DNA-based diagnosis of red cell enzymopathies: how we threw out the baby with the bathwater

Marinaki and colleagues (page 3327) have identified mutations in patients with pyrimidine-5′ nucleotidase (P-5′N) deficiency at the DNA level. This is a welcome finding, coming as it does years after identification of mutations in all of the more common red cell enzymopathies, such as glucose-6-phosphate dehydrogenase (G6PD) deficiency and pyruvate kinase deficiency, and even of the relatively rare ones, such as triosephosphate isomerase and hexokinase.

Demonstration of mutations at the DNA level is not only of academic interest; it makes possible, as Marinaki and colleagues point out, accurate detection of heterozygotes. It is also useful in diagnosis of patients with the disease: the unborn, women heterozygous for G6PD deficiency, and patients who have received red cell transfusions. In the case of P-5′-N deficiency, it will also facilitate differentiation of patients with inherited P-5′-N deficiency from those who have decreased P-5′-N activity because their red cell population is very old, as in transient erythroblastopenia of childhood or because the enzyme has been inhibited by lead poisoning.

In 1960 we began to recognize that there was great variability in the residual protein in red cell enzyme deficiencies. It was obvious that it would be very useful to be able to find the mutations of patients who had different enzyme variants and different clinical phenotypes. But only very small amounts of enzyme protein were available, and those of us working in this field looked forward to the time that protein sequencing methods could characterize the mutant proteins on a molecular basis.

That is where we threw out the baby with the bathwater.

We purified the enzyme and stored it for that day when technology would have advanced to the point that the protein could be sequenced, and threw out everything else, white cells and their DNA included. No long-term strategic plan had foretold that the decoding of the structure of the protein from the white cell DNA would be the key to understanding mutant red cell enzymes. Now, unfortunately, it will be difficult to apply DNA analysis to patients documented earlier, because usually neither the patient nor the DNA is available. Over the next few years, however, the definition of the structure of the gene and the proof that it is the one involved in the clinical disorder will make it possible to expand our knowledge of this enzyme and the disease that its deficiency causes.