Blood, 1 June 2001, Vol. 97, No. 11, pp. 3325-3325
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.
Ernest Beutler
Scripps Medical Institute
