Question

Question 3 You are doing genetic studies on a family that has several members with a particular disease. You identify a gene with a one base pair difference (C→T) between the patients (T) and a healthy relative (C). This gene encodes a protein that acts as an enzyme in cellular metabolism.

(A) Describe two ways in which a C→T mutation in the DNA could cause a loss-of-function disruption in a protein.

(B) Describe how it is possible that a loss-of-function mutation can be dominant to wild-type

(C) Describe three potential consequences of a loss-of-function mutation in the protein described in (A). (Answer could be at a cellular or organismal level.)

(D) Describe two ways in which a C→T mutation in the DNA could cause a gain-of-functiondisruption in a protein.

Answer 1

(A)

A C->T transition mutation could cause a loss of function disruption in the following ways:

1. Introduction of a premature-stop codon:
   A C->T mutation could convert the CAA codon, which encodes Glutamine to TAA, which is a stop codon. This mutant gene would produce a truncated, non-functional protein.
2. Introduction of an Amino-acid substitution:
   A C->T mutation could convert the CGG codon, which encodes Arginine to TGG, which encodes Tryptophan. These two Amino acids differ enough for there to be a change in the conformational structure of the Protein, disrupting its normal functionality.

(B)

A Loss-of-Function mutation, especially one described in A-2, above, can be dominant to a Wild-type protein as such a mutated protein could dimerize with either the Wild-type protein or itself. The Mutant-WT protein dimers may not have the necessary functional domains for proper function, resulting in a dominant loss-of-function phenotype.

(C)

1. Truncated Protein causes a complete loss of function: The loss of function due to a non-sense mutation would deprive the cell of control over the pathway the protein is a part of. This is especially true of non-redundant metabolic pathways. A nonsense mutation would only serve to add to the cellular burden for Transcription and Translation, without actually producing functional protein.
2. Loss of function affects metabolic flux: Cells tightly control the flux, or rate of flow, of different macromolecules. A loss of function in the regulatory pathways could significantly alter the flux of the pathway, altering the biochemical composition of the cell.
3. Loss of function affects regulation: A loss of function mutation in a transcription factor may cause errors in gene regulation since Transcription factors sit at the end of signal transduction pathways. A mutant Transcription factor would not drive transcription, or may even drive transcription constitutively, affecting the cells' ability to maintain homeostasis.

(D)

1. A C->T mutation may affect DNA binding: Many transcription factors rely on specific sequence recognition for their activity. A C->T mutation may alter the DNA binding motif such as to change the Amino acid sequence and consequently alter the DNA binding specificity of the mutant transcription factor, and changing the functionality of the transcription factor.
2. A C->T mutation may affect protein binding: A C->T mutation in a protein may alter the binding of the protein to partner proteins. Proteins interact with other proteins my means of surface-to-surface interactions. A mutation that alters the surface Amino acid composition can alter which proteins the mutant protein binds to, which in turn may affect functionality.