IHC stains:
Diagnosis: Cutaneous extramedullary hematopoiesis.
Cause?
DNA micrarray analysis of the peripheral blood:
50 kb loss at 11p15.4 that contains HBE1,HBG1, HBG2, and locus control region (LCR).
C/w epsilon gamma delta beta (εγδβ)0-thalassemia.
(εγδβ)0-thalassemia
(εγδβ)0-thalassemia results from variably sized large beta cluster deletions that abolish all protein expression on that allele. The lack of sufficient epsilon and gamma chains results in unpaired excess alpha globin chains which are toxic to RBCs and cause severe microcytic neonatal anemia. No electronically abnormal peaks or bands are detectable by electrophoresis or HPLC. In (εγδβ)0-thalassemia, the associated anemia resolves within the first year of life; however, the moderate to severe microcytosis persists throughout life. Hemoglobin fraction percentages are normal, including Hb A2 and Hb F (Table 1). (εγδβ)0-thalassemia deletions have not been reported in a homozygous state as it is presumed not compatible with life.
The deletions range from removing just a portion of the locus control region (LCR) (5’ HS1-5) to extending throughout the entire cluster (Figure 1). It was reported that deletions limited to the LCR region (such as the Hispanic and Puerto Rican subtypes) silenced the expression of all the beta-like globin genes, even though the genes were physically intact. Greater than 30 different large deletions have been reported that result in (εγδβ)0-thalassemia in a variety of ethnic populations. Due to the absence of abnormal protein findings, this diagnosis requires molecular confirmation using techniques specific for large deletion analysis, such as DNA microarray, multiplex ligation-dependent probe amplification (MLPA).
Beta Globin Cluster Deletions
The human beta-globin gene cluster is composed of five genes (epsilon-gamma G-gamma A-delta-beta) that are responsible for production of hemoglobin chains. Epsilon, gamma, delta and beta chain genes are present on chromosome 11 in the same order as their sequential expression from embryonic, fetal and adult maturity states (see Figure 1). The main regulatory element of this locus is known as the locus control region (LCR) which mediates a series of epigenetic and structural changes in the beta-globin gene cluster that allows expression of its genes. This LCR consists of five DNase I-hypersensitive sites with the furthest site being located ~22 kb upstream of the epsilon (HBE1) gene. Deletion of the LCR locus is equivalent to the deletion of the entire beta-globin gene cluster. When large deletions remove genes from this cluster, the findings of Hb electrophoresis will be the followings.
1) beta gene is deleted → results in increased Hb A2 because the delta gene is preferentially expressed (seen in β0-thalassemia).
2) delta and beta genes are deleted → results in increased Hb F levels, because the gamma genes are preferentially expressed (seen in HPFH or (δβ)0-thalassemia).
3) gamma, delta and beta genes are deleted → results in variably increased Hb F levels, because the gamma genes are preferentially expressed, although one is missing (seen in γ (γδβ)0-thalassemia).
when all four genes – epsilon, gamma, delta and beta - are deleted, there are no genes remaining to be preferentially expressed on the allele, and the hemoglobin fraction percentages reflect the other normal allele (seen in (εγδβ)0-thalassemia). This results in normal Hb A, A2 and F percentages and there is no abnormal variant peak because this is a thalassemia.
Cause?
DNA micrarray analysis of the peripheral blood:
50 kb loss at 11p15.4 that contains HBE1,HBG1, HBG2, and locus control region (LCR).
C/w epsilon gamma delta beta (εγδβ)0-thalassemia.
(εγδβ)0-thalassemia
(εγδβ)0-thalassemia results from variably sized large beta cluster deletions that abolish all protein expression on that allele. The lack of sufficient epsilon and gamma chains results in unpaired excess alpha globin chains which are toxic to RBCs and cause severe microcytic neonatal anemia. No electronically abnormal peaks or bands are detectable by electrophoresis or HPLC. In (εγδβ)0-thalassemia, the associated anemia resolves within the first year of life; however, the moderate to severe microcytosis persists throughout life. Hemoglobin fraction percentages are normal, including Hb A2 and Hb F (Table 1). (εγδβ)0-thalassemia deletions have not been reported in a homozygous state as it is presumed not compatible with life.
The deletions range from removing just a portion of the locus control region (LCR) (5’ HS1-5) to extending throughout the entire cluster (Figure 1). It was reported that deletions limited to the LCR region (such as the Hispanic and Puerto Rican subtypes) silenced the expression of all the beta-like globin genes, even though the genes were physically intact. Greater than 30 different large deletions have been reported that result in (εγδβ)0-thalassemia in a variety of ethnic populations. Due to the absence of abnormal protein findings, this diagnosis requires molecular confirmation using techniques specific for large deletion analysis, such as DNA microarray, multiplex ligation-dependent probe amplification (MLPA).
Beta Globin Cluster Deletions
The human beta-globin gene cluster is composed of five genes (epsilon-gamma G-gamma A-delta-beta) that are responsible for production of hemoglobin chains. Epsilon, gamma, delta and beta chain genes are present on chromosome 11 in the same order as their sequential expression from embryonic, fetal and adult maturity states (see Figure 1). The main regulatory element of this locus is known as the locus control region (LCR) which mediates a series of epigenetic and structural changes in the beta-globin gene cluster that allows expression of its genes. This LCR consists of five DNase I-hypersensitive sites with the furthest site being located ~22 kb upstream of the epsilon (HBE1) gene. Deletion of the LCR locus is equivalent to the deletion of the entire beta-globin gene cluster. When large deletions remove genes from this cluster, the findings of Hb electrophoresis will be the followings.
1) beta gene is deleted → results in increased Hb A2 because the delta gene is preferentially expressed (seen in β0-thalassemia).
2) delta and beta genes are deleted → results in increased Hb F levels, because the gamma genes are preferentially expressed (seen in HPFH or (δβ)0-thalassemia).
3) gamma, delta and beta genes are deleted → results in variably increased Hb F levels, because the gamma genes are preferentially expressed, although one is missing (seen in γ (γδβ)0-thalassemia).
when all four genes – epsilon, gamma, delta and beta - are deleted, there are no genes remaining to be preferentially expressed on the allele, and the hemoglobin fraction percentages reflect the other normal allele (seen in (εγδβ)0-thalassemia). This results in normal Hb A, A2 and F percentages and there is no abnormal variant peak because this is a thalassemia.
Figure 1. (εγδβ)0-thalassemia deletions. The beta globin cluster on chromosome 11 contains the 5’ hypersensitivity regions (also called the locus control region or LCR) and the epsilon (HBE), two gamma (HBG2 and HBG1), delta (HBD) and beta (HBB) globin genes. Over 30 mutation subtypes have been reported in diverse ethnic groups. Some involve only the LCR and others involve variable numbers of genes in the complex, but all are completely silent as no transcription occurs without a functional LCR. The Puerto Rican (top); Hispanic (second); and Dutch VII (third) subtypes are demonstrated here by the first three lines. Multiple different subtypes are represented by the last two lines, some of which leave the beta gene intact but non-functional (single barbed arrow) and many more in which the entire complex is deleted (double barbed arrow). Adapted from Thein S, Cold Spring Harb Perspect Med 2013;3:a011700
Table 1. Phenotypic features of large deletions in the beta globin gene cluster. BZT = (β)0-thalassemia; DBT = (δβ)0-thalassemia; HPFH = hereditary persistence of fetal hemoglobin (large deletional types); GDBT = Gγ(Aγδβ)0-thalassemia; EGDBT = (εγδβ)0-thalassemia. The anemia in (εγδβ)0-thalassemia improves with age; however, the MCV remains similarly low. From Hein MS et al. Blood 2015 126:3374.