Myelodysplastic syndrome (MDS) is an acquired bone marrow disorder characterized by dysplastic growth of hematopoietic progenitors, peripheral cytopenia, and progression to acute myelogenous leukemia (AML). It can be viewed as a transition state between normal hematopoiesis and AML, and is loosely analogous to intermediate states in other models of tumor progression (e.g., adenomas in pathogenesis of colon cancer). The molecular basis for MDS is unknown. The importance of understanding pathogenesis of this disorder is heightened by lack of effective therapy apart from bone marrow transplantation. The specific aims of this proposal are to (i) determine clonal involvement of hematopoietic lineages in MDS. Identification of somatic mutation which causes MDS requires analysis of clonally derived cells. Since cells are limiting in pancytopenic MDS patients, polymerase chain reaction (PCR) will be used to determine clonality. The method will be applied to clonal analysis of MDS, and other acquired bone marrow disorders where cell number has limited clonality analysis (e.g., aplastic anemia). (ii) determine whether homozygous deletion is responsible for pathogenesis of 5q- syndrome. 5q- is the most common chromosomal abnormality associated with MDS. Homozygous deletion of a gene on 5q has been proposed as a mechanism for disease pathogenesis in these patients. Clonally derived cells from patients with 5q- deletion will be analyzed for evidence of homozygous deletion. Analysis will include Southern blots using probes for known genes and for anonymous loci on 5q, analysis of expression of candidate 5q genes using competitive PCR, and amplification and direct sequence analysis of selected regions of candidate genes on 5q such as EGR-1. (iii) identify the genetic locus in a large kindred responsible for autosomal dominant inheritance of a functional platelet defect and strong propensity to develop AML. Affected family members in this kindred have a dysfunctional platelet syndrome, detectable at birth, that has similarities to that seen in MDS. Further, affected family members have a strong tendency to develop AML. In one patient for whom karyotype data is available, a 5q- and 11q- deletion was associated with progression to AML. The family thus provides an unusual opportunity to identify a genetic locus which causes a preleukemic dysfunctional platelet syndrome, and is associated with chromosomal abnormalities frequently seen in MDS. The locus will be identified by linkage analysis using cosmid probes for polymorphic loci on 5q and 11q, and will be expanded to other chromosomes if these regions are not informative.