Sequencing of Human Chromosome

Researchers at The Children's Hospital of Philadelphia Play Major Role in Genetic Milestone
First Full Sequencing of a Human Chromosome Is Announced

Philadelphia, Pa. -- Researchers at The Children's Hospital of Philadelphia, where hundreds of children are treated each year for genetic diseases, played a major role in today's landmark announcement of the first sequencing of a complete human chromosome. Building on their previous work in producing a general map of chromosome 22, The Children's Hospital of Philadelphia research team processed large numbers of DNA segments from that chromosome in preparation for the sequencing. The Hospital is considered a world leader in caring for children with a genetic disorder that results from a defect of chromosome 22.

A significant event in the emerging era of genetic-based medicine, the research study, published today in the journal Nature, announces that scientists have determined the order of 33 million DNA bases, the chemical components of the genetic code, for nearly 700 genes on chromosome 22. Chromosome 22 is the first human chromosome to be sequenced under the federally sponsored Human Genome Project, which expects to complete a working draft of the sequences of all 23 human chromosomes by March of next year.

The multicenter study is co-authored by over 200 collaborators from research institutions in 6 nations. Among those collaborators is the research group from Children's Hospital, led by Beverly S. Emanuel, Ph.D. In 1995, Dr. Emanuel's laboratory completed an important earlier step in the process, publishing a physical map of chromosome 22. That map, one outcome of a five-year, $10 million federal grant, established the locations of genes and other DNA markers along the chromosome. It laid a foundation for today's far more detailed findings: the specific order of the nucleotides, the four chemical units that make up the DNA alphabet and which, in different combinations, form the chromosome's genetic code. In recognition of earlier pioneering genetics research on chromosome 22 by Dr. Emanuel and her colleagues, the National Institutes of Health designated The Children's Hospital of Philadelphia, in collaboration with the University of Pennsylvania School of Medicine, as a Human Genome Cen

"Chromosome 22 is a hot spot for disease," said Dr. Emanuel, the chief of Human Genetics and Molecular Biology at The Children's Hospital of Philadelphia. Defects in genes on that chromosome are implicated in certain leukemias and other cancers, mental retardation, schizophrenia and numerous other diseases. One set of disorders, called the chromosome 22q11 deletion syndrome, includes heart defects, cleft palate, feeding problems, an abnormal or absent thymus gland, and learning disabilities. In 1992, Dr. Emanuel's laboratory was the first to develop a blood test to detect the syndrome, which is caused by a deletion or loss of genetic material on the chromosome, and which occurs in as many as 1 in 4,000 births. (It is also called velocardiofacial syndrome and DiGeorge syndrome.) Children's Hospital cares for more than 300 children with chromosome 22 deletion syndrome and will host an international conference on the syndrome in June 2000.

Portions of the DNA sequences on chromosome 22 have previously been made available to the scientific community as the information was collected, so that laboratories such as Dr. Emanuel's could use the results in their own genetic studies. The chromosome sequence announced today is "operationally complete," meaning that it provides researchers the letter-by-letter code of the gene-rich, functional portion of the chromosome (other, unsequenced portions of the chromosome contain many highly repetitive sequences, and encode genes involved in the chromosome's housekeeping tasks). Although the sequenced portion contains at least 679 genes, there may be 200 to 300 additional genes on the functional portion of chromosome 22 that have not yet been identified. Moreover, the actual functions of more than half the identified genes are yet to be discovered. "The sequencing is a first step," said Dr. Emanuel. "As with genes on all the other chromosomes, a great deal of further work needs to be done

The Children's Hospital of Philadelphia, the nation's first children's hospital, is a world-renowned leader in patient care, education and research. This 373-bed multispecialty hospital provides comprehensive pediatric services, including home care, to children from before birth through age 19. The hospital has more than 17,000 admissions, and provides care in more than 600,000 outpatient visits annually.

###