Q2: Causal Relationships – XCI Dysregulation and Tumorigenesis The lecturer describes a study where the critical factor Xist was removed from female mice several days after conception. Explain the causal chain of events that follows this deletion in highly proliferative cells (such as blood stem cells) versus post-mitotic cells (such as neurons). Specifically, address why this epigenetic failure leads to "fulminant blood cancer" in the former but has a different phenotypic impact in the latter, referencing the role of dosage compensation and the Barr body.

What is the causal chain of events after Xist deletion in female mice?

1) Loss of Xist → loss of X silencing mechanism. 2) Loss of Barr body. 3) Dosage compensation failure → inactive X reactivates → two active X chromosomes. 4) Overexpression of ~1,000 X-linked genes at double normal levels.

What happens in highly proliferative cells (e.g., blood stem cells) after Xist deletion?

Doubling of X-linked gene expression provides an oncogenic stimulus, leading to fulminant blood cancer in 100% of female mice, with abdominal masses and enlarged spleens.

Why does Xist deletion cause cancer in highly proliferative cells?

Because cells that divide many times are highly sensitive to gene dosage imbalances, and the extra dose of X-linked genes drives uncontrolled proliferation and tumorigenesis.

What happens in post-mitotic cells (e.g., neurons) after Xist deletion?

Neurons do not divide again, so they cannot transform into cancer. The animal appears runted with shortened lifespan, but brain tissue shows no increased tumorigenesis.

Why do post-mitotic cells not develop cancer after Xist deletion?

Because tumorigenesis requires active cell division. Without proliferation, the extra dose of X-linked genes causes developmental defects, not uncontrolled growth.

What is the Barr body?

The condensed heterochromatic structure of the inactive X chromosome in female somatic cells, which maintains transcriptional silencing.

What happens to the Barr body after Xist deletion?

Deleting Xist leads to loss of the Barr body because the repressive structure can no longer be maintain