부산대병원학술지통권제30 호, 2011 폐암의분자생물학적특징과임상적의의 부산대학교의학전문대학원내과학교실 Molecular biologic characteristics of lung cancer and its clinical implication Min Ki Lee Department of Internal Medicine, School of Medicine, Pusan National University Abstract * Lung cancer is a heterogeneous disease clinically, biologically, histologically and molecualry. Understanding the molecular cause of this heterogenitety is the focus of current lung cancer research. Identifying the genes and pathway involved, determining how they relate to the biological behavior of lung cancer, and their utility as diagnostic and therapeutic targets are important basic and translational research issues. This article reviews the important molecular steps in lung cancer pathogenesis and clinical implications. 1) Molecular etiology of lung cancer 2) Lung cancer in never smokers 3) Oncogenes and growth stimulatory pathways and targeted theraputics * 본연구는 2010년도부산대학교병원임상연구비지원으로이루어졌음 -63-
4) Tumor-suppressor genes and growth inhibitory pathways 5) Lung cancer stem cells 6) Angiogenesis 7) Metastasis and epithelial to mesenchymal transition 8) Activation of telomerase 9) Epigenetic changes in lung carcinogenesis There are significant molecular alterations known to be involved in the initiation and progression of lung cancer. Continued development of targeted therapies for the treatment of lung cancer is dependent on increased understanding of involved molecular pathways. Targeting multiple molecules at once to combat the interconnective and complex signaling complex signaling pathway will improve efficacy. Systematic understanding of the molecular basis of lung cancer through comprehensive characterization of aberrations in the cancer genome and their functionality will provide the means to evaluate their use in diagnosis, prognosis and therapy. Key words: molecular biology, lung cancer 서 론 폐암은미국에서남녀모두암사망원인의 1위를차지하고있는암이다. 폐암은정상폐상피세포에서유전적및후생유전학적인여러단계의변화과정을걸쳐 DNA가손상되어발생되게된다 2. 폐암의주된원인은흡연이며조직학적으로정상으로보이는상피세포라하더라도흡연으로인해분자수준에서전체폐상피세포에이상소견을보이며이를 field cancerization" 이라고한다. 이러한변화로인해폐암세포들은자기성장및성장억제신호에대한비감수성, 세포자멸사회피, 끊임없는복제, 혈관신생, 조직침범및전이등의암세포의특징을보이게된다 3. 폐암은임상적, 생물학적, 1 조직학적및분자생물학적으로다양한질환이며이러한다양성의원인을알아내기위해수많은연구가진행되고있다. 폐암에관련된유전자와그발현경로를확인하고어떻게생물학적으로연관관계가있는지알아내게되면이를활용하여진단과치료의표적으로사용하게될수있을것이며이에대해알아보고자한다. 본 론 1. 폐암의분자생물학적원인폐암은비소세포폐암과소세포폐암으로크게분류되며전체폐암에서비소세포폐암이약 80-85% 를차지한다. 비소세포폐암은 조직학적으로선암, 편평상피세포암, 대세 -64-
폐암의분자생물학적특징과임상적의의 포암및혼합형으로나누어진다. 이러한조직형에따라유전자의변화가다르게발현되며이를활용하여진단및치료에응용할수가있다. 예를들면선암에서 EGFR (Epidemal growth factor receptor) 유전자의돌연변이가 10-40% 발현되는반면소세포폐암이나편평상피세포암에서는거의발견되지않는다. 또한선암에서 EML4-ALK 융합유전자의빈도가 2-13% 발현되는반면다른암종에서는보고되지않고있다 4. 2. 비흡연자에서발생되는폐암폐암의주된원인은흡연이지만현재비흡연자폐암의빈도가점차높아지고있는추세이며이는보고에따라다르지만전체폐암환자의 15-25% 를차지한다. 비흡연자폐암은흡연자폐암과는다른역학적, 임상적및분자생물학적변화를보인다. 대부분여성이며동양인에호발하며젊은나이에호발하는양상을보인다. 또한폐말초부위에호발하며 EGFR 유전자돌연변이의빈도가높아이에대한치료에좋은반응을보이는특징이있다 5. 3. 종양유전자와성장관련경로및표적치료종양유전자의활성화는모든폐암에서발생되며유전자증폭, 과발현, 점돌연변이또는 DNA 재조합으로인해발생된다. 이로인해 oncogene addiction" 으로불리는지속적인세포분열을조장하는성장신호가발생하며세포는이런비정상적인종양유전자의신호에의존하여생존하게된다 6. 폐암에서흔히활성화되는종양유전자는 EGFR, ERBB2, MYC, KRAS, MET, CCND1, DCK4, EML4-ALK fusion 및 BCL2 등이있다. 1) 폐암에서 EGFR 신호 ErbB tyrosine kinase군은 4가지로구성되며 EGFR, ErbB-2 (HER2), ErbB-3, ErbB-4 등이 homodiemr 또는 heterodimer를형성하여리간드에부착하여수용체를활성화하게된다. EGFR은비소세포폐암에서 50-90% 의빈도로과발현되거나비정상적인활성화를보인다. 따라서이를표적으로한치료방법의개발에많은노력이집중되어왔다. 2004년 EGFR 유전자의 kinase domain에돌연변이가발견되고 gefitinib이나 erlotinib 같은 TKI의감수성과관련이있음을알게된후비소세포폐암특히선암의치료에많은발전을가져왔다 7. EGFR 유전자의돌연변이는선암, 여성, 비흡연자및동아시아인종에빈도가높게나타나는경향이있다 8. TKI에대한내성은 T790M 돌연변이와 KRAS 돌연변이또는 9 MET 유전자의증폭과관계가있다. 선암에서활성화된돌연변이 EGFR은 IL-6 레벨을증가시키는것으로알려져있으며 10 IL-6는 STAT3를활성화시킨다. IL-6는또한 JAK family tyrosine kinase를활성화시키는데중요한역할을하며 STAT3, MAPK와 PI3K등의다양한신호전달분자를통한경로를활성화시킨다 11. 2) RAS/RAF/MEK/MAPK 신호전달경로폐암에서 RAS/RAF/MEK/MAPK 경로가높은빈도로활성화되어있으며특히선암에서 20% 정도의빈도로 KRAS 유전자의돌연변이가발생하는것으로알려져있다 12. KRAS 유전자의돌연변이의위치는 80% 에 13 서 codon 12에위치한다. 이런돌연변이 는 EGFR 경로와는별개로 PI3K와 MAPK -65-
경로를활성화시키며이는 EGFR TKI와기 존의 항암치료에 내성을 보이게 된다. KRAS 돌연변이는 EGFR과 ERBB2 돌연변 이와는상호배제적이며주로흡연자선암 에서주로발견된다 14. KRAS 돌연변이대 한 치료 방법으로는 RAS 신호전달체계의 downstream 즉 RAF kinase와 MEK을표 적으로하는약제가시도되고있다 12. 3) MYC MYC 종양유전자는 RAS 신호전달의표적 으로세포증식에중요한조절인자이며 15, 비 소세포폐암에서는 MYC이흔히활성화되어 있고소세포폐암은 MYCN과 MYCL이활성 화되어있다 16. 4) EML4-ALK 융합단백 비소세포폐암의 소수의 환자에서 2007년 새로운융합유전자가보고되었다 17. 2번염 색체에 위치한 echinoderm microtubleassociated protein like-4 (EML4) 와 anaplastic lymphoma kinase(alk) 유전 자가융합되어지속적인세포분열을촉진하 고종양형성을하게된다. 최근의 13개의 연구를분석한메타분석을보면 2835건의 종양을분석한결과 EML4-ALK 융합유전 자의빈도는비소세포폐암에서 4% 에서발 견된다 18. EGFR 및 KRAS 돌연변이와는상 호배제적이며주로선암에서발견되고주 로비흡연자나흡연력이적은환자에서발 견된다. EML4-ALK 표적치료제에효과적 인임상적반응을보이며최근 crizotinib (PF-02341066) 이 3상 임상시험이 시작되 었다 19. 5) PI3K/AKT/mTOR 경로 PI3K는 lipid kinase 로세포의증식, 생 존, 부착과이동을조절한다 20.PI3K/AKT/ mtor경로는 EGFR을포함한다수의 tyrosine kinase 수용체의 downstream 신호전달경로에관여되며또한활성화된 RAS에 PI3K가결합함으로써활성화될수있다 21. LY294002나 rapamycin등의표적치료제가 AKT 경로가활성화된비소세포폐암과소세포암에서의미있는효과를보이고있다 22. 4. 종양억제유전자와성장억제경로 1) p53 경로 TP53은분열된세포에서유전적손상의축적을막는 phosphoprotein을인코딩한다. 세포의스트레스에반응하여 p53은 CDK inhibitor같은유전자의발현을야기시켜세포가 G1 기에머물도록하여 DNA를수리하거나자멸사에빠지게한다 23. 폐암에서 p53을불활성화시키는돌연변이는가장흔히발견되는유전적변화이다 24. 이러한이유로 p53을발현시키기위한방법으로유전자치료가시도된바있다 25. 2) 3p 염색체종양억제유전자 3번염색체단완의유전자소실은암의초기에가장흔한변화중의하나이며폐암에서 96% 의빈도로발견되고전암성병변에 26 서도 78% 의빈도로발견된다. 이부위에관련된유전자는종양억제유전자와관련된부분으로 FHIT (3p14.2), RASSF1A, TUSC2 등이포함된다. 3) STK11(LKB1) Serine/threonine kinase STK11(LKB1) 은종양억제유전자의역할을하며세포의극성과운동, 분화전이그리고대사에관여 한다 27. 폐암의약 30% 에서불활성화되며 -66-
폐암의분자생물학적특징과임상적의의 p53과 RAS 다음으로세번째로흔히돌연변이를보이는유전자이다. 비소세포폐암에서흔히발견되며소세포폐암은드물게발견되고남성과흡연자및분화가좋지않은선암에서흔히발견된다 28. 5. 폐암줄기세포 : 발견, 신호전달경로및 치료 암줄기세포모델은줄기세포와같은역할 을하는극소수의세포가자기증식과분열 을할수있는능력을가지고종양을형성 한다는것이다 29. 이런암줄기세포모델은 급성골수성백혈병에서처음보고되었으 며폐암을포함한고형성종양에서도그존 재가보고되었다. 그러나현재까지특이적 인마커가보고되고있지않으며이것은아 마도종양의다양성때문으로생각된다. 폐 암에서 암줄기세포의 조절은 Hedgehog (Hh), Wnt 그리고 Notch와같은줄기세포 신호전달체계에의해조절되는것으로생 각된다 30. 이러한 신호전달을 차단시키려는 시도가 있어왔으며특히 Notch 신호전달경로를 억제하는 r-secretase 억제제가유용한것 으로알려져현재 2상임상시험이진행되고 있다 31. 그외에도암줄기세포의바이오마커가진 단과예후를추정하는데유용할것으로생 각된다 32. 6. 혈관신생신생혈관은암의특징중의하나이며종양의성장에필수적이다. 따라서암발생초기에혈관성장인자가필수적으로필요하게되며이들의종류는 vascular endothelial growth factor(vegf), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), IL-8 및 angiopoietin 1,2 가있다. 이중특히 VEGF 가중요하며종양저산소증, 성장인자와사이토카인및종양유전자의활성화로인해자극된다 33. VEGF는소세포폐암과비소세포폐암모두높은빈도로발현되어있으며이의억제는중요한치료표적이다 34. 혈관신생을억제하는치료방법은 VEGF 에특이적인항체를투여하여차단하는방법과 TKI를투여하여 VEGFR의세포내부위를차단하는두가지방법이있다 35. Bevacizumab은 VEGF-A와결합하여효과를나타내며폐암치료에사용되고있다 36. 7. 전이와 epithelial to mesenchymal transition (EMT) 암세포가전이를일으키기위해서는다양한분자적변화가필요하며종양의진행과전이가정상세포발생과분열에필요한프로그램인 epithelial to mesenchymal transition (EMT) 이라는생물학적프로그램이암에서도관여되는것으로알려졌다 37. EMT는세포극성의소실로인해가동성이있는중간엽세포의양상을띄게되며특징적으로 E-cadherin 발현이소실된다 38. 폐암에서 mesenchymal marker와 EMT inducers (Vimentin, Twist, Snail) 은예후와강한연관관계가있다고알려져있다 39. EMT는또한 EGFR TKI의내성과연관이있다고알려져있다 40. 8. Telomerase 활성화 Telomere를연장시키는효소인 telomerase 는세포의지속적인분열에필요하다. 정상 -67-
세포에서는활성화되어있지않지만비소세포폐암의 80% 이상에서그리고소세포폐암에서는모두활성화되어있다 41. 이에대한치료방법으로서 imetelstat 같은 antisense oligonucleotide를투여하는방법과 42 면역치료방법이시도되고있다 43. 9. 폐암발생에서후생유전학적변화 1) Methylation과 histone modification 후생유전학적인변화(epigenetic change) 는 DNA 염기서열의변화없이유전자의표현형이변화되는것으로수복가능한변화이다 44. 폐암에서는흔히종양억제유전자의불활성화에관여되며 p16, MGMT, RASSF1, MTHFR 및 FHIT 유전자의전사부위의메틸화의빈도가높음이보고되고있다 45. 최근에는 methylation-specific microarray의개발로인해종양조직의 epigenome에대한분석이가능하게되었다 46. 후생유전학적인변화에관여되는효소로 DNA methyltransferase (DNMTs) 와 histone deacetylase (HDACs) 가있으며이런효소를억제하는치료방법이시도되고있으며폐암의치료의한방법으로써유용할것으로생각된다 47. 2) MicroRNA mirna는단백질전사에관계되지않으며유전자발현을조절하는 RNA 를지칭한다. 폐암과관련된 mirna는 let-7 mirna family로 N-RAS, MYC과 HMGA2 유전자를조절하는종양억제유전자의역할을하는것으로알려져있다 48. 최근의연구에의하면 mirna는진단과예후및치료에관련된중요한바이오마커의가능성이있다. 일례로말초혈액과객담 에서 mirna의검출은폐암의조기진단에유용한마커로서의가능성을보여주었다 49. 결 론 이상의결과들을보면폐암의발생과진행에중요한분자생물학적인변화가관여되어있음을알수가있다. 폐암의표적치료제의지속적인개발을위해서는이와관련된분자생물학적경로에대한관심과지속적인연구가필요하다. 최근의임상시험결과들을보면한가지표적만을억제하여서는충분한치료효과를얻을수가없으며다양한표적들을동시에목표로하는치료방법을개발하여야할것으로생각된다. 또이를위해폐암유전자를더잘검출할수있는신기술의지속적인개발이필요할것이다. 이로인해진단과치료및예후를더욱잘예측할수있게될것이며궁극적으로는환자별맞춤치료가가능해질것으로생각된다. 참고문헌 1. Jemal A., Siegel R., Xu Jl: Cancer statistics, 2010. CA Cancer J Clin 60. (5): 277-300.2010 2. Wistuba, II, II, Gazdar A.F.: Lung cancer preneoplasia. Annu Rev Pathol 1. 331-348.2006 3. Hanahan D., Weinberg R.A.: The hallmarks of cancer. Cell 100. (1): 57-70.2000 4.GazdarA.F.: Shouldwecontinueto use the term non-small-cell lung cancer? Ann Oncol 21. (Suppl 7): -68-
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