Page 377 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 26 Biology of Erythropoiesis, Erythroid Differentiation, and Maturation 313

and developmental stage–specific transcription factors contribute to circulation begins and 1 day before these cells are found in the yolk
this regulation. 447 sac. Long-term repopulating cells after their transplantation in adult
Recently genetic linkage and genome-wide association studies in recipients were detected only in the AGM area, leading to speculation
individuals with increased levels of HbF or hereditary persistence that this intraembryonic site is the main or only source of fetal liver
465
of fetal hemoglobin (HPFH) syndromes have provided new and colonization, in contrast to earlier experiments implicating the yolk
important insights in the control of fetal to adult globin switch- sac in that role. 460–462 Establishment of blood flow and the concentra-
ing. 448,449 Of interest, besides cis control of switching (deletions in tion of nitric oxide appear to play an important role in determining
β-globin cluster or mutations in the γ-globin gene promoters), trans the number of definitive hematopoietic stem cells generated in the
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control was revealed through direct and indirect interactions of AGM region. Presence of mesodermally derived hematopoietic
γ-globin with molecules (controlled by loci unlinked to the β-globin cells in two distinct anatomic sites, one intraembryonic and the other
cluster) exerting repressive fetal globin activity. These molecules were extraembryonic, has been seen in explant studies of Xenopus and after
467
BCL11A, a transcription factor involved in juvenile leukemia, and analysis of chick–quail chimeras. More recent experiments with
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HBS1L-Myb. The full-length BCL11A is expressed in adult but not human cells have led to similar conclusions. Although the presence
fetal erythroid cells, and adult individuals with high hemoglobin F/ of progenitors for definitive hematopoiesis in two independent sites
BCL11A genotype have reduced expression of full-length BCL11A. (extraembryonic and within the embryo proper) is indisputable, the
Suppression of its expression reactivates HbF expression in adult ery- extent to which these two sites contribute to fetal liver colonization
450
throid cells in vitro, and deletion of BCL11A interferes with fetal has been a matter of dispute. The conclusion that only the AGM area
hemoglobin silencing during development and rescues the phenotype contributes to fetal liver colonization was based on transplantation
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of a mouse model of sickle cell disease. These observations have experiments in adult recipients and has been challenged. Trans-
suggested that genome editing of BCL11A may represent a useful plantation experiments using newborn mice with active fetal liver
452
strategy to ameliorate the clinical picture of hemoglobinopathies. hematopoiesis as recipients showed that adult long-term repopulating
Genome editing approaches based on new editing tools (zinc fingers, cells are detectable in the yolk sac at day 9 postconception and are
TALENS and Crisper/cas technology) are rapidly being developed 37-fold greater in number than repopulating cells present in the P-Sp/
and allow precise targeting either of BCL11A or its newly identified AGM area at the same time. Therefore failure of yolk sac cells (or
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erythroid-specific enhancers. Myb-HBS1L is downregulated in AGM cells before day 10 postconception) to engraft adult recipients
individuals with elevated HbF levels and overexpression of Myb may be caused by either compromised homing or impaired survival
inhibits γ-globin in human erythroleukemia cells. The levels of Myb and proliferation within the adult bone marrow environment (because
were found to be controlled by micro-RNA 15a and 16-1 in patients of positive regulators or inhibition by negative regulators). In light of
with human trisomy 13 and high HbF levels. 453 this information, the 30-year-old theory that yolk sac colonizes the
460
In addition to the above molecules with γ-globin repressive fetal liver has been revived. A question that remains unanswered is
activity, recent data support a role of KLF1, the major erythroid whether stem cell activity 9 days postconception in yolk sac and P-Sp/
transcriptional regulator, as suppressor of fetal globin gene. KLF1 AGM is generated autonomously and independently or is derived
mutations were found relatively more common in a thalassemia- from a common precursor cell with migratory properties. Murine
endemic region and were associated with a milder β-thalassemia studies comparing newborn transplant outcomes before the onset of
454
phenotype. Valuable insights on the mechanism of KLF1 on the systemic circulation between the yolk sac site and the intraembryonic
469
regulation of globin genes were obtained from families with haplo- AGM site have emerged, and these, together with the identifica-
insufficiency of KLF1 (missense mutations affecting DNA binding) tion of placenta as an autonomous circulation-independent site with
470
and increased fetal hemoglobin levels. 455,456 However, this was not terminal erythroid maturation of primitive cells, have added an
always the case, and a compound heterozygosity was required in other additional layer of complexity.
457
families for high HbF expression. It appears that KLF1 targets A common precursor cell giving rise to erythroid cells with either
genes such as BCL11A, EPB4.9, and CD44, which are very sensitive yolk sac or fetal liver characteristics has been identified by culture of
to KLF1 activity, whereas its effects on other genes like γ-globin, or murine and human embryonic stem cells in vitro. 471,472 Environmen-
BCAM (carrying the Lutheran blood group antigens) are variable. tal regulation of specification to the primitive or definitive lineage
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Collectively, it turned out that KLF1 has a critical role in globin has been shown in Xenopus. However, because BFU-E present
gene switching both by directly activating β-globin and indirectly by in yolk sac, fetal liver, and fetal bone marrow have a definitive-
suppressing γ-globin through its control of BCL11A. like progeny and these progenitors were not present after ablation
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Overall, genetic data account for approximately 70% of the of core binding factor (CBF)-β despite the presence of normal
HPFH phenotypes observed in human populations, suggesting that embryonic erythropoiesis, the derivation of embryonic erythroblasts
new factors are yet to be identified. In this context, it is of interest from a distinct progenitor, not present in subsequent life, remains
0
that three β -thalassemia major patients who failed to engraft after a viable hypothesis. Of further interest is the observation that dele-
stem cell transplantation became transfusion-independent because tion of Mdm2 and Mdm4, two critical negative regulators of p53,
they expressed sustained levels of increased HbF in autologous red exerted distinct outcomes on primitive and definitive hematopoiesis.
cells posttransplantation without evidence of any known HPFH Whereas Mdm2 is required for primitive erythropoiesis, Mdm4 is
genotypes. 458,459 From all of these studies, it has become apparent required for massive expansion of definitive erythropoiesis in fetal
that switching is very complex, involving many players, cis and trans, liver and is dispensable for adult erythropoiesis. These data are also
with distinct and variable roles. The many models of globin switching consistent with the distinct molecular control between fetal and adult
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proposed previously (i.e., the competitive model, the chromosome cells discussed earlier.
looping, the gene silencing) are not mutually exclusive and may
complement each other.
In summary, throughout human development, waves of hema- TRANSCRIPTION FACTORS IN ERYTHROPOIESIS
topoiesis are initiated sequentially in newly recruited sites. The
first wave of erythropoiesis is seen in yolk sac between days 15 and Lineage-specific transcription factors are widely believed to be respon-
18 (7.5 days after conception in mice). In addition to erythroid sible for regulating the expression of erythroid genes during both
cells, uncommitted progenitors and progenitors for nonerythroid ontogeny and the course of erythroid differentiation. The majority
cells are present in the yolk sac and are thought to be the source of erythroid-specific transcription factors has been identified from
of cells colonizing the fetal liver. 211,460–462 However, in addition to cloning of breakpoints or translocations associated with human leu-
yolk sac, foci of hematopoietic activity have been detected within the kemias or from expression libraries obtained from erythroid cell lines.
embryo around the developing aorta (in para-aortic-splanchnopleura The precise role exerted by each of these factors in erythropoiesis was
[P-Sp] and aorto-gonad-mesonephros [AGM] area). 463–465 In fact, the later clarified by painstaking experiments with somatic cell fusions
P-Sp/AGM site in mice was shown to harbor progenitor cells before and in transgenic mice. 445,476,477 Some of the major transcription