Lecture 4: Apoptosis and Cancer

apoptosis is important in

pattern formation

why do duck hindlimbs show webbed feet but chick hindlimbs do not?

during early development, apoptosis cases programmed cell death between the toes for chicks but not ducks

apoptosis shapes our own

hands to form 5 digits

necrosis

death due to external damage to cell, specifically damages DNA

apoptosis

programmed cell death

cells may die by ____ or may _______ by _______

necrosis; self-destruct; apoptosis

apoptosis is a genetically programmed series of events that includes:

- cells first receive signal to die

- detachment of cell from its neighbors

- cytoplasmic "blebbing" to form apoptotic bodies

- fragmentation of nuclear DNA

- engulfment by neighbors

apoptosis in physiology

- white blood cells that recognize our own tissues (induces apoptosis)

- white blood cells that are no longer needed after an infection

- skin cells die by apoptosis and are sloughed off

- uterine cells during menstruation

- some cells harboring DNA damage

syndactyly

fused digits

apoptosis is essential for

normal development and life

breakthroughs in apoptosis came from genetics in C. elegans

- wild type has cells that separate from neighboring cells due to undergoing apoptosis

- homozygous ced-3 lof mutant: cells don't undergo apoptosis

- implies that ced-3 contributes to apoptosis

in ced-9 (lof) mutants, most cells

die by apoptosis, suggests that ced-9 prevents apoptosis from occurring

in ced-9 lof mutants, all cells undergo _____, so ced-9 normally

apoptosis; protects cells from death

in ced-3 lof mutants, no cells undergo _____, so ced-3 normally

apoptosis; promotes cell death

is the characteristic cell death of ced-9 mutants dependent on the activity of ced-3?

- create a double mutant by mating ced-9 mutant with ced-3 mutant

- isolate double het & mate with siblings to get homozygous for both genes and observe the resulting phenotype

- see if ced-9 is dependent on ced-3

- no cells die, means ced-9 negatively regulates ced-3 and represses ced-3

--| means

represses process

--> means

activates process

a conserved pathway of genes controls

the decision for cells to die

defects in apoptosis result in

developmental defects and disease

cancer is caused by

changes to our genes

carcinogen

mutagen that typically results in cancer

DNA damage causes cancer by

mutating the genes that control cell division and survival

oncogenes

required for cell division (gas), mutated version of proto-oncogenes

proto-oncogenes

normal version of oncogenes, normal function is to control cell growth and proliferation

tumor suppressors

required to stop cell division (brakes)

somatic mutations

mutation occurs in cells of the body

germ line mutations

mutation occurs in germ cells (sperm & egg)

RAS was the first known

- mutated human oncogene, specific mutation in RAS gene causes non-mediated cell proliferation

- when RAS doesn't turn off as it normally should, transcription does not stop, resulting in cancer bc transcription doesn't stop downstream pathways

humans and mice have 3 RAS genes

- HRAS, NRAS, KRAS

- gof mutations that will render RAS gene products to be constitutively on

- cells will continue to proliferate

mutation for 3 RAS genes

- all three can be mutated to gof via missense mutation at GLY12

- renders GTPase domain of RAS insensitive to inactivation, locking it in an active, GTP-bound "on state" (activates in GTP form)

proto-oncogenes become oncogenes with

the "right" mutation

RAS becomes an oncogen only after

it becomes mutated

how do tumor suppressor genes work

- "checkpoints" prevent inappropriate proliferation

- disruption of both copies of a tumor suppressor leads to cancer, loses function (lof), needs to be hom mutant

- in contrast, only one allele of an oncogene needs to be activated

retinoblastoma

- type of cancer that comes from tumor suppressor mutation

retinoblastoma: sporadic

- late onset

- single tumor

- limited

- smoking

- inherit two normal genes, lose one (rare event) and lose other, results in tumor

- late onset bc you need both alleles to be mutated

retinoblastoma: hereditary

- early onset

- multiple tumors

- secondary tumors

- inherit one normal/one defective and lose normal one

- results in tumor

- early onset, only need one more additional mutation

Knudson's Hypothesis

- two hit hypothesis

- occuring in your somatic cells (lof mutations)

- alleles can be different types of mutations but on the same gene & must be lof mutations (frameshift, missense, etc.)

famous tumor suppressors (cancer)

Rb and P53

Rb tumor suppressor

- encodes transcriptional repressor that acts to repress cell cycle progression by inhibiting transcription of genes required for S phase

P53 tumor suppressor

- encodes p53 protein that responds to DNA damage

- upon sensing DNA damage, p53 is activated, resulting in either G1 cell cycle arrest or apoptosis

- killing cells that have DNA damage, prevent cell proliferation of cells with DNA damage

BRCA1 and BRCA2 encode

DNA binding proteins required for DNA damage repair

why is cancer tough to eradicate?

- cancer is a disease of our own cells and genes that have become altered

- these cells look and behave like our cells, no foreign antigens for immune system to recognize

- drugs that kill tumor cells also kill our normal cells

BRCA1 associated cancers

usually inheritance of a mutant copy and then subsequent deletion/mutation of remaining copy

why would there be no BRCA 1 mutant homozygotes in a pedigree?

may not have survived past embryo stage

why do some daughters carry mutant BRCA1 but not get affected?

only 1 allele is mutated

why do some daughters not carrying the mutant BRCA1 still get affected?

sporadic mutation, late onset (2 random mutations)

cancers often involve the

activation of oncogenes and loss of tumor suppressors in the same cell

apoptosis and disease

homeostasis/balance between cell division and cell death