MLH1and Hereditary Non-polyposis Colon Cancer

Walter Ratchford

3/4/04

Outline

• Explanation of MMR in DNA replication

• Description of MLH1 and its role in MMR

• The link between hereditary non-polyposis colon cancer and MLH1

DNA Mismatch Repair (MMR)

• Correction of mismatched nucleotides and small insertion-deletion loops

• DNA replication errors thought to be 1/1000 nucleotides. Over 99.9% corrected by MMR

• Studied primarily in E.coli

• Involves three proteins MutS, MutL, and MutH in E. Coli

DNA MMRSingle base pair mismatch

DNA MMRMechanism of insertion-deletion loops

• Typically affect repetitive sections of DNA• Gain or loss of short repeated units• Capable of repairing 1,2, and 3 extra nucleotide loops• Caused by the slippage of DNA during replication

DNA MMR in E. Coli

•MutS/MutL complex activates MutH which locates a nearby methyl group, and nicks the newly synthesized DNA

• MutS recognizes mismatched DNA

•MutL acts as stabilizer for MutS/DNA interaction

•Excision is accomplished by cooperation between the UvrD (Helicase II) protein, which unwinds from the nick in the direction of the mismatch, and a single-strand specific exonuclease of appropriate polarity, followed by resynthesis and ligation by DNA ligase.

http://saturn.roswellpark.org/cmb/huberman/DNA_Repair/mmr.html

DNA MMR in HumansE. Coli Human Function

MutS MSH1 DNA Repair in Mitochondria

MSH2 Mismatch and loop repair w/MSH6 and MSH3

MSH3 Loop repair with MSH2

MSH4 Meitotic Recombination with MSH5

MSH5 Meitotic Recombination with MSH4

MSH6 Mismatch and loop repair with MSH2

MutL MLH1 Mismatch repair

PMS2 Mismatch repair w/ MLH1 (MutLalpha)

MLH3 Complex with MLH1 for meitotic recombination

PMS1 Not involved in MMR

MutH ? ?

DNA MMR in Humans

http://www.imr.unizh.ch/research/marra/HNPCC/hnpcc.html#

MutL homolog 1 (MLH1)• Human homolog of MutL in E. Coli

• Located on Chromosome 3p

• Encodes a 756 amino acid protein

• 19 exons

• Involved in the correction of base-base mismatches and insertion-deletion loops from errors induced during DNA replication and recombination

http://www.utoronto.ca/LabMedPathobiology/education/undergraduate/365/2004lecture%20notes/Elsholtz365-3(2004)_copy.pdf

MLH1

www.rcsb.org

Crystal Structure of 40 Kd segment of protein

MLH1 and hereditary non-polyposis colon cancer (HNPCC)

• Defined as inactivating germline mutations in genes encoding MMR proteins

http://www.imr.unizh.ch/research/marra/HNPCC/hnpcc.html

A benign neoplastic polyp - Adenoma

• No MLH1 = cancer predisposition....tumor formation• Autosomal dominant inheritance• Recessive cancer phenotype

HNPCC Inheritance

• Inherited mutation in one of the MMR genes• Subsequent loss of heterozygosity (Two Hit Hypothesis)• 20% of sporadic colorectal cancers due to MMR gene mutations

http://www.imr.unizh.ch/research/marra/HNPCC/hnpcc.html

HNPCC• Accounts for 3% of colon cancers• Tumors also develop in urinary tract, stomach, ovary, smallbowel, and endometrium• 85% penetrance• 70% HNPCC patients have mutation in MLH1 or MSH2

HNPCC General Population

Percent developing colon cancer

80 4

Average age of development

44 65