Download S3.Cell Signaling-Signaling and gene expression

Cell Signaling Pathways – A Case Study Approach
L. Emtage, L. Bradbury, N. Coleman, D. Davenport, A. Dunning and J. Grew
Signaling and Gene Expression
SCF
Plasma membrane
Kit
Grb2 Sos (Ras GEF)
(adapter)
MI
Ras
GTP
Ras
(G protein)
GDP
RAF
(MAPKKK)
P
MEK
(MAPKK)
P
ERK
(MAPK)
P
C
T F BP
M
ITF
P
The figure above shows how a series of proteins form a signal transduction pathway. This
pathway transmits a cellular signal from the receptor tyrosine kinase (RTK) Kit on the
plasma membrane, through the cytoplasm, to the cell nucleus, where it alters gene
expression.
The Kit protein is expressed in embryonic germ cells (precursors to sperm and eggs),
hematopoietic stem cells (blood cell precursors), and melanoblasts. Melanoblasts are the
precursors to melanocytes and retinal pigment epithelial (RPE) cells. Melanocytes are
found not only in the skin (where they produce the skin coloring pigment melanin), but
also in the inner ear, where they help form an important epithelial barrier in the cochlea.
Retinal pigment epithelial cells are found in the eye.
The transduction molecules GRB2, SOS, Ras, Raf, MEK and ERK are very widely expressed;
they may be activated by many different RTKs, and they may activate many different
transcription factors. In contrast, the transcription factor MITF is expressed only in
melanoblasts.
Questions
1. What do you think would be the effect on embryonic development if there was a
complete (homozygous) loss of the Kit receptor? A complete loss of SCF? A complete
loss of MITF? Complete loss of either the Kit receptor or its ligand SCF (also called
KITL or Steel Factor) is lethal due to the inability of the embryo to make blood. All
Cell Signaling Pathways – A Case Study Approach
L. Emtage, L. Bradbury, N. Coleman, D. Davenport, A. Dunning and J. Grew
three cell types listed above are affected in strains carrying milder alleles (mice are
anemic, infertile and have pigmentation anomalies). Homozygous severe loss-offunction alleles of MITF leads to complete loss of coat and eye pigmentation, small
eyes (due to fate changes in the retinal epithelium that disrupt eye formation), and
deafness (Nakayama et al, 1998; Bumsted and Barnstable, 2000).
2. Individuals with Waardenburg syndrome Type 2A often have a white forelock and
premature greying, unusual pigmentation of the iris, such as heterochromia iridis, and
hearing loss. Waardenburg syndrome is a congenital disorder, caused by dominant
loss-of-function mutations in a gene or genes in this pathway. Which gene or genes
above could be mutated to give rise to Waardenburg syndrome 2A? Explain your
answer. Loss-of-function mutations in MITF are also dominant, and cause either
Waardenburg or Tietz albinism-deafness, depending on the severity of the mutation
(OMIM, MITF). Neither syndrome is characterized by anemia or infertility, making the
possibility of Kit or SCF mutations as a cause seem less likely*.
3. Very pale hair, such as found in Icelanders, is thought to be due in part to a
polymorphism in SCF. If you were trying to identify the polymorphism responsible,
would you sequence the coding region or the regulatory region of SCF first, and why?
Blonde hair is recessive. There are no known recessive mutations in the coding region
of SCF that can be homozygous in humans, presumably because homozygous loss-offunction alleles are likely to be lethal (see notes on question #1 above).
A SNP (rs12821256), located approximately 350 kb upstream of SCF was found to
be associated with blonde (vs brown) hair in Icelanders, confirmed in a sampling of
Dutch. The SNP is believed to either modulate SCF expression directly, or to be linked
to another polymorphism that affects SCF expression (Sulem et al, 2007).
*Although actually, heterozygous loss-of-function mutations in both Kit and SCF
have been characterized and neither has the pleiotropic phenotype one might
expect!
Nakayama, A. et al. 1998. Mutations in microphthalmia, the mouse homolog of the
human deafness gene MITF, affect neuroepithelial and neural crest-derived
melanocytes differently. Mechanisms of Development, Volume 70, Issues 1–2, Pages
155–166.
Bumsted, K.M. and Barnstable, C.J. 2000. Dorsal Retinal Pigment Epithelium
Differentiates as Neural Retina in the Microphthalmia (mi/mi) Mouse. Retinal Cell
Biology Volume 41, Issue 3
Sulem, P. et al. 2007.Genetic determinants of hair, eye and skin pigmentation in
Europeans. Nature Genetics 39, 1443 - 1452