1983, but in the last five years, by the impulse of new molecular strategies for the study of de novo methylation CpG island, methylated DNA analysis has become a powerful biomarker for early detection of cancer, also allows to classify cancer subtypes considering histological grade of malignancy, differences in treatment response and various forecasts(Ramsden, 2010, pp 11:70). An important recent application is its use as a biomonitor response to therapy and prognosis predictor of cancer.

At present, prostate, liver, stomach, colon and breast cancers are detected with high specificity, using DNA obtained from body fluids (urine, plasma, serum, saliva, ...