Which of the following is DNA repair defect-
Ans. is 'a' i.e., Xeroderma Pigmentosum o All are familial cancer syndromes.o Xeroderma-pigmentosa is autosomal recessive, defective DNA repair syndrome,o Other three are autosomal dominant inherited cancer syndrome.o Whatever the etiology, cancer arises through a series of somatic alterations in DNA that result in uncontrolled cell division. Human cancers have following important etiological factors -Genetic predisposition to cancerNon-heriditary predisposing conditionsG Geographic and environmental factorsGenetic (Hereditary or inherited) predisposition to cancero A large number of cancers have hereditary predispositions,o Genetic predisposition may be of three types.1. Autosomal dominant Inherited cancer syndromeso This is the most common type of genetic predisposition.o The mechanism involves uncontrolled cell division due to germline mutation of cancer suppressor gene.GateInherited predispositionRBRetinoblastomap53Li-fraumeni syndromepl6INK4AMelanomaAPCFamilial adenomatous polyposis/colon cancerNF-LNF-2Neurofibromatosis 1 and 2BRAG-1, BRAC-2Breast cancerMEN 1, RETMultiple endocrine neoplasiaMSN 2, MLH 1, MSB 6Heriditary nonpolyposis colon cancerPATCHNevoid basal cell carcinoma syndrome2. Defective DNA repair syndromeo Beside ihe dominantly inherited precancerous conditions, a group of cancer predisposing conditions is collectively characterized by defects in DNA repair,o Normally, if the DNA damage is present it is repaired at cell-cycle checkpoints.o If DNA repair mechanism is defective, cells replicate with defective DNA and mutations or chromosomal breaks are transfered in the progeny of cells that can lead to uncontrolled replication.o Most of these conditions are inherited as autosomal recessive, e.g -1. Xeroderma pigmentosa3. Fanconisyndrome2. Bloom syn drome4. Ataxia telangectasiao One condition in this group is autosomal dominant - heriditory nonpotypid colon cancer (HNPCC).3. Familial cancerso Besides the inherited syndromes of cancer susceptibility', some cancers occur at higher frequency in certain families without a clearly defined pattern of transmission,o That means, there is familial clustering of cases, but role of inherited predisposition is not clear for each individual,o Example - Breast, ovarian, and pancreatic cancers.
Xeroderma-pigmentosum
Retinoblastoma
Neurofibromatosis
MEN-I
Cytogenetic Disorders