Genomic imprinting is a heritable yet reversible process by which a gene is modified, depending on which parent provides it. The mechanism is unclear but appears to operate at the chromatin organization level and involves heterochromatization and methylation of CpG (cytosine-phosphate-guanine) sites. Examples of genes that can be imprinted include the Wilms tumor–suppressor gene and the human SNRPN (small nuclear ribonucleoprotein polypeptide N) gene.
Prader-Willi and Angelman syndromes are examples of diseases resulting from abnormalities of imprinting. Approximately 70%–80% of patients with Prader-Willi syndrome harbor a deletion of the paternally derived 15q11–q13, resulting in the loss of this region’s normal contribution from the paternal line. About 70%–80% of patients with Angelman syndrome also have a deletion of 15q11–q13, but from the maternally derived chromosome, resulting in loss of the maternal contribution. Chromosome 15 uniparental disomy, wherein both copies of chromosome 15 are inherited from the same parent, can also cause each syndrome. The 2 chromosomes 15 in uniparental disomy are maternal in Prader-Willi syndrome and paternal in Angelman syndrome. The SNRPN gene maps to 15q11–q13 but appears to be expressed only from the paternally inherited allele.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.