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← Theoretical B

Q22 — Genomic Imprinting at the IGF2-H19 Locus

Theoretical B Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Extranuclear & Non-Mendelian Inheritance

Genomic imprinting is one mechanism by which sexes can encourage favourable traits in a fetus to maximize their own reproductive success. The most evident imprinting phenomenon occurs when the normal gene expression is disrupted.

Particularly, Beckwith–Wiedemann syndrome (BWS) is characterized by fetal macrosomia (birth weight >90th percentile) and an increased risk of congenital tumors. Meanwhile, babies with Silver–Russell syndrome (SRS) are born small for their gestational age and later exhibit dwarfism.

Both cases can be associated with genetic defects in clusters of imprinted genes on chromosome 11. IGF2 is a pro-proliferative regulator of fetal growth. H19 knockout does not have pathological consequences. Another paternally imprinted gene (active on a maternal chromosome) is IGF2R; its protein binds IGF2 and targets it for lysosomal degradation.

Two parallel gene-cluster diagrams — maternal chromosome (IGF2 silenced, H19 active, unmethylated ICR) above, paternal chromosome (IGF2 active, H19 silenced, methylated ICR) below, both with an enhancer region and looping arrows showing which gene the enhancer contacts. Figure 1. Maternal and Paternal structures of the IGF2-H19 cluster. ICR - imprinting control region; En - Enhancer, Met - methylated, unMet - unmethylated.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. The figure implies that male infants are more often prone to cancers.
B. Paternal uniparental disomy of chromosome 11, both copies of the pair are inherited from a father, might lead to the symptoms of SRS.
C. Individuals with the maternal IGF2R loss of function will have the phenotype resembling that of the BWS patients.
D. The genome imprinting in monogamous species is expected to occur at lower rates than in polygamous species.
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Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF