Download Candidate Gene Approach

Lecture 6
Candidate Gene Approach - 2
BSE652
24-1-2017
http://www.nobelprize.org/nobel_prizes/medicine/laureates/1995/wieschaus-lecture.pdf
Complementation analysis
http://www.nobelprize.org/nobel_prizes/medicine/laureates/1995/wieschaus-lecture.pdf
Which genes were missed and why?
Genes required for cleavage and cellularization
Development 104, 495-509 (1988)
Large deficiencies uncovered SEVEN genes
essential for cellularization (14th cell cycle). Three
of these seven genes are cytoskeleton related
genes.
Genes regulating morphological changes during
gastrulation
These genes were mostly missed in the Heidelberg screen
because the defects in gastrulation produced by them did
not have a cuticular phenotype that could be observed
The art and design of genetic screens: drosophila
melanogaster
Daniel St. Johnston
VOLUME 3 | MARCH 2002
Compulsory reading
Enhancer screen
Enhancer screen for sevenless
Some of the most successful screens of this type were carried out to find the components in the signal-transduction pathway downstream of
Sevenless (Sev), which is a receptor tyrosine kinase (RTK) that controls cell-fate choice between one of the eight photoreceptors in the eye,
the R7 cell, and non-neuronal cone cells30.A weak allele of sev provides just enough signalling activity for most of the R7 cells to form, but
these cells are transformed to cone cells if there is any further reduction in the efficiency of signal transduction. Under these conditions,
halving the dose of some of the downstream components in the pathway causes most of the R7 cells to become cone cells,without affecting
signalling through the other RTKs in the fly. This sensitized genetic background therefore allowed Simon et al.30 to identify mutants in
components of the RTK signalling pathway as dominant enhancers of a visible eye phenotype.
Genes are named after such screens e.g. suppressor of fused or enhancer of split etc.
What have we learned from the drosophila, C. elegans
and zebrafish genetic screens
1. Homeotic genes, co-linearity and establishment
of the body plan
 Ed Lewis and Walter Gehring, Denis
Duboule (vertebrates)
Homeotic genes confer
identity to a body
segment. Homeotic
mutations transform
certain parts or an
entire body segment
into the corresponding
structures of another
body segment, thereby
changing the
architecture of the
organism.
The concept of colinearity and posterior prevalence
Systematic analysis by Ed Lewis (http://www.nobelprize.org/mediaplayer/index.php?id=1704) helped Dennis Duboule (Trends Genet.
1994;10:358–364) propose the idea of “posterior prevalence”.
Elimination of entire bithorax complex converts all the segments into T2 -> thus second thoracic (T2) segment is proposed to be the “ground
state”. The three BX-C complex genes are expressed in a nested pattern - Ubx from T3 to A8, abd-A from A2 to A8 and Abd-B from A5 to A8.
Mutation in extra sex combs (esc), a transcriptional repressor gene that lies outside the BX-C locus, can apparently activate all of the BX-C
genes in all segments T2-A8, generating a mutant fly in which all segments resemble A8. Deletion of Ubx => A1 and T3 develop as T2;
deletion of abd-A => T2-A1 develop more-or less normally but A2-A4 develop as A1; Abd-B - T2-A4 develop normally but A5-A8 develop as
A4 (reference - http://www.ucl.ac.uk/~ucbzwdr/teaching/b250-99/homeotic.htm) . Based on genetics alone, Ed Lewis thought there were 910 genes in the BX-C complex but now it is known that of the different mutant loci characterized by Ed Lewis only three, Ubx, abd-A and AbdB code for proteins. It is now known that iab-2 to iab-8 are non-coding regulatory elements, possibly encoding miRNAs - Nat Rev Genet. Oct
2008; 9(10): 789–796.
Comparative anatomy and Hox gene expression
There is a correlation between the span of specific Hox genes in a given species and the identity of the vertebral element.
Gene targeting data also support the notion that Hox genes assign regional identity
HoxB6 is expressed
throughout the axial
skeleton
Unlike in flies in vertebrates
there are four copies of Hox
clusters and thus to realize
phenotype all the copies have to
be deleted in mouse.
First set of morphogens were identified
Morphogen gradient model According to the gradient
model, pattern is established
by one or a few
multifunctional regulatory
substances, called
morphogens, that are
distributed in monotonic
gradients. These
morphogens specify cell fate
in a concentrationdependent fashion such that
cells acquire different fates
based on the concentration
of morphogen that they
contain.
First set of morphogens were
identified – Bicoid, Dorsal, Nanos
and Dpp
The entire pathway for determination of dorsoventral polarity was uncovered from this screen
The concept of lateral specification/inductive
signaling and Notch/Glp-1/Lin-12 gene
Lateral specification
Inductive signaling
The concept of lateral specification/inductive
signaling and Notch/Glp-1/Lin-12 gene
Juxtacrine signaling