DBA is a congenital pure red cell aplasia characterized by normochromic macrocytic anemia and reticulocytopenia in the peripheral blood and the loss of erythroblast from bone marrow. Approximately 90% of affected individuals present in infancy or early childhood, although a “nonclassical” mild phenotype may not be diagnosed until later in life [2, 3].

Clinical manifestations of DBA are variable. In about 40% of patients, DBA is associated with physical anomalies and growth retardation, but in some patients, no congenital anomalies are found [3]. Table 7.1 summarizes the range of congenital abnormalities found in DBA patients.

Twenty-nine cases were reported among more than 700 patients in the literature [3]. A recent prospective study showed that among 608 patients, 15 solid tumors, two acute myeloid leukemias (AML), and two cases of myelodysplastic syndrome (MDS) were diagnosed at a median age of 41 years in patients who had not received a bone marrow transplant. The rate of development of solid tumors or leukemia was 5.4-fold higher in DBA patients than expected in a demographically matched comparison with the general population. Furthermore, specific tumors had significantly elevated incidence ratios for MDS, AML, adenocarcinoma of the colon, osteogenic sarcoma, and female genital cancer [138].

The diagnostic criteria for “classical” DBA include macrocytic (or normocytic) anemia with no other significant cytopenia presenting prior to the first birthday, reticulocytopenia, and normal marrow cellularity with a paucity of erythroid precursors, and it is accompanied by congenital anomalies in some cases. However, diagnosis of DBA is often difficult due to the incompleteness of the phenotypes and the wide variability in clinical expression. Even mutations in individual RP genes lead to widely variable phenotypic expression; family members with the same mutation in an RP gene can present with clinical differences. For example, RPS19 mutations are found in some first-degree relatives presenting only with isolated high erythrocyte adenosine deaminase activity and/or macrocytosis [24]. Therefore, it is very difficult to make a diagnosis on the basis of clinical phenotype alone. Molecular diagnosis enables the detection of carriers and the avoidance of hematopoietic stem cell transplantation from sibling donors with these mutations. However, about 40% have no known pathogenic mutations. The diagnostic and supporting criteria for diagnosis of DBA, including mild type, are described in Table 7.2. We have modified the criteria from the international clinical consensus conference [3] based on our results for the novel DBA biomarker reduced glutathione (GSH) [32].

Patients who do not respond to steroids may require chronic transfusion therapy every ~4–8 weeks. Although maintaining hemoglobin levels above 8 g/dL is recommended, a disadvantage of chronic transfusion therapy is hemosiderosis by iron overload. To avoid liver dysfunction, diabetes, and myocardiopathy due to iron deposition, chelation therapy with deferasirox or deferoxamine is recommended. However, methods for oral iron chelation therapy with deferasirox in neonates have not been established.

Approximately 80% of patients respond to steroids. The recommended starting dose is 2 mg/kg/day of prednisone at initial treatment. In over 20% of DBA patients, steroids eventually are stopped completely [3]. It should be mentioned that important steroid side effects include growth retardation, osteoporosis, obesity, hypertension, diabetes mellitus, cataract, and glaucoma. The therapy is not generally recommended in babies under 6 months of age. Alternative treatments, including cyclosporine, metoclopramide, EPO and a combination of prednisolone and cyclosporine, have been mentioned, but evaluations have not been provided.

Chronic transfusion dependence with refractoriness to steroids is considered an acceptable indication for hematopoietic stem cell transplantation (HSCT). The outcomes of HSCT for DBA in Japan are better than those reported in other countries. To date, 19 patients have received transplants of allogeneic hematopoietic stem cells (HSCs). Thirteen patients underwent BMT (six cases with HLA-matched siblings and seven cases with unrelated donors), and all have disease-free survival [33]. Five patients underwent cord blood transplantation (CBT), and two of these with transplants from siblings have survived. However, in three patients treated with unrelated donor cord blood, two were not successfully engrafted, and the third died of lymphoproliferative disease, despite confirmed engraftment. Consequently, bone marrow should be selected as a source of HSCs for transplantation at present. Better therapeutic outcomes have been observed with a conditioning therapy combining busulfan (oral administration, 16 mg/kg or 560 mg/m²) and cyclophosphamide (120–200 mg/kg). Although the reported number of patients is few, a conditioning therapy using a half dose of busulfan leads to good outcomes when using unrelated HSCs transplantation. When busulfan was omitted from pretreatment, the number of cases with engraftment failure was modestly higher. There is not sufficient evidence supporting the value of bone marrow nondestructive pretreatment therapy for HSCT in DBA [34].

Chromosome instability syndrome is a group of inherited conditions associated with chromosomal instability and breakage, and the following chromosome instability syndromes are known: xeroderma pigmentosum, ataxia-telangiectasia, Bloom syndrome, and Nijmegen syndrome.

Cell differentiation is controlled in part by cell lineage-restricted transcription factors. The transcription factor GATA1 is expressed within the hematopoietic hierarchy in erythroid, megakaryocytic, eosinophilic, and mast cell lineages [82–85], as well as in Sertoli cells of the testis [86, 87]. Gene targeting experiments have revealed that GATA1 is required for the terminal differentiation of definitive erythroid and megakaryocytic cells [88–90].

GATA1 mRNAs are abundantly expressed in blast cells from TAM patients, as well as ML-DS patients [91]. Almost all patients with TAM and ML-DS have GATA1 mutations [68–74]. The mutations lead to a loss of expression of the full-length GATA1 protein and the expression of a truncated GATA1 protein (GATA1s) that lacks the N-terminal transactivation domain. GATA1s, which is translated from the second methionine at codon 84, retains intact zinc finger regions, binds appropriately to the GATA consensus sequence, and interacts normally with its essential cofactor FOG-1.

Several cell culture-based rescue experiments have reported that the N-terminal activation domain appears to be dispensable during erythroid and megakaryocytic cell differentiation [92–95]. Furthermore, distinct regions in the N-terminus of GATA1, which regulates the proliferation of immature megakaryocytic progenitors, are required for terminal megakaryocyte differentiation and controlling the growth of immature precursors [96, 97]. GATA1, but not GATA1s, interacts physically with the transcription factor E2F, which can trigger reentry into the cell cycle. That observation suggests that the failure of GATA1s to repress E2F activity in cellular transformation of fetal progenitors may be caused by this failure in direct or indirect interaction [98]. Supporting this hypothesis, Toki et al. recently discovered novel GATA1 mutants in TAM that lack just the E2F interaction domain in the N-terminus of GATA1 [99].

On the other hand, mice harboring a heterozygous GATA1 knockdown allele frequently develop erythroblastic leukemia [100]. These observations indicate that the expression level of GATA1 is crucial for the proper development of erythroid and megakaryocytic cells and that compromised GATA1 expression is a causal factor in leukemia [101]. Recently, Kanezaki et al. showed that the spectrum of transcripts derived from the mutant genes affects GATA1s protein expression. Mutations resulting in low GATA1s levels are significantly associated with a risk of progression to ML-DS [102]. However, a subsequent report contended that the GATA1 mutational spectrum did not differ between TAM and AMKL and that the type of GATA1 sequence mutation is not a reliable tool and is not prognostic of which patients with TAM are likely to develop ML-DS [103]. To resolve this problem, a prospective study with a large series of TAM patients is necessary.

TAM is a disorder found mainly in patients with DS during the newborn period and is sometimes associated with liver fibrosis, which usually takes a lethal course. Based on pathological observation of TAM cases with fatal liver fibrosis, Miyauchi et al. have proposed a hypothesis that TAM blasts originate from fetal liver hematopoietic progenitors [104]. Analyses of GATA1 mutations provide direct evidence for this hypothesis. The presence of identical GATA1 mutations in the blasts of both TAM and ML-DS in an identical twin suggested that GATA1 mutations occur early during prenatal hematopoiesis [105]. GATA1 mutations were detected in hematopoietic tissues from DS fetuses that had no pathological evidence of leukemia [106] and in Guthrie newborn screening cards at birth from DS infants who later developed ML-DS [107].

In addition, mice expressing GATA1s presented with hyperproliferation of yolk sac and fetal liver progenitors [108]. Consistent with this finding, GATA1-deficient mice rescued with transgenic expression of GATA1s exhibited hyper-megakaryopoiesis during a limited embryonic and postnatal period, resembling the phenotype in human TAM cases [109]. These data from mouse experiments also indicated the possibility that fetal megakaryocytic progenitors are the target cells of TAM and ML-DS.

For the following reasons, trisomy 21 is thought to be the first genetic event in the development of ML-DS. First, TAM occurs almost exclusively in patients with DS [66]. Second, when TAM occurs in patients with mosaic trisomy 21, the TAM clones involve only the cells with trisomy 21 [110]. Third, an inherited mutation in humans leading to production of only GATA1s is associated with impaired erythropoiesis and granulopoiesis, but does not promote leukemia in the absence of trisomy 21 [111]. Fourth, trisomy 21 itself is associated with enhanced expansion of human fetal erythroid and megakaryocytic precursors, independent of GATA1 mutations [112, 113].