Screening of potential target genes for cataract by analyzing mRNA expression profile of mouse Hsf4-null lens

Cataract is the opacification of the eye lens, and is the leading cause of blindness
worldwide 30]. Cataractogenesis has multiple causes and is often associated with an abnormality
of the lens microarchitecture 31]. Hsf4 is prominently expressed in the lens compared with in other tissues and closely
related to the development of cataract 5], 32]. In the present study, we aimed to extend our understanding of the influence of lens
development caused by Hsf4 mutations. Results revealed that expression levels of 216 genes were altered in mHsf4-null lens compared with wide-type controls. Functional enrichment results showed
that response to DNA damage stimulus was the most significant function in mHsf4-null lens. In addition, we identified two key modules correlated with response to
DNA damage stimulus from the PPI network.

Cataracts may be caused primarily by the DNA damage, such as oxidized purines 33] and DNA single strand breaks 34]. Study has found that oxidative DNA damage is significantly high in the lens epithelial
cells (LECs) of cataract patients 35], 36]. Besides, loss-of-function mutations in TBC1D20 cause cataracts in blind sterile
mice 37]. The DNA damage response (DDR) is a signal transduction pathway that senses DNA damage
and sets a response to protect the cell and moderate the threat to the organism 38], 39]. Mice with the knock-down of HSF4 have cataract because of an increased proliferation
of LECs in the lens as well as an abnormal lens fiber cell differentiation 8]. Two modules which were closely related to DDR, were isolated from the PPI network.
Ubc (ubiquitin C/polyubiquitin-C), the module-related gene, was also showed to be
the hub node in the PPI network. Ubc is one of the sources of ubiquitin during cell
proliferation and stress that cannot be compensated by other ubiquitin 40]. Ubiquitin is a normal component in the lens and a ubiquitin-dependent proteolytic
system exists in lens 41], which consists of fiber cells that differentiate from epithelial cells and undergo
programmed organelle degradation during terminal differentiation 42]. Furthermore, ubiquitination is a reversible post-translational modification of cellular
proteins and is considered to play key roles in the regulation of varieties of cellular
processes, such as protein degradation, cell-cycle regulation, DNA repair, apoptosis
and signal transduction 43]. Moreover, the ubiquitin proteasome system is found to be essential to cell proliferation
of the lens epithelium and required for differentiation of lens fiber cells in zebrafish
44]. The overexpression of ubiquitin affects ubiquitin proteasome system and thus disorders
cell proliferation and differentiation of lens. Thus our results suggested that the
lack of Hsf4 up-regulated the expression of Ubc, which might be involved in the development
of cataract by regulating the cellular processes of lens.

Ptgs2 (prostaglandin-endoperoxide synthase 2) is also known as cyclooxygenase-2 or
COX-2, which is involved in the conversion of arachidonic acid prostaglandin H2. In
human fibroblasts, Ptgs2 has been showed to interact with Cav1 (Caveolin 1) 45], which is the main component of the caveolae plasma membranes. Caveolae is cholesterol-rich
lipid rafts that are likely to play important roles in lens 46]. What is more, Cav1 was found to participate in repair of DNA damage through regulating
the important molecules involved in maintaining genomic integrity 47]. Besides, redundant Cav1 has been reported to play a role in age-dependent hyporesponsiveness
to growth factors in vitro and may act as an indicator of wound-healing capacity in aged human corneal epithelium
48]. Therefore, Ptgs2 plays a role in the response to DNA damage and may be related to
the repair of DNA damage in lens through the interaction with Cav1.

Egr1 (early growth response 1) belongs to EGR family of zinc finger proteins and functions
as a transcriptional regulator. It has been reported that the mRNA expression of Egr1
can be used as a marker for the direction of mammalian ocular growth 49]. In addition, Fos (FBJ murine osteosarcoma viral oncogene homolog), also named c-FOS,
can be induced by a variety of extracellular stimuli 50] and interact with Jun (jun proto-oncogene, c-JUN) to form the transcription factor
AP-1 (activating protein 1) 51], which regulates cell adaptation to environmental changes 52]. Furthermore, Fos and Jun are differentially regulated during terminal differentiation
of lens fiber cells 53]. Thus, Egr1 and Fos may be involved in the cell cycle and apoptosis of lens.

However, there were some limitations in this study. For example, there were no experiments
to confirm our predictions. The number of samples were also small. Considering these
issues, the experimental studies will be subsequently conducted later using more samples.