Mitochondria Damage Leading to High Frequency of Lymphoma and Myeloproliferative Disorder in Transgenic Mice Overexpressing a Human 8-Oxoguanine DNA-Glycosylase Gene.

Alterations of nuclear genes in human lymphoma and leukemias have been well investigated in past several decades and established a predominant role in the pathogenesis. However, the relationship of mitochondrial genome alteration or dysfunction and human lymphoma and leukemias remains large unknown. Mitochondria are dynamic organelles that play critical roles in oxidative phosphorylation, energy metabolism, cell growth and apoptosis. We have successfully generated a novel transgenic (TG) mouse model of mitochondrial disorder by overexpressing human hOGG1, a base excision DNA repair gene, in the mitochondria of a wide variety of tissues in mice. We reported here the high frequency of essential thrombocythemia (ET)-like myeloproliferative disorder and lymphoma in the TG mice. TG mice were generated via pronuclear microinjection of a mammalian expression construct of a mitochondrial isoform of hOGG under the control of a mouse metallothionein-1 promoter. Peripheral blood smears were prepared from TG and non-TG control mice for platelet counts and morphologic evaluation. TG mice were sacrificed and various organs were harvested for histologic, biochemical and molecular studies. Two of the male founder mice successfully transmitted the transgene to their offspring at an expected frequency of 50%. All the female mice failed to reproduction. All TG mice expressed high levels of hOGG mRNA in their liver by RT-PCR and direct DNA sequencing. Over-expression of this gene produced a wide range of adverse biological phenotype, manifesting early-onset obesity, metabolic disturbance, female infertility and high frequency of ET-like myeloproliferative disorder and lymphoma (>90%) in nodal and extranodal sites. Eight TG mice (ranging from 1 to 2 years) become moribund and subsequently sacrificed. Upon histologic examination, the TG mice displayed moderate to severe splenomegaly. The spleen weights were 2-, 4- and 10 times respectively in the 3 TG founder mice as compared to wild type, age-matched mice. Extensive abdominal lymphadenopathy and numerous enlarged nodules involving liver, spleen, peritoneum, lung and diaphragm were identified. Microscopically, the red pulp in the enlarged spleens is markedly expanded with aggregates of large, mature but dysplastic-appearing megakaryocytes, focally disrupting the normal splenic structure. Various lymphomas ranged from low-grade lymphoma to high-grade Burkitt-like or lymphoblastic lymphomas were seen. Peripheral blood from these aged TG mice showed marked thrombocytosis and platelet clumping. In bone marrow, the aged TG mice displayed marked myeloid and megakaryocytic hyperplasia with relative erythroid hypoplasia in the absence of marrow fibrosis, indicative of myeloid and megakaryocytic proliferation. Bone marrow involvement by B-cell lymphoma was also seen. Of note, none of the above phenotype was seen in younger TG mice (4 months), indicating that the full development of this pathologic process is age-dependent. The molecular basis for this process is currently under investigation. Our preliminary data showed that mitochondrial DNA deletion or decrease in DNA copy numbers due to overexpressed hOGG1 and imbalance of base excision repair resulted in defects in mitochondrial respiration and increased ROS production. We thus hypothesize that oxidative stress caused by mitochondrial malfunction may play important part in the development of hematopoietic malignancies.