Q3: Molecular Deduction – Epigenetic Editing of the CDKL5 Locus In the study by Halmai et al. (2020), a dCas9-TET1 fusion was used to reactivate the silenced CDKL5 allele. Predict the outcome if an investigator replaced the TET1 "eraser" with a DNMT3A "writer" targeted to the same promoter on the active X chromosome (Xa). How would this substitution affect the total cellular "dosage" of CDKL5 protein, and why would the synergistic use of a VP64 transactivator likely fail to overcome this specific change?

What would happen if you replaced TET1 with DNMT3A targeted to the active X chromosome (Xa) CDKL5 promoter?

Silencing of the CDKL5 allele on Xa. DNMT3A adds methyl groups to CpG dinucleotides on the previously unmethylated promoter, creating a hypermethylated silenced state.

How would replacing TET1 with DNMT3A affect total cellular CDKL5 protein dosage?

Critical reduction to near-zero expression. Xa becomes silenced by methylation, and Xi remains methylated and silent (TET1 removed), so the cell loses its only functional source of CDKL5.

Why would VP64 transactivator fail to overcome DNMT3A-mediated silencing?

1) DNA methylation is a potent barrier making chromatin non-permissive; VP64 cannot access DNA or recruit transcription factors. 2) VP64 cannot demethylate DNA, so it cannot remove methyl groups installed by DNMT3A.

What evidence from Halmai et al. (2020) shows VP64 cannot overcome methylation?

VP64 alone targeted to a methylated promoter (on Xi) failed to induce significant reactivation, and the authors explicitly showed VP64 does not contribute to DNA demethylation.

What is the functional difference between TET1 and DNMT3A?

TET1 is an eraser that removes methyl groups (demethylation). DNMT3A is a de novo writer that adds methyl groups to unmethylated DNA. TET1 clears the methylation barrier for transcription; DNMT3A establishes a methylation barrier that silences expression