STAT3 and RORγT in hyper IgE syndrome

Reason for Correct Answer:

The first paragraph explains how STAT3 is activated by membrane receptors and then functions as a transcription factor.

STAT3 activation in the cytoplasm involves phosphorylation and dimerization, according to the passage.

As a transcription factor, STAT3 must then move to the nucleus where it can activate RORγt expression.

Transcription factors like STAT3 contain nuclear localization signals that acts as tags, indicating that they should be moved to the nucleus.

The only answer choice that would NOT reduce RORγt expression (and therefore activity) in a T cell is the loss of kinase activity of STAT3. STAT3 is a transcription factor, not a kinase (enzyme that phosphorylates targets).

Reason for Correct Answer:

AD-HIES is caused by mutations in the STAT3 gene, not in RORγT.

In Figure 1, STAT3 mRNA levels in AD-HIES cells are half of STAT3 mRNA levels in the control.

The STAT3 mutation associated with AD-HIES is a heterozygous mutation, meaning that it affects one chromosome.

Thus, the data is consistent with STAT3 expression by both chromosomes in control and only one chromosome in AD-HIES.

Reason for Correct Answer:

The mutation associated with AD-HIES is heterozygous, and data in figure one shows that AD-HIES cells have about 50% as much non-mutated STAT3 mRNA as control cells.

Individual STAT3 protein has to dimerize with another STAT3 protein to have the intended biological effect.

This means that you’d expect close to 75% of dimers to have at least one mutated subunit.

If STAT3 exhibits a dominant negative phenotype, you’d therefore expect only 25% of normal dimer activity in AD-HIES cells.

Reason for Correct Answer:

Transcription factors are proteins that help control the activity of genes by entering the nucleus and binding to specific DNA sequences in the gene’s promoter region or enhancer elements.

Some transcription factors activate transcription of genes, while others prevent transcription of genes. The same is true for other regulatory elements.

Many other gene regulatory elements, such as enhancers, silencers, and insulators, also play important roles in gene regulation but do not bind directly to the DNA. Instead, they can influence gene expression by interacting with transcription factors and other proteins to modulate the transcriptional activity of specific genes.

Reason for Correct Answer:

You’re looking for a mutation that is most likely to be a silent mutation, not affecting STAT3 expression or activity and therefore not affecting RORγT expression.

Mutations in an intron would be more likely to remain silent (although they could have significant impact if they affect a splice site).

Single, double, or triple nucleotide deletions differ in their ability to cause frameshift mutations.

A single nucleotide deletion, even in an intron, would result in a frameshift, or a shift in the reading frame of the mRNA transcribed from the gene. This drastically affects the regions of the protein downstream of this mutation, in addition to the region where the deletion occurs.

However, a trinucleotide deletion (or deletion of a multiple of 3) will not affect the reading frame.

A trinucleotide deletion (deletion of 3 nucleotides) in an intron would therefore be the LEAST likely to affect STAT3 expression or activity, as long as it does not affect a splice site. It is possible that this mutation would result in the lost of 3 mRNA nucleotides that are cut out of the intron.

This deletion would have a less certain effect than it would in the DNA binding domain, where it would more certainly affect STAT3’s ability to activate RORγT expression.

Reason for Correct Answer:

The graphs in Figure 2 show RORγT (A) and c-Maf (B) expression for wild type cells (white bars) versus cells deficient in STAT3 (black bars).

The passage says that c-Maf is a transcription factor that promotes RORγt expression and the differentiation of intestinal RORγt⁺ T cells.

Graph B shows that c-Maf expression is retained in cells that are STAT3 deficient (and even elevated in STAT3 deficient mLN cells).

Graph A shows that, despite this, RORγT is still significantly reduced in cells that are STAT3 deficient.

This suggests that c-Maf expression alone is not sufficient to induce RORγT expression, as RORγT expression is reduced in STAT3 deficient cells, even when c-Maf levels are normal.

Reason for Correct Answer:

To estimate the length of a protein, use the approximation of 110 g/mol per amino acid.

88 kDa means 88,000 Da (equivalent to 88,000 g/mol). Therefore, the length can be estimated as 88,000/110 = 800 amino acids.

The isoelectric point (pI) of a protein is the pH at which the net charge is zero.

At pHs above the pI, the environment is less positive, and the net change of the protein is negative; at pHs below the pI, the net charge is positive.