Markus Grompe

Cell fusion is the principal source of bone-marrow-derived hepatocytes.

Evidence suggests that haematopoietic stem cells might have unexpected developmental plasticity, highlighting therapeutic potential. For example, bone-marrow-derived hepatocytes can repopulate the liver of mice with fumarylacetoacetate hydrolase deficiency and correct their liver disease. To determine the underlying mechanism in this murine model, we performed serial transplantation of bone-marrow-derived hepatocytes. Here we show by Southern blot analysis that the repopulating hepatocytes in the liver were heterozygous for alleles unique to the donor marrow, in contrast to the original homozygous donor cells. Furthermore, cytogenetic analysis of hepatocytes transplanted from female donor mice into male recipients demonstrated 80,XXXY (diploid to diploid fusion) and 120,XXXXYY (diploid to tetraploid fusion) karyotypes, indicative of fusion between donor and host cells. We conclude that hepatocytes derived form bone marrow arise from cell fusion and not by differentiation of haematopoietic stem cells.

Myelomonocytic cells are sufficient for therapeutic cell fusion in liver

Liver repopulation with bone marrow–derived hepatocytes (BMHs) can cure the genetic liver disease fumarylacetoacetate hydrolase (Fah) deficiency. BMHs emerge from fusion between donor bone marrow–derived cells and host hepatocytes. To use such in vivo cell fusion efficiently for therapy requires knowing the nature of the hematopoietic cells that fuse with hepatocytes. Here we show that the transplantation into Fah−/− mice of hematopoietic stem cells (HSCs) from lymphocyte-deficient Rag1−/− mice, lineage-committed granulocyte-macrophage progenitors (GMPs) or bone marrow–derived macrophages (BMMs) results in the robust production of BMHs. These results provide direct evidence that committed myelomonocytic cells such as macrophages can produce functional epithelial cells by in vivo fusion. Because stable bone marrow engraftment or HSCs are not required for this process, macrophages or their highly proliferative progenitors provide potential for targeted and well-tolerated cell therapy aimed at organ regeneration.

The ploidy-conveyor of mature hepatocytes as a source of genetic variation

Mononucleated and binucleated polyploid hepatocytes (4n, 8n, 16n and higher) are found in all mammalian species, but the functional significance of this conserved phenomenon remains unknown. Polyploidization occurs through failed cytokinesis, begins at weaning in rodents and increases with age. Previously, we demonstrated that the opposite event, ploidy reversal, also occurs in polyploid hepatocytes generated by artificial cell fusion. This raised the possibility that somatic ‘reductive mitoses’ can also happen in normal hepatocytes. Here we show that multipolar mitotic spindles form frequently in mouse polyploid hepatocytes and can result in one-step ploidy reversal to generate offspring with halved chromosome content. Proliferating hepatocytes produce a highly diverse population of daughter cells with multiple numerical chromosome imbalances as well as uniparental origins. Our findings support a dynamic model of hepatocyte polyploidization, ploidy reversal and aneuploidy, a phenomenon that we term the ‘ploidy conveyor’. We propose that this mechanism evolved to generate genetic diversity and permits adaptation of hepatocytes to xenobiotic or nutritional injury.

Repair Kinetics of Genomic Interstrand DNA Cross-Links: Evidence for DNA Double-Strand Break-Dependent Activation of the Fanconi Anemia/BRCA Pathway

ABSTRACT The detailed mechanisms of DNA interstrand cross-link (ICL) repair and the involvement of the Fanconi anemia (FA)/BRCA pathway in this process are not known. Present models suggest that recognition and repair of ICL in human cells occur primarily during the S phase. Here we provide evidence for a refined model in which ICLs are recognized and are rapidly incised by ERCC1/XPF independent of DNA replication. However, the incised ICLs are then processed further and DNA double-strand breaks (DSB) form exclusively in the S phase. FA cells are fully proficient in the sensing and incision of ICL as well as in the subsequent formation of DSB, suggesting a role of the FA/BRCA pathway downstream in ICL repair. In fact, activation of FANCD2 occurs slowly after ICL treatment and correlates with the appearance of DSB in the S phase. In contrast, activation is rapid after ionizing radiation, indicating that the FA/BRCA pathway is specifically activated upon DSB formation. Furthermore, the formation of FANCD2 foci is restricted to a subpopulation of cells, which can be labeled by bromodeoxyuridine incorporation. We therefore conclude that the FA/BRCA pathway, while being dispensable for the early events in ICL repair, is activated in S-phase cells after DSB have formed.

Prospective isolation of a bipotential clonogenic liver progenitor cell in adult mice

The molecular identification of adult hepatic stem/progenitor cells has been hampered by the lack of truly specific markers. To isolate putative adult liver progenitor cells, we used cell surface-marking antibodies, including MIC1-1C3, to isolate subpopulations of liver cells from normal adult mice or those undergoing an oval cell response and tested their capacity to form bilineage colonies in vitro. Robust clonogenic activity was found to be restricted to a subset of biliary duct cells antigenically defined as CD45(-)/CD11b(-)/CD31(-)/MIC1-1C3(+)/CD133(+)/CD26(-), at a frequency of one of 34 or one of 25 in normal or oval cell injury livers, respectively. Gene expression analyses revealed that Sox9 was expressed exclusively in this subpopulation of normal liver cells and was highly enriched relative to other cell fractions in injured livers. In vivo lineage tracing using Sox9creER(T2)-R26R(YFP) mice revealed that the cells that proliferate during progenitor-driven liver regeneration are progeny of Sox9-expressing precursors. A comprehensive array-based comparison of gene expression in progenitor-enriched and progenitor-depleted cells from both normal and DDC (3,5-diethoxycarbonyl-1,4-dihydrocollidine or diethyl1,4-dihydro-2,4,6-trimethyl-3,5-pyridinedicarboxylate)-treated livers revealed new potential regulators of liver progenitors.

Liver Repopulation and Correction of Metabolic Liver Disease by Transplanted Adult Mouse Pancreatic Cells

The emergence of cells with hepatocellular properties in the adult pancreas has been described in several experimental models. To determine whether adult pancreas contains cells that can give rise to therapeutically useful and biochemically normal hepatocytes, we transplanted suspensions of wild-type mouse pancreatic cells into syngeneic recipients deficient in fumarylacetoacetate hydrolase and manifesting tyrosinemia. Four of 34 (12%) mutant mice analyzed were fully rescued by donor-derived cells and had normal liver function. Ten additional mice (29%) showed histological evidence of donor-derived hepatocytes in the liver. Previous work has suggested that pancreatic liver precursors reside within or close to pancreatic ducts. We therefore performed additional transplantations using either primary cell suspensions enriched for ducts or cultured ducts. Forty-four mutant mice were transplanted with cells enriched for pancreatic duct cells, but only three of the 34 (9%) recipients analyzed displayed donor-derived hepatocytes. In addition, 28 of the fumarylacetoacetate hydrolase-deficient mice were transplanted with cultured pancreatic duct cells, but no donor-derived hepatocytes were observed. Our results demonstrate for the first time that adult mouse pancreas contains hepatocyte progenitor cells capable of significant therapeutic liver reconstitution. However, contrary to previous reports, we were unable to detect these cells within the duct compartment.

Adeno‐associated virus gene repair corrects a mouse model of hereditary tyrosinemia in vivo

Adeno‐associated virus (AAV) vectors are ideal for performing gene repair due to their ability to target multiple different genomic loci, low immunogenicity, capability to achieve targeted and stable expression through integration, and low mutagenic and oncogenic potential. However, many handicaps to gene repair therapy remain. Most notable is the low frequency of correction in vivo. To date, this frequency is too low to be of therapeutic value for any disease. To address this, a point‐mutation–based mouse model of the metabolic disease hereditary tyrosinemia type I was used to test whether targeted AAV integration by homologous recombination could achieve high‐level stable gene repair in vivo. Both neonatal and adult mice were treated with AAV serotypes 2 and 8 carrying a wild‐type genomic sequence for repairing the mutated Fah (fumarylacetoacetate hydrolase) gene. Hepatic gene repair was quantified by immunohistochemistry and supported with reverse transcription polymerase chain reaction and serology for functional correction parameters. Successful gene repair was observed with both serotypes but was more efficient with AAV8. Correction frequencies of up to 10−3 were achieved and highly reproducible within typical dose ranges. In this model, repaired hepatocytes have a selective growth advantage and are thus able to proliferate to efficiently repopulate mutant livers and cure the underlying metabolic disease. Conclusion: AAV‐mediated gene repair is feasible in vivo and can functionally correct an appropriate selection‐based metabolic liver disease in both adults and neonates. (HEPATOLOGY 2010.)

Fancd2−/− mice have hematopoietic defects that can be partially corrected by resveratrol

Progressive bone marrow failure is a major cause of morbidity and mortality in human Fanconi Anemia patients. In an effort to develop a Fanconi Anemia murine model to study bone marrow failure, we found that Fancd2(-/-) mice have readily measurable hematopoietic defects. Fancd2 deficiency was associated with a significant decline in the size of the c-Kit(+)Sca-1(+)Lineage(-) (KSL) pool and reduced stem cell repopulation and spleen colony-forming capacity. Fancd2(-/-) KSL cells showed an abnormal cell cycle status and loss of quiescence. In addition, the supportive function of the marrow microenvironment was compromised in Fancd2(-/-) mice. Treatment with Sirt1-mimetic and the antioxidant drug, resveratrol, maintained Fancd2(-/-) KSL cells in quiescence, improved the marrow microenvironment, partially corrected the abnormal cell cycle status, and significantly improved the spleen colony-forming capacity of Fancd2(-/-) bone marrow cells. We conclude that Fancd2(-/-) mice have readily quantifiable hematopoietic defects, and that this model is well suited for pharmacologic screening studies.

Isolation of major pancreatic cell types and long-term culture-initiating cells using novel human surface markers

We have developed a novel panel of cell-surface markers for the isolation and study of all major cell types of the human pancreas. Hybridomas were selected after subtractive immunization of Balb/C mice with intact or dissociated human islets and assessed for cell-type specificity and cell-surface reactivity by immunohistochemistry and flow cytometry. Antibodies were identified by specific binding of surface antigens on islet (panendocrine or alpha-specific) and nonislet pancreatic cell subsets (exocrine and duct). These antibodies were used individually or in combination to isolate populations of alpha, beta, exocrine, or duct cells from primary human pancreas by FACS and to characterize the detailed cell composition of human islet preparations. They were also employed to show that human islet expansion cultures originated from nonendocrine cells and that insulin expression levels could be increased to up to 1% of normal islet cells by subpopulation sorting and overexpression of the transcription factors Pdx-1 and ngn3, an improvement over previous results with this culture system. These methods permit the analysis and isolation of functionally distinct pancreatic cell populations with potential for cell therapy.

Tempol Protects against Oxidative Damage and Delays Epithelial Tumor Onset in Fanconi Anemia Mice

Fanconi anemia (FA) is a genetic disorder characterized by congenital abnormalities, bone marrow failure, and marked cancer susceptibility. FA patients have an elevated risk of developing hematologic malignancies and solid tumors. Using Fancd2(-/-) knockout mice as a model of FA, we examined the potential of tempol, a nitroxide antioxidant and a superoxide dismutase mimetic, as a tumor-delaying agent for solid tumors. Dietary tempol increased the mean tumor-free survival time of Fancd2(-/-) Trp53(+/-) mice by 27% (P < 0.01), from 308 to 390 days, without changing the overall tumor spectrum. More strikingly, tempol delayed the onset of epithelial tumors and increased the mean epithelial tumor-free survival time by 38% (P < 0.0001), from 312 to 432 days, in Fancd2(-/-) Trp53(+/-) mice. These results show that tempol can significantly delay tumor formation in Fancd2(-/-) Trp53(+/-) mice. Furthermore, tempol treatment did not adversely affect the repopulating ability of FA hematopoietic stem cells. The reduction in oxidative DNA damage in tempol-treated FA fibroblasts and mice suggests that its tumor-delaying function may be attributed to its antioxidant activity.