Unmethylated cytidines are susceptible to deamination. In this process the amino group at position 4 of the ring is removed from the cytidines. A deaminated non-methylated cytidine is an uracil. This is not one of the 4 physiological DNA bases "adenine, cytosine, guanine or thymine". Therefore an uracil in the DNA is recognized as a defect and eliminated.
If a 5-methylcytidine is deaminated, thymine is formed. Thymine is a DNA building block. This means that the DNA repair system does not recognize whether this thymine or the opposite guanine was inserted incorrectly. The error is not recognized. The thymine is not eliminated. If this methylation has taken place in a germ cell, this mutation is also inherited.
DNA methylation and epigenetics: A number of environmental factors such as stress, smoking (influence on histone acetylation), various drugs(glucocorticoids, theophylline) can change the methylation pattern of a person, so that certain genes are activated while others are inactivated. Epigenetic mechanisms play a role in tumour development, e.g. by silencing tumour suppressor genes. In allergological diseases, connections between environmental pollution and DNA methylation behaviour are assumed.