Question

Questions 6: Gene Expression Regulation (9 pts) Heart defects from genetically inherited mutations affect about 1-2% of children and cause a significant number of stillbirths. They can also lead to heart disease in adults. Many genetically inherited heart defects are due to mutations in genes for transcription factors that control expression of genes that are required for normal heart development. For example, over 30 different mutations have been found in the gene for transcription factor Tbx5 in patients with genetically inherited heart defects. To carry out its functions, Tbx5 must bind to another transcription factor called Nkx2-5, which also plays a role in heart development and is mutated in many individuals with genetically inherited heart defects. Imagine that you can isolate developing cardiac cells with a Tbx5 mutation and study the molecular defects caused by the mutations. In each of the questions I though III, please circle the correct multiple-choice answer and explain your reasoning - include an explanation for why your choice is correct AND a brief sentence or two explaining why the other choices are incorrect.

I) In one mutant, you observe that the full-length “mature” Tbx5 mRNA is present at its usual levels, but rather than full-length Tbx5 protein being detected, a shorter version of it is detected. What kind of mutation most likely accounts for this phenotype?

A. A mutation that blocks binding of Tbx5 to Nkx2-5

B. A mutation that causes a splicing defect

C. A mutation that disrupts the normal folding of Tbx5

D. A mutation that introduces a premature stop codon

II) In another mutant cell line, you find that full-length Tbx5 mRNA and protein are present at normal levels, yet individuals carrying this mutation have severe defects in heart development. What kind of mutation could cause this phenotype?

A. A mutation that blocks association of the Tbx5 mRNA with the ribosome

B. A mutation that blocks export of the Tbx5 mRNA from the nucleus

C. A mutation that causes a splicing defect

D. A mutation that disrupts binding of Tbx5 with Nkx2-5

III) Which one of the following is a plausible mechanism by which Tbx5 could control initiation of transcription?

A. Activation of polyA tail addition

B. Recruitment of RNA polymerase to promoters

C. Recruitment of splicing factors to RNA polymerase

D. Regulation of 5’ cap formation

Answer 1

I)

A nonsense mutation that leads to the addition of a premature stop codon in the m RNA would mean that even though the mRNA is of full length, at some point in the middle rather than at the end, the process of translation would stop.

in cases of mutations that affect the binding of the protein Tbx5 to Nkx2-5, the most probable mutation here would be any amino acid substitution leading to conformational changes which inhibits the binding of the two proteins. However, the full-length peptide would be formed. A mutation leading to a changed splicing pattern would result in an mRNA with altered length, which is not the case here. Also, defective folding also would not have affected the peptide length.

II)

Here options A, B, and C are all associated with mRNA. Any defect in mRNA synthesis or translation would result in the protein being absent. Since the mRNA and protein are both detected, this mutant has a defect that affects the protein function, i.e. binding of Tbx5 to Nkx2-5.

III)

Addition of poly-A tail, 5'cap and splicing is done at the end or immediately after transcription is complete. A mechanism for control of initiation of transcription would need to control the polymerase attachment to the DNA.