Stress Erythropoiesis and Genetic Regulation of Fetal Hemoglobin in Inherited Bone Marrow Failure Syndromes

Abstract 2401

Patients with inherited bone marrow failure syndromes (IBMFS) frequently have manifestations of what has been called “stress erythropoiesis”. This includes macrocytosis (increased mean cell volume, MCV), increased fetal hemoglobin (Hb F) and erythropoietin (Epo) levels higher than predicted by the degree of anemia (red blood cell count, RBC). In patients with hemoglobinopathies (sickle cell disease, thalassemia, and hereditary persistence of fetal hemoglobin), Hb F levels are regulated by 3 quantitative trait loci (QTL), located at HBSIL-MYB on chromosome 6q, BCLIIA on chromosome 2p and XMN1-Gg representing HBB cluster on chromosome 11p. The role of these QTLs in the elevated Hb F levels in patients with an IBMFS has not been previously reported. Percent Hb F was measured by HPLC in blood from 97 untransplanted individuals with an IBMFS. Absolute Hb F (g/dL) was calculated by multiplication of Hb F% times total Hb in order to include data from transfused patients, and log-transformed to approximate a normal distribution. Epo levels were also log-transformed due to the wide range (8 to 1800 mU/mL). DNA was extracted from leukocytes, and candidate regions were amplified and genotyped by TaqMan. The candidate QTLs were evaluated by genotyping of tagging single nucleotide polymorphisms (SNPs): five for HBSIL-MYB, two for BCLIIA, and one for XMN1-Gg. Data were modeled using a generalized linear model (GLM), appropriate for data with a constant coefficient of variation. There were 31 patients with Diamond-Blackfan anemia (DBA), 35 with dyskeratosis congenita (DC), 25 with Fanconi anemia (FA), and 6 with Shwachman-Diamond syndrome (SDS). Hb F was elevated in 70% of the total group of patients: 48% of DBA, 83% of DC, 76% of FA, and 83% of SDS. In the pooled group of 97 IBMFS patients, 68 (70%) had Hb F >1 % (upper limit of normal), 59 (61%) were macrocytic, 55 (57%) were anemic for age, and 70 (77%) had elevated Epo. The frequencies of heterozygosity or homozygosity for the alternative alleles for the QTLs were 50% for HBSIL-MYB, >90% for BCLIIA, and 52% for XMN1-Gg. The multivariate model for Hb F in the total goup of IBMFS included the alternative allele for the XMN1-Gg SNP (p = 0.04), younger age (p<0.001), male sex (p=0.04), and increased Epo (p<0.001). In this model, the alternative allele for the XMN1-Gg QTL was associated with a 32% increase in the level of Hb F. Subset analyses indicated that the strongest association of the XMN1-Gg QTL was in FA and DC (increased Hb F by 68% and 48% respectively, p-values 0.02 and 0.09) and had no effect in DBA (decreased Hb F by 18%, p = 0.6). Data including the other QTLs were not significant. These results suggest that the alternative allele at XMN1-Gg is associated with the increased level of Hb F in FA and DC, but not DBA, after adjustment for age, sex, and Epo level. A low level of Hb F should not exclude the diagnosis of an IBMFS in a patient who has other signs of stress erythropoiesis (anemia with increased MCV and Epo), since that patient may not have the variant allele associated with increased Hb F. The degree of elevation of Hb F in FA and DC depends on the alleles at the XMN1-Gg QTL. A strength of this study is the sample size of almost 100 patients with an IBMFS who are well-characterized. A limitation is that the number within each syndrome is still small; the role of the other QTLs may be identified in future larger studies. Of major interest is that this is the first study to show regulation of Hb F by the same QTL in FA and DC as the common hemoglobinopathies, thus linking Hb F regulation across disparate hematologic disorders.