Answer:
Gene mutations
Low level of transcription
Not enough amino acids
Problems with protein trafficking
Explanation:
Mutations in genes or their surrounding regulatory elements could affect the transcription of the gene or the sequence of the mRNA. For example, mutations in the gene regulatory elements could affect the binding of transcription factors that affect gene transcription. This would in turn affect the abundance of mRNA. Alternatively, a deletion in the gene could result in an incomplete and non-functional protein being produced. While a mutation would only directly affect one protein, it could have a knock on effect on a variety of others
In the nucleus, if gene transcription is low, the cell will not produce enough mRNAs, which are translated into protein. This could happen if, for example, the level of RNA polymerase or another important factor in transcription, are low. If the transcription machinery is not producing enough mRNA, then the protein synthesis machinery in the cytoplasm will not have templates from which to make proteins
Amino acids are important building blocks of proteins. Not all of the amino acids can be produced by the cell, some of them have to be obtained from our diet. For example, valine. If there is not a sufficient pool of valine in the cell, then the tRNA will not be able to deliver valine to the ribosome quickly enough to growing mRNA chains. This means the protein synthesis machinery can't keep up with the demands of the cell, resulting in less protein being synthesised.
The Golgi apparatus is an important organelle in the trafficking of proteins. Proteins reach the Golgi apparatus, and are enclosed in membrane-bound vesicles, where they are delivered to the sites they are required, for example, proteins are secreted outside the cell in this way. A problem with the Golgi apparatus would mean proteins cannot be secreted outside the cell properly. The Golgi apparatus is known to be dysfunctional in Alzheimer's disease, for example.
