Lecture Notes: Genomic Imprinting

Introduction to Genomic Imprinting

• Genomic Imprinting is the process where certain genes are expressed in a parent-of-origin-specific manner.
• Normal genetic inheritance involves two chromosomes per cell, one from each parent.
• Usually, one allele is dominant and determines the phenotype.

Basic Concepts

• Chromosomes: For example, Chromosome 1 from mother and father.
• Gene Locus: Specific position on a chromosome where the gene for a characteristic (e.g., color) is located.
• Alleles: Variants of a gene. Example: light colored skin allele (mother), dark colored skin allele (father).
• Dominance: The dominant allele's trait is expressed.

Mechanism of Genomic Imprinting

• Some alleles are silenced due to methylation, a process that inhibits gene expression.
  • Methylation: Silences genes.
  • Acetylation: Activates genes.

Examples of Imprinting in Chromosome 15

• SNORP Gene:
  • Located on Chromosome 15.
  • Maternal SNORP: Silenced by methylation, not expressed.
  • Paternal SNORP: Expressed.
• UBE3A Gene:
  • Also on Chromosome 15.
  • Maternal UBE3A: Expressed.
  • Paternal UBE3A: Silenced by methylation.

Diseases Related to Genomic Imprinting

Prader-Willi Syndrome

• Causes:
  • Deletion of paternal segment on Chromosome 15 (particularly on the PR arm).
  • Maternal Disomy 15: Two Chromosome 15s inherited from the mother.
• Symptoms:
  • Obesity and hyperphagia (excessive eating).
  • Short stature, hypotonia (reduced muscle tone).
  • Hypopigmentation.
  • Low impulse control.

Angelman Syndrome

• Causes:
  • Deletion of maternal segment on Chromosome 15.
  • Paternal Disomy 15: Two Chromosome 15s inherited from the father.
• Symptoms:
  • "Happy puppet" appearance due to characteristic smiling.
  • Seizures and intellectual disability.
  • Wide spaced teeth.

Conclusion

• Genomic imprinting can lead to specific genetic disorders depending on which parent's gene is expressed or silenced.
• Key Disorders: Prader-Willi Syndrome and Angelman Syndrome.

The lecture covered the physiological basis of genomic imprinting and the genetic and clinical implications of imprinting disorders.