You slowly develop eyelid drooping and muscle weakness. You go to the doctor and she believes you display classic symptoms of KSS (Kearns Sayre Syndrome), a mitochondrial disease. The doctor orders a test where a southern blot of your mitochondrial DNA is performed using a blood sample. The southern blot shows one large band in the disease-negative control lane and a similar large band in your sample lane. The doctor is not convinced that the results rule out KSS and orders more tests. Why is the doctor not convinced that the results rule out KSS?

A) The disease-negative control lane did not have the proper normal mitochondrial DNA sample

B) A nonsynonymous or missense mutation may be present in the mitochondrial DNA

C) Your mitochondria may contain a nonsynonymous or missense mutation in the mitochondrial DNA and another biopsy from a distinct site may show a different result

D) Another biopsy from a distinct site may show a different result

E) Two bands should have been present in the negative control sample

In molecular biology southern blot is used to detect a specific sequence of DNA. Mitochondria are energy producing organelles in the cell and they possess DNA that is inherited only from the mother. Although Southern blot is the classic method used to detect large deletions in mitochondrial DNA, it is not always sensitive enough to detect heteroplasmic deletions at low levels leading to results that are not always reliable. Heteroplasmy refers to the presence of more than one mitochondrial DNA in an individual. The presence of a mutation in the mitochondrial DNA can create an intracellular heteroplasmic mix of mutant and normal mitochondrial DNA. Cells with a higher rate of cell division like blood can yield inconclusive results in southern analysis because heteroplasmy favors wild type mitochondrial DNA. Thus, non-dividing cells such as muscle cells are preferred for the analysis.