Large Deletions Involving the Beta Globin Gene Complex: Genotype-Phenotype Correlation of 119 Cases

Background: Large deletions involving the beta globin complex are relatively rare. They can be categorized generally into five groups by deletion size and/or location: 1) beta zero thalassemia (BZT); 2) delta beta thalassemia (DBT); 3) hereditary persistence of fetal hemoglobin (HPFH); 4) gamma delta beta thalassemia (GDBT); and 5) epsilon gamma delta beta thalassemia (EGDBT). These deletions are not well understood but often have significant clinical impact, either when present alone or in combination with other hemoglobin mutations. In this study, we analyze phenotypic and molecular data on a large number of cases with deletions in the beta globin gene complex to better classify these five groups of deletions as they occur in isolation.

Methods: A query of the routine clinical testing patient files from the Mayo Clinic Metabolic Hematology and Molecular Genetics Laboratories from 2010 to 2015 identified 179 patients with a deletion confirmed by a Multiplex Ligation-dependent Probe Amplification (MLPA) assay. Twenty-four probes sets were placed from the 5' locus control region (LCR) to the 3' hypersensitivity region, spanning the beta globin gene complex. Using a Luminex LX200 flow cytometer, a gene dosage ratio was calculated for each probe set using the median fluorescent intensity value collected. The size and location of the deletion and patient phenotype were compared.

Results: Of the 179 total cases, the following large deletions were identified: beta gene (HBB) (n = 47), delta (HBD) through HBB (n = 105), A-gamma (HBG2) through HBB (n = 20), and locus control region (LCR) through HBB (n = 7). One case had a deletion involving the LCR epsilon with the rest of the complex left intact. A subset (n = 60) of cases had compound hemoglobin mutations that altered the phenotype. The BZT cases had relatively high Hb A₂ levels and variable Hb F levels consistent with promotor region loss. The main differences between DBT and HPFH included Hb F and Hb A₂ levels. GDBT cases presented with median Hb F levels higher than that observed in DBTs, normal Hb A2, and microcytic anemia. EGDBT cases had variable features according to age of the patient and Hb F level; severe microcytic anemia was observed in neonates, milder microcytic anemia in young children, and microcytosis without anemia in an adult case. The phenotypic features of 119 patients with isolated large deletions are compiled in table 1.

Conclusion: In general, all five categories of large deletions in an isolated heterozygous state can present with microcytic anemia and are typically benign with the exception of transient severe microcytic anemia in neonatal EGDBT cases. Although phenotypes associated with large deletions involving the beta globin gene complex are frequently distinctive, significant phenotypic overlap can be seen in a subset of cases. These cases require molecular analysis due to their clinical importance when in combination with another beta globin gene complex mutation for an adequate diagnosis and treatment approach.