A 30-year-old man developed liver cirrhosis and diabetes. High serum ferritin and high transferrin saturation levels led to the hypothesis that iron toxicity to the liver and pancreas contributed to his cirrhosis and diabetes. A blood specimen was tested for mutation in the HFE gene that is responsible, at least in part, for hereditary hemochromatosis. Polymerase chain reaction (PCR) followed by melt curve analysis was performed, and a pathologist interpreted the findings as homozygous HFE C282Y mutation. The HFE gene mutation is predicted to cause an amino acid substitution in the encoded protein, predisposing the patient to iron overload via overabsorption of iron from the diet. A straightforward treatment is available to overcome iron overload in affected patients. This patient was treated with therapeutic phlebotomy until his serum iron levels returned to the normal range. He remains at risk for recurrence of iron overload, and he should be periodically monitored for evidence of high serum ferritin. This is a heritable disorder, and a genetic counselor educated the patient about the increased risk of iron overload faced by blood relatives who may also have inherited two mutant alleles of the HFE gene.
The practice of molecular pathology capitalizes on analysis of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) to inform medical decision making. Each nucleated cell in the body contains a complete set of DNA inherited from parents, constituting the person’s genome. DNA is the blueprint by which cells catalog, express, and transmit genetic information. The human genome is composed of 3 billion pairs of nucleotides divided among 46 chromosomes containing about 20,000 different protein-coding genes plus additional genes encoding RNAs that can function without translation to protein. Expression of each gene is regulated by a network of transcription factors and chromatin modifications such as DNA methylation and histone acetylation. There are two copies of nearly every gene—one of paternal and the other of maternal origin.
Molecular assays are used to characterize or quantify particular DNA or RNA molecules. Typically, this is accomplished by using a single stranded “nucleic acid probe” whose sequence is complementary to the gene of interest. The probe binds to its target through a process called hybridization, and then the probe is detected or its effects (e.g., priming DNA synthesis) are evaluated using various analytic strategies (see Figure 6-1).
DNA is composed of complementary strands of nucleotides that are connected by hydrogen bonds between adenine (A) and thymine (T), or between guanine (G) and cytosine (C). In the laboratory, the two strands of a patient’s DNA may be dissociated from one another by heating them to near boiling (94°C) or by treating with an alkaline solution (high pH). Then a single-stranded probe may hybridize to its complementary target sequence in the patient’s DNA. In the examples depicted here, two different probes have been labeled to facilitate their ...