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710 Part VI: The Erythrocyte Chapter 47: Erythrocyte Enzyme Disorders 711

and PGK. 333,564 Myoglobinuria has been encountered in patients with states of patients with common G6PD variants or in G6PD-deficient
352
PGK, 261,565 aldolase, and G6PD deficiency. Table 47–2 summarizes patients with chronic hemolysis. Moreover, “bite cells” have been noted
539
the clinical features of enzyme deficiencies causing nonspherocytic in G6PD-replete patients. 576,577
hemolytic anemia. The presence of small, densely staining cells has often been noted
in the blood films of patients with hereditary nonspherocytic hemolytic
GENETIC MODIFIERS OF THE PHENOTYPES anemia with defects other than G6PD deficiency. Particularly when
The clinical phenotype of both acute and chronic hemolysis can be manifesting an echinocytic appearance, such cells have been thought to
be common in PK deficiency. In one reported case, spectacular num-
578
modified by coinherited (although unrelated) other defects of the red bers of such cells were observed. However, cells of this type are seen
cells. Combined deficiencies of, for example, GPI and G6PD, of PK in many blood films both from patients with other glycolytic enzyme
316
and band 3, 566–568 of PK and α-thalassemia, and of PK and G6PD deficiencies and from those with other disorders and it is hazardous to
570
569
have been documented. attempt to make an enzymatic diagnosis on the basis of such findings.
The inheritance of polymorphic UGT1A1 promoter alleles exacer- Basophilic stippling of the erythrocytes is prominent in most patients
bates the icterus both in neonates and in adults with G6PD deficiency with pyrimidine 5′-nucleotidase deficiency but is on itself an unspecific
(see also “Mechanism of Hemolysis” above). Overt iron overload and finding, and may not be apparent in blood that has been collected in eth-
472
iron-related morbidity in PK deficiency has been attributed to coin- ylenediaminetetraacetic acid anticoagulant. Leukopenia occasionally is
heritance of mutations in HFE, the gene associated with hereditary observed in patients with hereditary nonspherocytic hemolytic anemia,
hemochromatosis. 571 possibly secondary to splenic enlargement. Other laboratory stigmata
A striking example of complex interplay defining the differences of increased hemolysis may include increased levels of serum biliru-
between the genotype and the phenotype was described in a Hungar- bin, decreased haptoglobin levels, and increased serum LDH activity
ian family with TPI deficiency. Two adult germline-identical compound (Chap. 33). Reticulocytosis is frequently observed, which may result in
heterozygous brothers displayed strikingly different phenotypes. Both increased mean corpuscular volume of erythrocytes. In PK deficiency,
had the same severe decrease in TPI activity and congenital hemolytic splenectomy increases reticulocyte counts even further because in
anemia, but only one suffered from severe neurologic disorder. Studies particular the younger PK-deficient red blood cells are preferentially
aimed at the pathogenesis of this differing phenotype indicated func- sequestered by the spleen. Also in P5′N1 deficiency reticulocytes tend
579
tional differences between the two brothers in lipid environment of to be higher in splenectomized patients compared to non-splenecto-
the red cell membrane proteins influencing the enzyme activities, as mized patients. 420
562
well as differences in TPI1 mRNA expression, and protein expression Diagnosis of red cell enzyme deficiencies usually depends on the
levels of prolyl oligopeptidase, the activity decrease of which has been demonstration of decreased enzyme activity either through a quantita-
reported in well-characterized neurodegenerative diseases. 572 tive assay or a screening test. 580–583 Assay of most of the enzymes gen-
The variety of clinical features associated with the various enzy- erally is carried out by measuring the rate of reduction or oxidation of
mopathies, regardless of the underlying molecular mechanism, do nicotinamide adenine nucleotides in an ultraviolet spectrophotometer,
unequivocally demonstrate that the phenotype of hereditary red blood and a number of screening tests that depend upon the development or
cell enzymopathies, is not solely dependent on the molecular properties loss of fluorescence have been devised. 584
of mutant proteins but rather reflects a complex interplay between phys- However, difficulties arise when the patient has been transfused so
iologic, environmental, and other (genetic) factors. Putative phenotypic that the blood drawn represents a mixture of the patient’s own cells and
modifiers include differences in genetic background, concomitant those obtained from the blood bank. Under the circumstances, DNA
functional polymorphisms of other glycolytic enzymes (many enzymes analysis may prove invaluable, because the DNA is extracted from blood
are regulated by their product or other metabolites), posttranslational leukocytes and transfused leukocytes do not persist in the circulation.
modification, ineffective erythropoiesis, and different splenic function. Alternatively, density fractionation has been applied to isolate fractions
As an example, persistent expression of the PK-M2 isozyme has been of patient’s red cells, in which an enzyme deficiency can be detected. 585
reported in the red blood cells of patients (and animals) with severe Although detection of G6PD deficiency in the healthy, fully
PK deficiency. 29,573 The survival of these patients, though not in all cases affected (hemizygous) male can be achieved readily through either
may be enabled by this compensatory increase in PK activity. 574 assay or screening tests, difficulties arise when a patient with G6PD defi-
ciency of the A– type has undergone a hemolytic episode. As the older,
LABORATORY FEATURES more enzyme-deficient cells are removed from the circulation and
are replaced by young cells, the level of the enzyme begins to increase
Varying degrees of anemia and reticulocytosis are the main hemato- toward normal. Under such circumstances, suspicion that the patient
logic laboratory features of patients with hereditary nonspherocytic may be G6PD deficient should be raised by the fact that enzyme activity
hemolytic anemia. Heinz bodies often are found in the erythrocytes is not increased, even though the reticulocytes count is elevated. 586,587 It
of G6PD-deficient patients undergoing drug-induced hemolysis. In is helpful to perform DNA mutational analyses, carry out family studies,
the absence of hemolysis, the light-microscopic morphology of G6PD- or to wait until the circulating red cells have aged sufficiently to betray
deficient red cells appears to be normal. Differences in the texture of their lack of enzyme.
the membrane of the cells have, however, been observed under elec- Even greater difficulties are encountered in attempting to diagnose
tron microscopy. When a hemolytic drug is administered to a G6PD- heterozygotes for G6PD deficiency. Because the gene is X linked, a
588
575
deficient patient, Heinz bodies (Chap. 31) develop in the erythrocytes population of normal red cells coexists with the deficient cells. This
immediately preceding and in the early phases of the hemolytic episode. may mask the enzyme deficiency when screening tests are used. Even
If the hemolytic anemia is very severe, spherocytosis and red cell frag- enzyme assays carried out on erythrocytes of heterozygous females fre-
mentation may be seen in the stained film. Despite the fact that “bite quently may be in the normal range. Here DNA mutational analyses and
cells” may be noted in the blood of a G6PD-deficient patient under- histochemical methods that depend upon individual red cell enzyme
going drug-induced hemolysis, the association with G6PD deficiency activity may be useful. 589,590 In addition, the ascorbate cyanide test, in
591
is doubtful because such cells are usually lacking in acute hemolytic which screening is carried out on a whole-cell population rather than

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