第四章 総括
4.2 今後の展望
本研究では、合成生物学的アプローチを用いて環境刺激応答型遺伝子発現シ ステムを開発した。さらに開発したシステムの再生医療分野への応用を目指し て、移植用三次元組織の構築を行った。しかしながら、遺伝子発現システムを細 胞内に維持した状態で移植を行うためには、開発した遺伝子発現システムをゲ ノム上に組込む必要がある。この際、遺伝子の組込みによって細胞ががん化して しまうというリスクがある。iPS細胞の樹立においても、初期は山中因子と呼ば れる遺伝子群をウイルスベクターによってゲノム上に組込み樹立していたが [97]、研究が盛んになるにつれ、ゲノムに組込まないプラスミドベクターやRNA による導入、またタンパクとして山中因子を導入する方法が報告されている
[98]–[100]。このように、再生医療分野への応用を目指した研究においては、が
ん化のリスクを低減するためのアプローチが行われている。がん化リスクを低 減したうえで、遺伝子発現システムを細胞に安定導入する方法として、人工染色 体（human artificial chromosome; HAC）を用いた方法があり得る。HACはその名 の通り人工的に作製した染色体であり、細胞に導入すると、核内に安定して存在 し、細胞分裂の際も本来の染色体と同じく複製されるため、宿主細胞の染色体に 組込まれることなく、外来の遺伝子を安定維持するのに優れている[101]。本研 究では、ウイルスベクターを用いて遺伝子発現システムを細胞に導入したが、臨 床応用を考慮した際、HACのような別の遺伝子導入法を検討する必要もある。
環境刺激応答型遺伝子発現システムのうち温熱応答型遺伝子発現システムに ついては、磁場照射によって細胞にダメージを与えることなく EGFP 遺伝子の 発現誘導が可能だと明らかにした。一方で、本システムを移植用三次元組織に応 用する際は、EGFP遺伝子ではなく、移植組織の機能を増幅するような治療遺伝 子を発現させることができる。本研究では HepG2-HSP 細胞を用いているため、
肝機能を向上させるための肝特異的転写因子の発現を誘導するような遺伝子発
91 現システムも有用だと考えられる。
低酸素応答型遺伝子発現システムについては、本システムが低酸素環境に応 答して自律的に遺伝子発現することを明らかにした。本システムの強みは低酸 素環境が克服されると、それ以上過剰な遺伝子発現が起こらないため、移植後に 副作用のリスクが低減できる点である。一方で、三次元組織の乏血管性克服の試 みとして、血管内皮細胞を共培養した三次元組織を構築することで、組織内の血 管網構築を促進する研究も行われている。自律的遺伝子発現システムを導入し た細胞と血管内皮細胞を共培養することで更なる血管網構築を促進する手法も 組織工学の課題解決に有用だと考えられる。
本論文で述べたように、合成生物学的アプローチを用いることで細胞の遺伝 子発現を制御可能になり、医療分野、エネルギー分野などへ広く応用されている。
こうした研究を加速させるためには、従来の生物学を利用した解析的な研究に よる遺伝子・タンパクの機能解析と、合成生物学的な遺伝子発現システムの開発 を並行して行う必要がある。さらに、コンピューターシミュレーションによる遺 伝子発現システムの発現挙動予測なども行われており、専門分野にとらわれな い多くの研究者の協力が行われている。こうした背景も踏まえ、合成生物学の研 究は今後ますます発展していくと考えられる。
92
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