Homozygous deletion of the major alpha-globin regulatory element (MCS-R2) responsible for a severe case of hemoglobin H disease

To the editor:

Alpha-thalassemia commonly results from deletions or point mutations in one or both alpha-globin genes, located on chromosome 16p13.3. Rarely, alpha-thalassemia is caused by deletions in a region, located 30 to 70 kb upstream of the alpha-globin genes, containing 4 remote, multispecies conserved sequences (MCS-R 1-4), required to regulate alpha-globin expression.¹  Natural deletions in humans, analysis of interspecific hybrids, stable transfectants, and studies of transgenic mice indicate that deletion of MCS-R 2 leads to almost complete down-regulation of alpha-gene expression.^2,3 

Hemoglobin H (HbH) disease, the clinically significant intermediate form of alpha-thalassemia, is characterized by mild to moderate (sometime severe) microcytic, hypochromic hemolytic anemia, jaundice, hepatosplenomegaly, and occasionally mild thalassemia-like bone modifications. Most commonly HbH disease results from deletion or dysfunction of 3 of 4 alpha-globin genes, and rarely from deletions in the upstream regulatory region.^2,4-6 

Here we describe a severe case of HbH disease in a 11-year-old Italian boy due to deletions of variable extent of both upstream regulatory regions, with all 4 downstream alpha-globin genes intact. The hematologic characteristics of the proband and his family are reported in Figure 1. The patient had moderate jaundice, marked hepatosplenomegaly, and mild but typical facial thalassemia-like modifications; he maintained the hemoglobin level between 6.0 and 8.0 g/dL and needed several red blood cell transfusions for worsening of anemia. The patient lacks MCS-R2 in both chromosomes, and MCS-R1 and MCS-R3 in one chromosome (Figure 1).

A series of naturally occurring human deletions that remove MCS-R elements, reducing the expression of the remote alpha-globin genes, have been identified.^2-7  It has been shown experimentally that deletion of MCS-R2 alone is sufficient to down-regulate alpha-globin expression to less than 3% of normal, consistent with the notion that MCS-R2 is the most important regulatory element.^8,9  In our patient, the homozygous deletion of MCS-R2 is associated with HbH disease, a phenotype less severe than expected from the predicted reduction of alpha-globin chain expression. Moreover, it should be pointed out that in the paternal chromosome, MCS-R1 and MCS-R3 are also deleted. Therefore, in this patient the residual production of alpha-globin chains is due only to the presence of paternal MCS-R4 and maternal MCS-R1, -R3, -R4, which seem therefore to play a significant role in the control of alpha-globin gene expression. The clinical findings of our patient (in particular the grade of anemia and percentage of HbH) are more severe compared with those in similar patients with deletions removing the upstream regulatory region.²  All of these patients with HbH disease have a combination of deletions of the upstream regulatory region with the common 3.7- or 4.2-kb alpha-globin gene deletion, whereas in our patient all 4 alpha-globin genes are intact.

In conclusion, the patient here described represents the first true human “knockout” MCS-R2 mutation, with a clinically relevant phenotype despite the presence of all 4 alpha-genes, but less severe than expected. Despite their rarity, these mutations should be investigated in the gene-mapping screening programs for alpha-thalassemia. This report adds significant information on the control of the alpha-gene cluster, proving that the complete loss of the major regulatory MCS-R2 element severely down-regulates the expression of alpha-globin genes but is not associated with a complete absence of alpha-chain production.