Broadly speaking, the study of epigenetics focuses on how changes in the letters of the dna affect protein structure
Answers
Epigenetics is the study of regulatory mechanisms of genes. Point mutations are not the only focus here. It studies how mutations can have an effect on the regulatory mechanisms. Such as, methylation; which when deviated from the normal pathway results in over or undermethylation. This might lead to silencing of genes that normally should be expressed or expression of genes which are normally silenced.
Now, if a gene that is normally silenced is expressed, it will lead to synthesis of proteins that should not be produced in the body and how this protein thus effects the body.
In case of cancers, generally, these kind of situations result in chain reactions. Ex: EGFR pathway.
The epigenetic mutations have very wide effects. One regulation if disturbed leads to innumerable effects it is supposed to regulate.