Page 1054 - Williams Hematology ( PDFDrive )

1028 Part VII: Neutrophils, Eosinophils, Basophils, and Mast Cells Chapter 66: Disorders of Neutrophil Function 1029

of this oxidase system reside in the cytosol of the resting phagocyte. expression of the other subunit. 376
Upon stimulation, translocation of p47 phox takes place. Phosphory- Genetic Alterations Affecting Cytosolic Proteins Two other
lated p47 phox together with two other cytoplasmic components of the proteins have been identified as being vital to the function of the
oxidase, p67 phox , and a low-molecular-weight guanosine triphosphate NADPH-oxidase system. Their absence results in the syndrome of
405
Rac-2, translocate to the membrane, where they interact with cyto- CGD. These proteins have molecular masses of 47 kDa and 67
plasmic domains of the transmembrane cytochrome b to form the kDa, respectively, and are located in the cytosol of resting cells.
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active oxidase. 391,392 Both p47 phox and p67 phox contain SH3 (Src homol- Defects in the genes for p47 phox , termed NCF1, which is found on
ogy 3) domains that may participate in intramolecular and inter- chromosome 7q11, are responsible for the majority of all cases of
.
molecular binding with consensus proline-rich regions in p47 phox 392 autosomal recessive CGD, whereas inherited defects for the gene for
Phosphorylation, which occurs on serines in the cationic C-terminal neutrophil p67 phox , termed NCF2, account for a small subgroup of
region of p47 phox , serves to disrupt this intermolecular interaction, autosomal recessive CGD. The function of p47 phox and p67 phox in
376
making the SH3 regions available for binding to p22 phox . Another regulating the respiratory burst oxidase is thought to involve activa-
cytoplasmic component with homology to p47 phox has been identi- tion of the electron transport function of cytochrome b . The muta-
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fied as p40 phox . p40 phox , like p47 phox , contains a PX domain, a motif tion analysis in patients with p47 phox -deficient forms of CGD reveals
that supports the binding to phosphoinositides on the cytosolic side an unusual pattern, in that more than 90 percent of mutant alle-
of membranes. The p40 phox component stabilizes the cytoplasmic les have guanine-thymine dinucleotide deletion at the start of exon
393
complexes of p67 phox and p47 phox on phagosomes. Its binding of phos- 2, resulting in frameshift and premature stop. 402,406 The truncated
phatidylinositol 3 phosphate also potentiates superoxide production protein is unstable in that it cannot be detected immunologically.
394
upon neutrophil activation. Cytochrome b spans the membrane, The majority of patients appear to be homozygous for this muta-
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permitting NADPH to be oxidized at the cytoplasmic surface and tion without any history of consanguinity. The p47 phox gene occurs
–
oxygen to be reduced to form O on the outer surface of the plasma in an area of chromosome 7 that has a high degree of evolution-
2
membrane or on the inner surface of the phagosomal membrane. 395 ary duplication in normal individuals because a pseudogene highly
Genetic Alterations Affecting Cytochrome b The most frequent homologous to the normal p47 phox gene exists in the normal genome
form of CGD occurs in 70 percent of patients and is caused by muta- in this region of duplication. The pseudogene contains the same GT
tions in the gp91 phox gene, termed CYBB, which is located on chromo- deletion associated with most cases of p47 phox CGD. This implies that
some Xp21.1. 376,396 These mutations lead to the X-linked form of the recombination of the normal gene and pseudogene with conversion
disease. Large interstitial deletions causing other X-linked disorders of the normal gene to partial pseudotype sequence in that region
such as retinitis pigmentosa, Duchenne muscular dystrophy, McLeod may be responsible for the high relative rate of this specific mutation
hemolytic anemia, and ornithine transcarbamylase deficiency, have in diverse racial groups, which proved to be the case. 407
been reported in a few patients with X-linked CGD. 383,397–399 Mutation A second rare form of CGD is caused by mutations in the gene
401
analysis of the gene encoding gp91 and a large group of X-linked CGD for the p67 phox cytosolic component. The p67 phox gene, which has been
kindreds has documented many distinct defects, including point muta- mapped to the long arm of chromosome 1, spans 37 kb and contains
tions, inversions, deletions, or insertions that disrupt the reading frame 16 exons. The mutations identified in p67 phox -deficiency CGD have
396
and nonsense mutations that create a premature stop codon. Some included missense mutations and spliced junction mutations affecting
splice-site defects have also been identified. In this situation, short dele- mRNA processing, which led to nondetectable p67 phox protein by immu-
tions in gp91 phox mRNA are caused by point mutations that produce nologic means. 402
400
partial or complete exon skipping during mRNA splicing. This abnor- Mutation of NCF4, the gene encoding p40 phox , was reported in a
mality is a common cause of X-linked CGD. In the remaining patients, child with granulomatous colitis. One allele had a frameshift mutation
point mutations have been identified that generate either premature with a premature stop codon. The other had a missense mutation pre-
stop codons or amino acid substitutions that apparently disrupt pro- dicting an R105Q substitution in the PX domain which is responsible
tein stability or function and lead to a complete lack of detectable cyto- for binding to phosphatidylinositol 3 phosphate. The functional defect
chrome b protein in phagocytic cells in most patients with X-linked was inability to assemble the NADPH oxidase in the membrane of pha-
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CGD. In some situations, low levels of functional cytochrome b are gosomes but not on the plasma mambrane. 408
present, whereas in others, normal levels of dysfunctional cytochrome Predisposition to Infection Mutations in the gene for cyto-
401
b occur. In the latter situation there is some clustering of defects chrome b or the cytosolic factors involved in activating the
558
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in regions of known function, such as the NADPH- or flavin-binding cytochrome are associated with the CGD phenotype. Figure 66–7
402
consensus regions. Approximately 10 to 15 percent of X-linked CGD shows schematically the manner in which the metabolic deficiency
arises from new germline mutations. 403 of the CGD neutrophil predisposes the host to infection. Normal
A similar array of mutations has been identified in the 5 percent neutrophils accumulate H O and other oxygen metabolites in the
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of CGD patients who have abnormalities in the p22 phox gene, termed phagosomes containing ingested microorganisms. MPO is delivered
CYBA, which is located on chromosome 16q24. 376,402,404 In this autoso- to the phagosome by degranulation and in this setting H O acts as
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mal disorder, mutations in the p22 phox gene result in deletions, frame- a substrate for MPO to oxidize halide to HOCl and chloramines,
shifts, and/or missense mutations. Patients with a defective p22 phox gene which kill the microbes. The quantity of H O produced by the nor-
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do not express the other cytoplasmic unit polypeptide. In one patient, mal neutrophils is sufficient to exceed the capacity of catalase, a
p22 phox peptide was associated with normal amounts of cytochrome b H O -catabolizing enzyme produced by many aerobic microorgan-
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with normal heme spectrum, but p47 phox translocation membrane did isms, including S. aureus, most Gram-negative enteric bacteria, C.
not occur and there was no oxidase activation because the mutation albicans, and Aspergillus spp. In contrast, H O is not produced by
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affected a proline-rich region thought to mediate binding to one of the CGD neutrophils, and any generated by the microbes themselves
SH3 domains of p47 phox . In gp91 phox -deficient patients, p22 phox mRNA is may be destroyed by their own catalase. Thus, catalase-positive
present, but it is not translated, which is consistent with the notion that microbes can multiply inside CGD neutrophils, where they are pro-
either cytochrome subunit polypeptide is dependent upon the stable tected from most circulating antibiotics, and can be transported to
Kaushansky_chapter 66_p1005-1042.indd 1029 9/21/15 10:48 AM