Chapter 3: Epigenetics and Disease

DNA methylation, the addition of methyl groups to cytosine bases, silences genes and establishes stable transcriptional repression, while histone modifications reshape chromatin structure to activate or suppress transcription of target genes. These processes are normally essential for development, including X-chromosome inactivation that equalizes gene dosage between males and females, but aberrant epigenetic changes underlie numerous genetic disorders and malignancies. Genomic imprinting demonstrates how the parental origin of an allele determines its expression pattern independent of DNA sequence, creating parent-of-origin effects that produce distinct phenotypes when disrupted. Paternal deletion of chromosome 15q11–q13 causes Prader-Willi syndrome characterized by hypotonia, severe obesity, and cognitive impairment, while identical maternal deletion results in Angelman syndrome featuring intellectual disability, seizures, and ataxia. Altered imprinting of the IGF2 growth factor gene produces Beckwith-Wiedemann syndrome with predisposition to childhood tumors and Russell-Silver syndrome with intrauterine growth restriction. Environmental factors including nutritional deprivation, temperature exposure, maternal behavioral patterns, and prenatal toxin exposure create lasting epigenetic signatures that modify disease susceptibility across the lifespan and potentially transmit across generations. Twin studies reveal that initially identical epigenomes accumulate divergent methylation patterns with age and lifestyle exposure, demonstrating the dynamic interplay between environment and epigenetic regulation. Fragile X syndrome results from expansion of CGG trinucleotide repeats that trigger methylation and silencing of the FMR1 gene, while facioscapulohumeral muscular dystrophy arises from DUX4 gene hypomethylation and aberrant expression. Cancer cells characteristically exhibit hypermethylation of tumor suppressor genes including RB1, BRCA1, and MLH1 that silences protective mechanisms, coupled with hypomethylation of oncogenes that promotes proliferation. These discoveries have spawned clinical applications including liquid biopsy techniques for early cancer detection and therapeutic agents such as DNA demethylating drugs like 5-azacytidine for hematologic malignancies and histone deacetylase inhibitors for solid tumors, offering potential pathways to reverse disease through epigenetic intervention.