医学遗传学 16肿瘤遗传学eng v.ppt
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1、16 遗传与肿瘤发生 Cancer Genetics,The ancient Greeks believed that cancer was caused by too much body fluid they called black bile.,Doctors in the seventeenth and eighteenth centuries suggested that parasites caused cancer.Today,doctors understand more about the link between cancer and genetics.,Viruses,ul
2、traviolet(UV)radiation,and chemicals can all damage genes in the human body.If particular genes are affected,a person can develop cancer.Understanding how genes cause cancer,though,first requires a basic understanding of several genetic terms and concepts.,1.General,Cancer is a very common disease,a
3、ffecting about 1 in 3 individuals,and about half the people that contract cancer will die as a direct result of their disease.,For the most part,cancer arises from a single cell,that is,cancer is a clonal disease.The average human being contains about 1014 cells(i.e.,100,000,000,000,000 cells),any o
4、ne of which could,in principle,become a cancer cell,if it acquired the right sort of mutations while it still had the potential to proliferate.,Therefore,the cancer cell arises and progresses once out of a possible 1014 cellular targets.That only happens in 1 in 3 people.Even then it usually takes 6
5、0 or 70 years to occur.,Tumors are hereditary Hereditary retinoblastoma is an autosomal dominant trait in which susceptibility to retinoblastoma is inherited.This is an unusual dominant trait in that a mutation in one RB gene is not sufficient to cause symptoms,but mutations in the second allele oft
6、en arise during development.,Offspring have a 50%chance of receiving the mutant gene from a heterozygous parent,and 90%of carriers will develop retinoblastoma,usually in both eyes.The hereditary form is also associated with a high risk for other cancers especially of the bone and fibrous tissues(ost
7、eosarcomas and fibrosarcoma.,Sporadic retinoblastoma is a trait in which the affected individual has not inherited any mutant alleles of the retinoblastoma gene.,The mutations occur after birth and result in tumor formation.Tumors usually develop in only one eye and patients are not at high risk for
8、 other cancers.Both alleles need to be mutated in a single cell,and that is why this form typically occurs only in one eye.,Chromosome and tumors Detailed studies of the Philadelphia chromosome show that most of chromosome 22 has been translocated onto the long arm of chromosome 9.In addition,the sm
9、all distal portion of the short arm of chromosome 9 is translocated to chromosome 22.This translocation,which is found only in tumor cells,indicates that a patient has chronic myelogenous leukemia(CML).In CML,the cells that produce blood cells for the body(the hematopoietic cells)grow uncontrollably
10、,leading to cancer.,The connection between this chromosomal abnormality and CML was clarified by studying the genes located on the chromosomes at the sites of the translocation breakpoints.,In one of the translocated chromosomes,part of a gene called abl is moved from its normal location on chromoso
11、me 9 to a new location on chromosome 22.This breakage and reattachment leads to an altered abl gene.The protein produced from the mutant abl gene functions improperly,leading to CML.,2.oncogene,Oncogenes are mutated forms of genes that cause normal cells to grow out of control and become cancer cell
12、s.They are mutations of certain normal genes of the cell called proto-oncogenes.,Proto-oncogenes are the genes that normally control how often a cell divides and the degree to which it differentiates(or specializes).When a proto-oncogene mutates(changes)into an oncogene,it becomes permanently turned
13、 on or activated when it is not supposed to be.When this occurs,the cell divides too quickly,which can lead to cancer.,It may be helpful to think of a cell as a car.For it to work properly,there need to be ways to control how fast it goes.A proto-oncogene normally functions in a way that is similar
14、to a gas pedal-it helps the cell grow and divide.An oncogene could be compared to a gas pedal that is stuck down,which causes the cell to divide out of control.,The pathway for normal cell growth starts with growth factor,which locks onto a growth factor receptor.The signal from the receptor is sent
15、 through a signal transducer.A transcription factor is produced,which causes the cell to begin dividing.If any abnormality is detected,the cell is made to commit suicide by a programmed cell death regulator.,More than 100 oncogenes are now recognized,and undoubtedly more will be discovered in the fu
16、ture.Scientists have divided oncogenes into the 5 different classes.,Growth factors These oncogenes produce factors that stimulate cells to grow.The best known of these is called sis.It leads to the overproduction of a protein called platelet-derived growth factor,which stimulates cells to grow.,Gro
17、wth factor receptors These are normally turned on or off by growth factors.When they are on,they stimulate the cell to grow.Certain mutations in the genes that produce these cause them to always be on.In other cases,the genes are amplified.,This means that instead of the usual 2 copies of the gene,t
18、here may be several extras,resulting in too many growth factor receptor molecules.As a result,the cells become overly sensitive to growth-promoting signals.,The best known examples of growth factor receptor gene amplification are erb B and erb B-2.These are sometimes known as epidermal growth factor
19、 receptor and HER2/neu.HER2/neu gene amplification is an important abnormality seen in about one third of breast cancers.Both of these oncogenes are targets of newly developed anti-cancer treatments.,Signal transducers These are the intermediate pathways between the growth factor receptor and the ce
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