Chromosome 7q Hemizygosity Recapitulates MDS-Related Cellular Phenotypes In Genetically Engineered Human Pluripotent Stem Cells

Loss of the entire or part of one copy of chromosome 7 [del(7/7q)] is a recurrent cytogenetic abnormality in MDS. Its strong association with previous exposure to alkylating agents, consistently poor response to therapy and discrete gene expression profile strongly suggest that del(7/7q)-MDS is a distinct disease in the MDS spectrum whose pathogenesis is intimately linked to loss of chr7q genetic material. Understanding the role of chr7q loss in cell biology can provide key insights into the pathogenesis of MDS and leukemogenesis.

Narrowing down the responsible region on chr7q presents a challenge that has proved intractable with existing approaches. Chr7q deletions physically mapped in large cohorts of patients are typically very large and dispersed along most of the length of chr7q. Modeling in the mouse is problematic for reasons of synteny. Deletion of the syntenic region of one commonly deleted region (7q22 CDR) failed to demonstrate a phenotype.

Several lines of evidence point to haploinsufficiency of chr7q genetic material – rather than a 2-hit model – as the underlying mechanism in del(7q)-MDS. No inactivating mutations of candidate 7q genes have been detected by resequencing the remaining allele and haploinsufficiency of coding and miRNA genes has been strongly linked to the pathogenesis of del(5q)-MDS. Haploinsufficiency can only be assessed through functional studies (and not genomic technologies), but these are currently hindered by the lack of a clearly described del(7q)-associated phenotype.

With recent advances in human pluripotent stem cell (hPSC) research and genetic engineering technologies, reverse human genetics in an isogenic setting by disruption of genomic elements into their cognate genomic and cellular context - hitherto unthinkable for the human genome – are now a realistic prospect. To determine the impact of hemizygous chr7q loss on the cellular phenotype, we have generated isogenic del(7q)- and normal hPSCs by engineering deletions spanning variable overlapping regions encompassing the entire length of chr7q using adeno-associated virus (AAV)-mediated gene targeting combined with Cre-lox technology. Specifically, we targeted two inverted loxP sites together with a positive (puro) and a negative (HSVtk) selection marker in a near-telomeric region of chromosome 7q (7q36.3) into the H1 hESC line, as well as a karyotypically normal iPSC line (line 2-12) derived from BMMCs of a patient with del(7q)-MDS. Following transient expression of Cre recombinase and ganciclovir selection, clones were screened by qPCR probing different regions along the length of chr7. Four H1-derived and four 2-12-derived clones were selected following screening of 24 and 34 clones, respectively, and after excluding clones with additional chromosomal abnormalities by karyotyping and the exact extent of their chr7q deletions was mapped by aCGH.

We focused our phenotypic characterization on two cellular phenotypes that we recently reported in del(7q)-iPSCs derived from MDS BMMCs: cell proliferation and in vitro hematopoietic differentiation potential. 7 of the 8 clones harboring deletions spanning variable lengths along the entire chr7q had a lower (by ½ log) proliferation rate than their corresponding isogenic parental lines. All these clones also exhibited a markedly reduced differentiation potential along all hematopoietic lineages and almost absent clonogenic capacity in methylcellulose. These cellular phenotypes are highly similar to those we find in our del(7q)-MDS-iPSCs. Notably, one of the eight clones (2-12.Cre-44), harboring a smaller deletion spanning 7q11.21-7q.31.1 retained comparable proliferation and differentiation capacity to that of normal isogenic and non-isogenic hPSCs.

In conclusion, our results demonstrate that hemizygous loss of chr7q material recapitulates the cellular phenotypes of impaired proliferation and hematopoietic differentiation that we find in del(7q)-MDS-iPSCs, supporting a haploinsufficiency pathogenesis of del(7q)-MDS. Correlation of the phenotypes with the boundaries of chr7q deletions in our collection of hESC and iPSC clones points to a region spanning 7q31.1-7q36.1 as the critical region in del(7q)-MDS. Further studies in additional clones harboring smaller chr7q deletions will further narrow down the responsible region and guide prioritization of candidate genes.