Leonard Shultz

Education

• PhD, University of Massachusetts.1972

Biosketch

Spontaneous mutations that cause defects in immunological function in mice have provided important research tools with which to investigate the development and regulation of the immune system. A number of these mutations serve as models for specific human diseases and provide critical tools to increase the understanding of human immunodeficiency diseases, autoimmunity and leukemia. Spontaneous as well as targeted immunological mutations also provide mouse models that support engraftment with human stem cells and with human peripheral blood lymphocytes. These “humanized” mice have provided critical models for experimental investigation of human immune diseases without putting individuals at risk. Our recent research has leveraged these mouse models for studies on diabetes, bone development, autoimmunity, malignancy, anemia, thrombocytopenia, infectious diseases, transplantation tolerance and asthma.

Research Interests

Development of Humanized SCID Mice and Studies of Single Gene Models for Human Hematological Diseases
Advances in our understanding of the development and regulation of the immune system in normal and pathologic states rely heavily on studies of mice with genetically determined immunodeficiency and autoimmunity. Research in our laboratory encompasses (1) basic studies focused on elucidating the mechanisms underlying immunodeficiency, autoimmunity, and hematological diseases; and (2) applied studies focused on the development of effective immunodeficient mouse models that support heightened engraftment with human hematopoietic stem cells. These “humanized SCID mice” will facilitate studies of human immunity, regenerative medicine, and neoplasia. Examples of the studies carried out over the last year are summarized below:

Development of Humanized Mice for Translational Biomedical Research
Complex biological processes often require in vivo analysis, and important research advances have been obtained using mice as a model system for the study of many biological systems. However, mice are not humans, and the study of human biology in vivo is severely limited by ethical and technical constraints. There is a growing need for animal models to carry out in vivo studies on human cells, tissues, and organs without putting individuals at risk. Humanized mice, or mouse–human chimeras, have been developed to overcome these limitations and have become an important research tool for the in vivo study of human cells and tissues. We have developed a NOD-Prkdc^scid (NOD/SCID) mouse strain harboring a null mutation of the common cytokine receptor γ chain (Il2rgtm1Wjl) (NOD/SCID/IL2rγ^null) (full name NOD.Cg-Prkdc^scid Il2rg^tm1Wjl/SzJ). In collaboration with Dale Greiner and his colleagues at The University of Massachusetts Medical School along with Fumihiko Ishikawa and his colleagues at The Riken Research Center for Allergy and Immunology, we have shown that these mice can efficiently support development of all myelo-erythroid components and functional lymphoid cells following human hematopoietic stem cell (HSC) engraftment. Development of mice that are ‘humanized’ by engraftment of human tissues, HSCs, and peripheral-blood mononuclear cells (PBMCs) provides an opportunity to study human biological processes in vivo that would otherwise not be possible. The new generations of humanized mice are proving to be powerful tools in preclinical testing and in the investigation of many human biological processes. The culmination of decades of research on humanized mice is leading to advances in our understanding of human hematopoiesis, innate and adaptive immunity, autoimmunity, infectious diseases, cancer biology, and regenerative medicine. The potential for new advances in our understanding of human biological systems provided by studies in humanized mice remains promising. The development of humanized mice based on immunodeficient IL2rγ^nullhosts has overcome many of the limitations and constraints of previously available models.

The Mouse Mutation “Thrombocytopenia and Cardiomyopathy” (trac) is within the Abcg5 Gene: A Single Gene Model for Human Mediterranean Macrothrombocytopenia
Homozygosity for a new spontaneous mouse mutation named “thrombocytopenia and cardiomyopathy” (trac) results in thrombocytopenia, dilated cardiomyopathy, and infertility. A/J-trac/trac mice show a precipitous drop in platelet numbers and increases in platelet volume by 4 weeks of age. By 2-3 months of age, trac/trac mice have a 20-fold decrease in platelet number, a 3-fold increase in platelet volume, and a greatly increased bleeding time. Blood smears showed abnormally large platelets and megakaryocytoid cells. The trac/trac mice also developed mild microcytic anemia accompanied by the presence of stomatocytes, a doubling of reticulocyte numbers, and a two-fold decrease in WBC counts. Increased numbers of megakaryocytes were present in bone marrow, spleen, and lungs. Ultrastructural studies of trac/trac megakaryocytes showed a poorly developed demarcation system and a failure to form platelet territories. Thetrac/trac platelets were enlarged, spherical, and contained numerous small alpha granules. The thrombocytopenia was not associated with defects intrinsic to bone marrow progenitor cells. Although thrombopoietin (TPO) levels were decreased, TPO treatment failed to reverse the thrombocytopenia. To identify the responsible gene, we produced a fine-structure genetic map of a 5-megabase interval containing the trac locus on mouse chromosome 17. The human syntenic region is Chr 2p21-p22. Analyses of 1100 F2 progeny from intercross matings of (A/J x C57BL/6J) F1 +/trac mice narrowed the interval to 0.3 Mb containing 4 genes. Sequencing of these genes revealed a G-to-A mutation at base 1435 (refseq nm031884) of Abcg5 (ATP-binding cassette sub-family G, member 5). This G>A base change results in a tryptophan codon (UGG) at amino acid position 463(uniprot) being changed to a premature stop codon (UAG). The transmembrane helices prediction program, TMHMM, predicts that the premature stop codon would truncate the last four of the six transmembrane domains of the ABCG5 protein. No DNA alterations were found in any of the other candidate genes. Genetic crosses of +/trac mice with mice doubly transgenic for the closely linked human ABCG5 and ABCG8 genes (stock B6SJL-Tg(ABCG5/ABCG8)14-2Hobb/J) showed that the transgenes normalized platelet counts and volumes in trac/trac mice. ABCG5 (sterolin-1) functions as part of a heterodimer, with ABCG8, that regulates plant sterol uptake. Thetrac/trac mutant mice have greatly elevated plasma levels of plant sterols. When placed on a phytosterol-free diet, the thrombocytopenia was reversed. Recent studies have shown that Mediterranean Macrothrombocytopenia is caused by mutations in ABCG5 or ABCG8. Identification of the molecular basis of the mouse Abcg5trac mutation provides a new model for studying the role of phytosterols in pathogenic changes in the hematopoietic, cardiovascular, and reproductive systems.

Selected Publications

Service

Institutional Committees (The Jackson Laboratory)

Genetic Resources Committee
Human Subjects IRB, Chair
RAF Advisory Committee
Research Grants Committee