A mouse model of Zhu-Tokita-Takenouchi-Kim syndrome reveals indispensable SON functions in organ development and hematopoiesis
Rare diseases are underrepresented in biomedical research, leading to insufficient awareness. Zhu-Tokita-Takenouchi-Kim (ZTTK) syndrome is a rare disease caused by genetic alterations that result in heterozygous loss-of-function of SON. While ZTTK syndrome patients suffer from numerous symptoms, the lack of model organisms hampers our understanding of SON and this complex syndrome. Here, we developed Son haploinsufficiency (Son+/-) mice as a model of ZTTK syndrome and identified the indispensable roles of Son in organ development and hematopoiesis. Son+/- mice recapitulated clinical symptoms of ZTTK syndrome, including growth retardation, cognitive impairment, skeletal abnormalities, and kidney agenesis. To obtain a more comprehensive understanding of the changes within HSPCs at the molecular level, we employed single-cell RNA-sequencing (scRNAseq) using both WT and Son+/-  bone marrow HSPCs isolated and sorted based on their Lin- cKit+ (LK) cell markers (24,355 total LK cells from 2 WT mice; 31,232 total LK cells from 2 Son+/- mice. The linage-marker-negative (Lin-) c-kit+ population containing hematopoietic stem and progenitor cells was sorted from dissociated mouse bone marrow cells by flow cytometry. Sorted single-cell suspension, 10x barcoded gel beads and oil (10xGenomics Chromium Next GEM Single Cell 3' Kit v3.1 PN 1000268) were loaded into 10x Chromium™ Single Cell Chip to capture single cells into nanoliter-scale oil droplets by 10xChromium X Controller and generate Gel Bead-In-EMulsions (GEMs). cDNA from single cells were synthesized and barcoded by incubation of the GEMs in a thermocycler machine and then purified from GEMs by DynaBeads (10xGenomics PN 2000048). cDNA was pre-amplified by PCR to generate sufficient mass for sequencing library construction. The single-cell cDNA libraries were constructed by following 10xGenomics 3' NextGem 3.1 version kit instructions. The final constructed 3'-biased single-cell libraries were sequenced by the Illumina Novaseq6000 machine, targeting 50,000 read pairs/cell. We identified hematopoietic abnormalities in Son+/- mice, including leukocytopenia and immunoglobulin deficiency, similar to those observed in human patients. Surface marker analyses and single-cell transcriptome profiling of hematopoietic stem and progenitor cells revealed that Son haploinsufficiency inclines cell fate toward the myeloid lineage but compromises lymphoid lineage development by reducing genes required for lymphoid and B-cell lineage specification. Additionally, Son haploinsufficiency causes inappropriate activation of erythroid genes and impaired erythroid maturation. These findings highlight the importance of the full gene dosage of Son in multiple organs. Our model serves as an invaluable research tool for this rare disease and related disorders associated with SON dysfunction. Overall design: The linage-marker-negative (Lin-) c-kit+ population containing hematopoietic stem and progenitor cells was sorted from dissociated mouse bone marrow cells by flow cytometry. Sorted single cell suspension, 10x barcoded gel beads, and oil (all were included in 10xGenomics Chromium Next GEM Single Cell 3' Kit v3.1 PN 1000268) were loaded into 10x Chromium™ Single Cell Chip to capture single cells into nanoliter-scale oil droplets by 10xChromium X Controller and generate Gel Bead-In-EMulsions (GEMs). cDNA from single cells were synthesized and barcoded by incubation of the GEMs in a thermocycler machine and then purified from GEMs by DynaBeads (10xGenomics PN 2000048). cDNA was pre-amplified by PCR to generate sufficient mass for sequencing library construction. The single cell cDNA libraries were constructed by following 10xGenomics 3' NextGem 3.1 version kit instruction, briefly by following steps of cDNA fragmentation, end repair & A-tailing, and size selection by SPRIselect beads (Beckman Coulter, PN B23318), adaptor ligation, and sample index PCR amplification, and then SPRIselect beads size selection again. The final constructed 3'-biased single cell libraries were sequenced by Illumina Novaseq6000 machine, targeting 50,000 read pairs/cell. The sequencing cycles consisted of 28bp for read 1, 90bp for read 2, and 10bp for i7 and i5.