Download Laboratory diagnosis of rare red cell enzyme disorders

Laboratory diagnosis of rare red cell enzyme disorders
The red blood cell is optimally adapted to perform the binding and transport of oxygen, and
its delivery to all tissues. This is the most important task of the erythrocyte during its 120-day
journey in the blood stream. The biconcave shape of the red blood cell provides an optimal
area for respiratory exchange. The latter requires passage through microcapillaries, which is
achieved by a drastic modification of its biconcave shape, made possible only by the loss of
the nucleus and cytoplasmic organelles and, consequently, the ability to synthesize proteins.
Red blood cells possess an active metabolic machinery that provides energy to pump ions
against electrochemical gradients, to maintain red cell shape, to keep hemoglobin iron in the
reduced form, and to maintain enzyme and hemoglobin sulfhydryl groups. The main source of
metabolic energy comes from the anaerobic conversion of glucose by the Embden-Meyerhof
pathway (or direct glycolytic pathway) and the oxidative hexose monophosphate pathway (or
pentose phosphate shunt). 2,3-bisphosphoglycerate, an important regulator of the oxygen
affinity of hemoglobin, is generated during glycolysis. Numerous red cell enzymes are
involved in these pathways, thereby providing the cell with the necessary high-energy
phosphates (ATP) and reducing power (NADPH). The maturation of reticulocytes into
erythrocytes is associated with a rapid decrease in the activity of several enzymes. However,
the decrease in activities of other enzymes occurs much more slowly or not at all with aging.
Deficiencies in the activities of a number of red cell enzymes may lead to hemolytic
anemia. By far the majority of these disorders are hereditary in nature. Most defects are
transmitted as autosomal recessive disorders, while glucose-6-phosphate dehydrogenase and
phosphoglycerate kinase deficiencies are X-linked. The continous lack of sufficient energy
and other metabolic impairments results in a shortened lifespan of the red blood cell:
hereditary nonspherocytic hemolytic anemia (HNSHA). The degree of hemolysis is often
exacerbated by infection. The most common cause of HNSHA is pyruvate kinase deficiency.
In some cases, red cell enzymopathies may lead to pathologic changes in other cell lines such
as myopathies (phosphofructokinase deficiency) and neuromuscular abnormalities
(deficiencies of triosephosphate isomerase and phosphoglycerate kinase).
Diagnosis is best achieved by determining red blood cell enzymatic activity with a
quantitative assay, followed by DNA analysis. Except for the basophilic stippling of
erythrocytes that is characteristic of pyrimidine 5′ nucleotidase deficiency, red cell
morphology is unremarkable. A variety of molecular lesions have been defined in most of
these enzyme deficiencies but in many cases a genotype-to-phenotype correlation has proven
to be difficult to establish. Accurate diagnosis is necessary for genetic counseling and is
helpful in recommendations for treatment.