Pathobiology, classification, and diagnosis of myelodysplastic syndrome.

Recent advances in molecular genetics have advanced the knowledge regarding the mechanisms leading to myelodysplastic syndrome (MDS), secondary acute myeloid leukemia, and therapy-induced MDS. Unfavorable cytogenetics associated with this group of disorders includes monosomy or deletion of the long arm of chromosomes 5 or 7, inversions of chromosome 3, translocations, deletions, and trisomies involving several other chromosomes. These unbalanced chromosomal aberrations result in hemizygosity and unmasking of oncogenes, changes in levels of expressed genes, or inactivation of tumor suppressor genes. It is evident that the cytogenetics associated with MDS is highly complex and heterogeneous, leading to an equally heterogeneous manifestation of the disease. Classifications, initially defined by the French-British-American group followed by the World Health Organization, and now by the International Prognostic Scoring System, have determined prognosis and helped develop treatment strategies for these patients, thus reducing their potential to develop acute leukemia. To date there are seven different prognostic schemas. These are constantly being improved so that MDS patients, who tend to be elderly, can be suitably treated. Additionally, treatment considerations and prognosis are different for patients who develop therapy-related MDS or for the juvenile population than for those with de novo MDS. The genetic alterations in MDS bone marrow and blood cells have been identified and possible models have been proposed for the development and progress of MDS, from the early stage to late-stage MDS evolving to acute myeloid leukaemia. As the functional mechanisms behind these chromosomal changes are being revealed, new therapies based on these mechanisms are currently being made and tested.