A 30-year-old man developed liver cirrhosis and diabetes. Laboratory studies showing high serum ferritin and high transferrin saturation 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 HFE C282Y mutation without wild-type DNA at that locus. Homozygous HFE gene mutation with the predicted amino acid substitution predisposes to iron overload by overabsorption of iron from the diet. He was treated with therapeutic phlebotomy until his serum iron levels returned to the normal range. He remains at risk for iron overload, and he should be periodically monitored and managed accordingly. A genetic counselor educated him about the increased risk of iron overload faced by blood relatives if they, too, inherited two mutated 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 25,000 different protein-coding genes plus additional genes encoding RNA that is not translated into protein. There are two copies of every gene—one of paternal and the other of maternal origin. Transcription factors act on gene promoters to regulate gene transcription, as do chromatin modifications such as DNA methylation and histone acetylation.
Molecular assays rely on the ability to find a specific nucleotide sequence in DNA or RNA by using a nucleic acid probe targeting that sequence. A probe is a single-stranded segment of nucleic acid whose nucleotide sequence is complementary to the target sequence of interest. A 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 detection strategies (see Figure 5-1).
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 DNA may be dissociated from one another by heating them to near boiling (94°C) or by treating with an alkaline solution (high pH). Single-stranded patient nucleic acid may then hybridize to a complementary probe. In the examples depicted here, labeled probes hybridize to target sequences in patient DNA, and then a detection system deposits black or red chromagen at the locus where the probe is bound.