探索Rtl1於小鼠腦中的表現機制與功能

Abstract

基因印痕是一種在表觀遺傳學中的特殊現象：透過表觀遺傳的調控，使特定一群基因並非表現父母雙方的基因，而是僅表現父方或母方的基因，這些特別的基因稱為印記基因，過去在胎盤與胎兒中被大量發現，而他們的功能多與發育相關。此外，這些印記基因未必在其他組織中也有基因印痕現象，但近年來發現有部分印記基因在大腦也有基因印痕現象。Retrotransposon-Like 1(Rtl1)是一個與胎盤、胎兒發育息息相關的基因，位於小鼠的第12套染色體上、人類的第14套染色體上，它在胎盤的微血管內皮細胞中大量表現。在Rtl1缺失小鼠模型中，發現父方的Rtl1缺失會造成胎鼠與胎盤的異常，小鼠在發育後期或出生一兩天內死亡，僅有少部分能夠存活，且這些存活下來的胎鼠有發育遲緩的現象，而母方的Rtl1缺失小鼠被發現胎盤會稍大，但出生率與體型並未受到影響。在人類單親源二倍體的患者中，當兩個第14套染色體均來自父方時，這些患者為鏡-緒方症候群(Kagami-Ogata syndrome)，反之則稱為Temple syndrome。這些Temple syndrome的患者的病徵包含肌肉無力、身高矮小與低出生體重、性早熟，我進而推測Rtl1的功能可能是與掌管性腺發育的神經內分泌系統：下視丘－腦垂腺－性腺軸之調節有關。藉由CRISPR-Cas9的系統製造出的Rtl1剃除小鼠中我發現其性腺激素釋放素與濾泡刺激素均有顯著的上升，而性腺激素釋放素的受器表現量則有顯著的下降，但詳細的分子機制仍須進一步探討。在這個研究中藉由測量基因與激素的表現量可以知道，Rtl1的缺失會影響下視丘－腦垂腺－性腺軸的表現，進而影響小鼠的性發育。

Imprinted genes are a subset of genes which show parent-of-origin-specific expression through epigenetic modulation. These genes are known to function for placenta and fetal development and many of them are placenta-specifically imprinted and biallelic expressed in other tissues. Some of the imprinted genes are found imprinted in the brain, and the function is still not fully understood. Retrotransposon-Like 1 (Rtl1), which is imprinted in the brain, has been known for its critical function in endothelial cells of the fetal capillaries of the placenta. Previous studies have shown that Rtl1 paternal knockout mice tend to have phenotypes such as prenatal and neonatal death and growth retardation. Rtl1 maternal knockout mice showed no lethality in pre- or postnatal but are associated with placentomegaly. Human patients with Uniparental disomy (UPD) of chromosome 14 are associated with Temple syndrome (maternal UPD) and Kagami-Ogata syndrome (paternal UPD). It is known that deletion of Rtl1 may relate to less birth weight and height, hypotonia, and premature puberty. I then hypothesized that Rtl1 is involved in regulating sexual maturation. In this thesis, I am interested in studying whether RTL1 plays a role in the hypothalamus-pituitary gland-gonadal (HPG) axis, which is involved in the regulation of puberty. I investigated the potential role with the Rtl1 knockout mice generated by CRISPR/Cas9 system. The experimental outcomes showed that Rtl1 paternal knockout mice have higher Gonadotropin-releasing hormone (GnRH) and Follicle-stimulating hormone (FSH) and lower Gonadotropin-releasing hormone receptor (GnRHR) expression. However, whether these observations are associated with precocious puberty still needs further investigation.