Page 734 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 44 Red Blood Cell Enzymopathies 621

splenectomy if possible until after the age of 3 years when the risk of The accumulation of pyrimidines in the RBCs because of a
infections with encapsulated organisms declines and because in most deficiency of P5′N1 is presumed to be toxic although the exact
cases the degree of hemolysis declines after infancy, by a not fully mechanism by which P5′N1 causes hemolysis is unknown. The
understood pathophysiologic mechanism. One PK-deficient boy ribosomal aggregates are visible as the characteristic coarse basophilic
with severe hemolysis was apparently cured by an allogeneic marrow stippling seen on the peripheral smear. However, deficiency of P5′N1
transplant. is at least partly compensated in vivo by other nucleosidases or other
A small molecule AG-348 (Agios Pharmaceuticals Inc.) can nucleotide metabolic pathways. Acquired P5′N1 deficiency occurs
2+
increase the activity of many PK mutants perhaps by increasing the in acute lead toxicity because of Pb outcompeting the essential
2+
stability of the PK enzyme. This oral compound is currently in clini- cofactor Mg .
cal trials and may be able to restore glycolytic pathway activity and
normalize ATP and 2,3 BPG levels in PK deficiency.
Clinical and Laboratory Features
Prognosis P5′N1 deficiency causes chronic hemolytic anemia, with severity
ranging from compensated hemolysis without anemia to transfusion
The clinical course is highly variable ranging from fatal hydrops dependent anemia. Marked basophilic stippling of RBCs is a labora-
fetalis, transfusion dependency from birth, and a high risk of kernic- tory hallmark and thus morphologic examination of the peripheral
terus to normal childhood development with no or rare transfusions, blood smear provides simple and inexpensive screening. However
or to compensated hemolysis without anemia. However, hemolysis basophilic stippling is not a specific finding, as it is also found in
that is fully compensated because of excessive erythropoiesis may hemolytic anemia caused by acute lead toxicity and sideroblastic
be deceptively benign. In addition to the usual complications of anemia. Confirmation of the diagnosis requires demonstration of
chronic hemolysis such as gallstones and parvovirus-induced aplas- decreased P5′N1 activity, normal blood lead levels and, if available,
tic crisis, the excessive number of erythroblasts in these patients high concentrations of pyrimidine nucleotides in red cells.
produces erythroferrone, which mediates low hepcidin and may
result in typical hemochromatosis induced cardiac and hepatic
dysfunction. Therapy
Chronic transfusion support may be necessary for severe cases. Milder
Future Directions cases may require transfusion only periodically, during pregnancy,
infection or other stressors. Iron overload may occur, as in any
Small molecular activators of PK are promising, but are being vali- chronic hemolytic condition, and iron chelation may be needed.
dated in ongoing trials. Splenectomy was reported to be beneficial in several cases.
New gene therapeutic techniques are also being evaluated in severe
PK deficiency. These include the zinc finger nucleases and CRISPR/
Cas9 technology (clustered regularly interspaced short palindromic OTHER ENZYMOPATHIES OF THE
repeats) that uses noncoding RNAs to guide the Cas9 nuclease to GLUTATHIONE PATHWAY
induce site-specific DNA cleavage. The resultant DNA damage is
repaired by cellular DNA repair mechanisms that may correct the PK γ-Glutamylcysteine Synthase Deficiency
mutation. These technologies hold early promise that will need to be
validated by future clinical trials. γ-glutamylcysteine synthase (GCL) catalyzes the first metabolic step
of glutathione synthesis. Deficiency of this enzyme is rare and only
Pyrimidine 5′ Nucleotidase-1 Deficiency a few cases, usually in consanguineous families, have been reported.
All patients had hemolytic anemia and approximately half also had
neurologic defects, including spinocerebellar degeneration, peripheral
Introduction neuropathy, and mental retardation. Some patients have been char-
acterized at the molecular level and in all these cases the causative
P5′N1 deficiency is the third most common cause of hemolytic mutation affected the catalytic domain of GCL.
anemia caused by a red cell enzymopathy, after G6PD and PK
deficiency. It is inherited in an autosomal recessive manner.
Glutathione Synthetase Deficiency
Pathobiology Glutathione synthetase (GS) deficiency is a slightly more common
enzymopathy of glutathione metabolism than GCL deficiency. The
As the reticulocyte matures, ribosomes and RNA are degraded. clinical severity is variable, but approximately 25% of the patients die
P5′N1 assists in the process by catalyzing the dephosphorylation of in childhood. Mild GS deficiency can manifest as only mild chronic
pyrimidine nucleoside monophosphates into cytidine and uridine, hemolysis. Moderately affected individuals present in the neonatal
which can diffuse across the cell membrane. P5′N1 activity is specific period with both hemolytic anemia and severe metabolic acidosis
for pyrimidines and is much higher in reticulocytes than mature caused by the accumulation of the γ-glutamylcysteine metabolite
red cells; P5′N1 activity rapidly declines during the first few days 5-oxoproline, which results from decreased feedback inhibition of
2+
of red cell maturation. P5′N1 requires Mg for its activity and GCL by the decreased levels of GSH. Severely affected patients have
2+
is inhibited by a number of heavy metals, including Pb . P5′N1 progressive neurologic defects and animal experiments suggest that
also has phosphotransferase properties, suggesting an additional role 5-oxoproline has a direct neurotoxic effect. The different clinical
of this enzyme in nucleotide metabolism. Thus far, 27 different manifestations are likely compounded by environmental factors as
mutations have been reported in P5′N1 deficiency. Most patients phenotypic variability among siblings with the same mutation has
are homozygous. been encountered.
P5′N2 is another nucleotidase present in RBCs. Although the High levels of 5-oxyproline in the urine (found in all severely
activity of this enzyme is generally measured together with that of and moderately affected but in only some of the mildly affected
P5′N1, it is encoded by a separate gene, is not strictly pyrimidine- patients) suggest the diagnosis, which is confirmed by documenting
specific, and is unable to compensate for deficient function of P5′N1. deficiency of the enzyme or by demonstrating mutations in the GS
Only P5′N1 deficiency is associated with hemolytic anemia. gene. Correction of the metabolic acidosis and early supplementation