Date of Award

1-1-2000

Embargo Period

2-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Molecular and Cellular Biology and Pathobiology

College

College of Graduate Studies

First Advisor

Demetri D. Spyropoulos

Second Advisor

Ioanna Maroulakou

Third Advisor

Alexander Awgulewitsch

Fourth Advisor

Tien Hsu

Fifth Advisor

Dennis K. Watson

Abstract

Homeotic genes encode transcription factors, believed to regulate cell proliferation, differentiation, as well as cell migration to create positional identities during embryogenesis. In the mouse, the Hox complex consists of at least 39 genes, organized into four clusters, in four different chromosomes. Hoxc5 and Hoxc6 are adjacent genes located in chromosome 15, and are expressed in the developing somites of the thoracic region and central nervous system with overlapping patterns. Both Hoxc5 and Hoxc6 are also expressed in the mammary gland. This study demonstrates that a targeted disruption of the Hoxc6 gene results in a failure of ductal development in a graded pattern along the A-P axis. It also demonstrates that Hoxc6 responds to ovarian hormone regulation. Ovarian hormones regulate the development of ductal and lobuloalveolar structures in the cycling mammary gland. Mice homozygous for this mutation also exhibited a homeotic transformation of the second thoracic vertebra into the first at 60% penetrance, corresponding to both the gene's anterior boundary of expression and the most extreme appearance of mammary defects. Histological and whole mount analyses of mammary glands from adult Hoxc6-/- females revealed the absence of mammary epithelium in thoracic glands, and dilated ducts in inguinal glands at 100% penetrance. Mammary gland defects were also reflected by the neonatal lethality of pups from Hoxc6-/- mutant females. Histological analysis of mammary glands from ovariectomized Hoxc6-/- females show the same phenotype observed in intact females, suggesting hormonal dysregulation. A coordinate regulation of Hoxc6 expression by ovarian hormones was further observed. Despite overt defects in adult mammary glands, mammary buds in E12.5 Hoxc6-/- embryos appeared normal. In the newborn Hoxc6-/- female however, the thoracic primary duct was observed but branching, ductal structures, and fat pad development were greatly reduced. A 14 bp linker was later introduced into the homeobox of Hoxc5, producing a frameshift mutation and a homeodomain-truncated protein. Mice producing the Hoxc5 truncated protein could partially restore the Hoxc6 homozygous phenotypes. Together, these results suggest a position-specific role for Hoxc6 in mammary gland morphogenesis after the mammary bud stage in response to hormonal signals, as well as parallel regulatory pathways for Hoxc5 and Hoxc6 gene products.

Rights

All rights reserved. All rights reserved. Copyright is held by the author.

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