Lecture 4 January 30, 2006 End of Chapter 3: codominance through complementation

HLA (MHC) genes are codominant and have multiple alleles  HLA’s are cell-surface proteins involved in the recognition of self and non-self by the immune system – important in determining histocompatibility for transplantation  HLA’s present foreign antigens to the immune system – figure in the resistance to viral and bacterial pathogens  HLA’s are codominantly expressed  Multiple alleles of HLA in a population increases the likelihood that the population will survive a pathogen threat, also cause histoincompatibility in organ and tissue transplants

Polymorphism and polygeny MHC genes are polymorphic: that is, there are large numbers of alleles for each gene MHC genes are polygenic: that is, there are a number of different MHC genes.

Class II MHC genes –pairs of class II  and  chains are DR, DP, DQ – with extra DR-  chain, up to four types of class II MHC Class III MHC genes – complement proteins C2, C4, factor B, cytokine TNF- , lymphotoxin TNF-  Class I MHC genes – three types of class I MHC  -chains are called HLA-A, HLA-B and HLA-C “Non-classical” genes labeled in black; DM catalyzes peptide binding to MHC class II molecules, DN and DO regulate DM, TAP and LMP are components of the proteasome Mouse and Human genomic map of MHC genes

Simplified comparison of MHC genes in mouse and human: Class II MHC genes: encode glycoproteins expressed primarily on APC’s (macrophages, dendritic cells and B cells) where they present processed antigenic peptides to T H cells Class III MHC genes: encode secreted proteins that have immune functions e.g. components of the complement system and molecules involved in inflammation, and other proteins Class I MHC genes: encode glycoproteins expressed on the surface of nearly all nucleated cells; present peptide antigens to T C cells   2-microglobulin, which forms a complex with the MHC class I  -chains is encoded on a separate chromosome Chromosome 17 Chromosome 6

Haplotype: the particular combination of MHC alleles found on one parental chromosome Expression of MHC alleles is codominant, with protein products expressed from both haplotypes in an individual The many combinations of possible haplotypes contributes to the difficulty of finding compatible donors for tissue transplantion

HLA genes are codominant: A protein from each parental gene is expresed on cell- surfaces

MHC molecules expressed on APC’s in a heterozygous mouse. Both maternal and paternal genes are expressed.

Class I MHC presents processed endogenous antigen to CD8 (cytotoxic) T cells: CD8 cells are Class I restricted Class II MHC presents processed exogenous antigen to CD4 (helper) T cells CD4 cells are Class II restricted Professional APC’s: B cell macrophage dendritic cell Class I MHC Class II MHC Most nucleated cells “Target cell” T C cell T H cell CD4 CD8 TCR

Complementation  A method to determine if two mutants are due to a defect within single gene, i.e. within a single genetic locus

Mutant screen: A group of organisms are purposely mutagenized with radiation or chemical mutagens A great example: the Heidelberg mutagenesis screen performed by Christiane Nusslein-Volhard and Eric Wieschaus, which won the Nobel Prize for Medicine in 1995  They established and tested 27,000 fly lines (saturation screen of 5000 expected genes in Drosophila)  They found 18,000 lethal mutations  4332 mutations causing embryonic lethality  580 mutations causing embryonic phenotypes  139 complementation groups (genes)

Genetic Complementation A genetic complementation test allows one to determine if two mutations with the same phenotype are in the same gene or in 2 different genes.

A genetic complementation grid showing results for 6 mutations in all pair wise combinations

Representation of the results of complementation tests by a line diagram

Molecular explanation of genetic complementation

Next time: Chapter 4 Chromosomes At right: A newt lung cell in culture at an intermediate stage of mitotic spindle formation, when most of the chromosomes (blue) have already connected to spindle microtubules emanating from the centrosomes but have not yet congressed to the spindle equator. Immunofluorescence highlights: alpha-tubulin (green) gamma-tubulin (magenta) keratin (red) Image: A. Khodjakov