Genetic Mosaicism and Hemoglobinopathies; Uniparental Isodisomy

Recently we discovered three independent cases of “severe late onset beta-thalassemia”, all presenting with the mild phenotype of beta thalassemia minor up to adult age and developing a severe transfusion dependent phenotype in the third and fourth decade of life when a presumed homozygosity for the beta-thalassemia mutation is observed. We demonstrate that uniparental isodisomy (UPD) of part of chromosome 11p, a mechanism also seen in 20% of Beckwith-Wiedemann syndrome patients, accounts for the observed mosaicism in all three independent cases. Clonal selection for hematopoietic stem cells containing the UPD for the mutant beta-globin gene during life may account for the progressive development of the disease. Recently we discovered another case showing a similar mosaic UPD of 11p, presenting as a regular beta-thalassemia carrier.

Direct sequencing of the beta-globin genes. Multiplex Ligation dependent Probe Amplification (MLPA) analysis of the HBB (beta-globin) gene cluster. Affymetrix GeneChip Human Mapping 262K NspI array (Santa Clara, CA, USA). Illumina OmniExpress 730K SNP Beads array (San Diego, CA, USA).

In all cases molecular analysis shows sequences in which a near to complete homozygosity for the beta-gene mutation in DNA extracted from leucocytes is seen. Loss of heterozygosity due to allele drop-out or a deletion of one allele was excluded by direct sequencing and MLPA analysis. Affymetrix and/or Illumina SNP-array analysis revealed incomplete homozygosity for SNP’s along almost the entire short arm of chromosome 11 containing the beta-globin gene, indicating mosaicism for a partial uniparental isodisomy of chromosome 11p. Three patients were born asymptomatic as beta-thal carriers and developed a severe blood-transfusion dependent beta-thalassemia major at different ages and with different percentages of mosaicism. The fourth patient however did not develop the clinical severity despite of an almost 50% mosaicism determined from the DNA isolated from leucocytes.

We demonstrate that uniparental isodisomy of part of chromosome 11 accounts for the observed mosaicism in all four independent cases. The most probable mechanism seems clonal selection for hematopoietic stem cells containing the uniparental isodisomy for the mutant beta-globin gene during life, this may account for the progressive development of the disease. However, there seems to be no correlation between the percentage of mosaicism measured in the DNA isolated from the white cells and the severity of the clinical phenotype related to the expression in red cells, which strongly suggests hematopoietic tissue heterogeneity for the observed UPD containing cell lineages. This may have serious consequences for disease prediction and counseling, as this is largely dependent upon DNA isolated from leucocytes.

No relevant conflicts of interest to declare.