Development of a mouse model for Hutchinson-Gilford progeria syndrome reveal defects in adult stem cell maintenance

Abstract: Hutchinson-Gilford progeria syndrome (HGPS) is a very rare genetic disease that presents some features of accelerated aging. Children with the disease are born appearing healthy but start to develop signs of the disease within their first years of life. The disease affects multiple tissues, and the symptoms include growth retardation, alopecia, lack of subcutaneous fat, scleroderma-like skin changes, bone abnormalities and joint stiffness. Heart disease and other atherosclerotic complications are the most common cause of death, which occurs at a median age of 13 years. Ninety percent of HGPS cases are caused by a de novo point mutation in the LMNA gene (c.1824C>T, p.G608G). The LMNA gene encodes, by alternative splicing, the different isoforms of A-type lamins. These intermediate filaments are the main proteins of the nuclear lamina, a meshwork of proteins located underneath the inner nuclear membrane. The nuclear lamina serves important functions in determining the shape and size of the nucleus as well as being involved in fundamental cellular processes such as DNA replication and transcription. This thesis is focused on gaining a deeper understanding of the cellular effects caused by the most common HGPS mutation. For this purpose, we developed an inducible mouse model that carries a human mini gene for lamin A with the c.1824C>T mutation. The mini gene caused over-expression of both wild-type human lamin A and human progerin, the mutated form of the lamin A. Targeting the expression of the transgene to keratin 5-expressing tissues resulted in a progressive skin phenotype. The phenotype evolved from an intermediate stage with hyperplasia of the interfollicular epidermis and increased proliferation to an end stage with hypoplastic epidermis, fibrotic dermis and loss of subcutaneous fat. The end stage of the condition resulted in mice that had similar abnormalities to those seen in the skin of HGPS patients. This mouse model was used to further investigate the molecular effects of the HGPS mutation in the skin. To examine whether expression of the lamin A c.1824C>T mutation influences the hair cycle or the expression pattern of lamin B, we first characterized the normal hair cycle and expression patterns for lamin A/C and B. Immunohistochemical stainings of the dorsal skin of FVB/NCrl wild-type skin from animals of different ages showed strong expression of lamin A/C and B in the basal cells of the epidermis, the outer root sheath of the hair follicle and the dermal papilla in all phases of the hair cycle. Analyzing the progression of the phases of the first hair cycle and the expression of lamin B in our HGPS mouse model, we could not distinguish any effects of progerin expression. The lamin A c.1824C>T expressing mutant mice were also used to study the effects of the expression of this mutant lamin during embryogenesis. Animals with embryonic expression of progerin developed a rapidly progressing phenotype. In summary, our inducible models will be useful for testing different treatments and could also be used for tissue-specific expression in other tissues affected in HGPS. The work presented in this thesis provides insights into the molecular mechanism underlying the skin symptoms seen in patients with HGPS.