Temporal and spatial expression of Hoxa2 gene in the developing mouse palate and the effects of valproic acid on Hoxa2 expression during murine palatogenesis
Date
2003-06
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ORCID
Type
Degree Level
Masters
Abstract
Hoxa2, a second arch selector gene, when deleted results in many cranial
abnormalities including a cleft plate. The occurrence of cleft plate in the Hoxa2-/- mice is
reported to be due to a secondary effect and Hoxa2 gene has been assumed not to be
present in the palate. However, Hoxa2 gene and protein, have recently been
demonstrated to be expressed in the developing palate. This finding was extended and a
detailed temporal and spatial pattern of expression of Hoxa2 was identified in the
developing murine palate. Hoxa2 protein expression is detected in palatal epithelium as
early as embryonic stage 12 (E12). From E12 to E13.5, Hoxa2 expression extends to the
mesenchyme, where intensity of expression is enhanced. At E13-13.5, Hoxa2 protein
and mRNA expression is the highest in the outer lateral half of the elongating palatal
shelves. At E14-14.5 when the palatal shelves elevate above the tongue to a horizontal
position, Hoxa2 expression is down regulated in the mesenchyme, Hoxa2 continues to be
expressed in the palatal epithelium and the medial edge of the epithelium at this stage.
The temporal and spatial pattern of Hoxa2 expression in the developing palate is in
concordance with the cytoarchitectural changes occurring in the growing palate. In vitro
whole organ palatal cultures show that palates from Hoxa2-/- mice exhibit a much lower
fusion rate (44.4%) compared to their heterozygous or wild type counterparts (78.7%,
90% respectively). In the wild-type palate organ culture group exposed to valproic acid
(VPA), a clinically used anticonvulsant drug and a known teratogen that can induce a
cleft palate and other abnormalities in mice and humans, Hoxa2 mRNA is downregulated
within the palatal shelf as determined by RT-PCR in a dose dependent manner.
These results provide evidence that cleft palate induced in Hoxa2-/- mice is not
only secondary to an altered positioning of the tongue but is also due to an absence of
Hoxa2 gene expression which results in delayed palatal development. The percentage of
cleft palate calculated as a result of delayed palatal development is 45.6% in Hoxa2-/-
mice, whereas the percentage of cleft palate secondary to the abnormal position of the
tongue accounts for approximately a third in these mice. However, both of the factors, a
delay in palatal growth and an abnormal tongue position act combinatorially to give the
reported incidence of 82% cleft palate in the Hoxa2 knockout mice. These results
demonstrate a direct role for Hoxa2 in palatogenesis. In addition, the ability of VPA to
alter the Hoxa2 expression in the palate during palatogenesis suggests one possible
mechanism of VPA induced cleft palate.
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Degree
Master of Science (M.Sc.)
Department
Toxicology
Program
Toxicology