1. Search and Find Anti-Cancer Genes

The naked mole rat (Heterocephalus glaber)

Living strictly in subterranean colonies in the semiarid regions of East Africa including parts of Kenya, Ethiopia and Somalia, the naked mole rat is the only known eusocial mammal (Honeycutt et al. 1991). A single breeding female individual, also referred as queen, maintains her subordinates in a reproductively quiescent state. The behavioural division of labor can be compared to that of bee colonies (Jarvis, 1981).

Figure 1: The spread of the naked mole rat is limited to east Africa (displayed in blue colour)

Due to the extreme constraints of its natural subterranean habitat including full darkness, low oxygen and high carbon dioxide concentrations, the naked mole rat adapted striking physiological features that distinguish it from other rodents. Based on the average body weight centering around 35 g, naked mole rats remarkably exceed their theoretical life span of 10 years, since captive individuals of 30 years could frequently be observed (Buffenstein and Jarvis, 2002). Compared to the life span of other similarly sized rodents such as mice, the life span of a naked mole rat is 9 times higher. Furthermore naked mole rats show a delayed ageing process mainly resulting from a characteristic low metabolic and respiratory rate. The latter probably evolved as an adaption to the challenging food acquisition and the life in the oxygen poor environment (Buffenstein, 2005). The delayed aging process, also called negligible senescence, offers an interesting opportunity for studies on age related research (Buffenstein, 2008). Another outstanding feature of the naked mole rat is its unusual resistance to both spontaneous cancer and experimentally induced carcinogenesis (Liang et al. 2010, Selunov et al. 2009). The cause of this cancer resistance has recently been identified by Tian et al. (2013). It is attributed to the secretion of an extremely high molecular mass hyaluronic acid (HMM-HA) in naked mole rat fibroblasts. HMM-HA is playing a central role in our Project.

• Read More about The high molecular mass hyaluronic acid of the naked mole rat

Missing the neuropeptide Substance P in the skin the naked mole rat additionally features a reduced sense of pain (Park et al., 2008). All together the naked mole rat can be referred as an uncommon model organism in scientific research, although it is in possession of several promising traits that can lead to a more profound understanding of important biological and biomedical questions.

Properties and Synthesis of hyaluronic acid

Hyaluronic acid (HA) is a linear polysaccharide that is naturally occurring in various living organisms. The macromolecular chain consists of repeating disaccharide units of alternating D-glucuronic acid and N-acetyl-D-glucosamine and can reach variable chain lengths of up to 20.000 disaccharide units resulting in molecular weights of up to 107 Da.

HA is synthesised by the integral membrane protein hyaluronan synthase (Has). There are three different hyaluronan synthases known in vertebrates which are Has1, Has2 and Has3 (Necas, 2008). The enzyme catalyzes the linkage of UDP-D-glucuronic acid and UDP-N-acetyl-D-glucosamine to the long, linear HA chain (Garg and Hales, 2004). The energy for this process is delivered by the activated carbohydrate UDP which is used as a substrate by the hyaluronan synthases. The synthesis of the HA itself is a highly regulated process. Hyaluronidases (HAase) play also a crucial role in the regulation of HA amounts as they consequently lower the HA concentration in tissues (Necas, 2008).

As a main component of the pericellular matrix and extracellular matrix HA is ubiquitously found in human and animal connective tissues with significant amounts in skin, umbilical cord, synovial fluid, and vitreous humor. High concentrations of HA are also found in lung, kidney, brain, and muscle tissues (Garg and Hales, 2004; Necas, 2008). Due to its unique properties concerning viscoelasticity and excellent moisture retention capacity, HA is functionally involved in many biological processes mainly including hydration of tissues and lubrication of moveable parts of the body such as intervertebral discs and joints. These exceptional physiologic properties along with its high biocompatibility and non-immunogenicity have contributed to the fact that HA has found success in an extraordinarily broad range of biomedical and cosmetic applications. Local intra articular injections of HA in patients suffering from osteoarthritis for example restore the shock absorbance, reduce the degeneration of cartilage and lead to an alleviation of the painful symptoms in affected joints (Uthman et al., 2003). The glycosaminoglycane is additionally used in medicine to faciliate wound healing and dermal regeneration in skin cosmetics (Necas, 2008). Apart from this a receptor-mediated role in gene expression, proliferation, migration, tumor development and inflammation has been described (Turley et al., 2002; Toole et al., 2002; Hascall et al., 2004).

The high molecular mass hyaluronic acid of the naked mole rat

The high molecular mass hyaluronic acid (HMM-HA) of the naked mole rat shows the same basic structure composed of a repetitive disaccharide unit, built up of D-glucuronic acid and N-acetyl-D-glucosamine, compared to the human HA. However, the HMM-HA has a molecular weight of 6-12 MDa which is significantly higher than the human pendant ranging between 0,5 and 2 MDa.
The Has of the naked mole rat is in possession of 7 putative transmembrane domains and one large cytoplasmic loop which is supposed to be the catalytic center for HA Synthesis.

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In contrast to the highly conserved catalytic domain of hyaluronan synthases in other vertebrates the naked mole rats HA synthase features two unique changes in the amino acid sequence as two asparagines have been substituted by serines. This might be the cause for the raised activity of the naked mole rat´s Has.
In the recently published paper of Tian et al. (2013) the exceptionally HMM-HA of the naked mole rat is postulated to mediate the cancer resistance of the naked mole rat. In fact the resistance seems to be a side effect of the evolutionary adaptation. Thereby the anti carcinogenic effect is based on the hypersensitivity of contact inhibition, also referred as early contact inhibition (ECI) (Seluanov et al., 2009). Contact inhibition is a cellular mechanism inducing cell cycle arrest and consequently disabling cell divisions in order to counteract uncontrolled cell proliferation during tumor formation. A proposed signaling pathway for induction of ECI by HMM-HA is shown in Figure 3.

Figure 3: ECI is regulated by the HA-CD44-NF2 pathway. First of all the HMM-HA binds to the cytoplasmic membrane receptor CD44 found in human and mouse cells. CD44 activates the tumor suppressor protein Merlin encoded by the Neurofibromatosis-2 gene (NF2) which in turn mediates the early contact inhibition (Morrison et al., 2001). In contrast to contact inhibition in human and mouse ECI is though not induced by p27Kip1 but the activation of the p16INK4a pathway. Thereby cyclin dependent kinases CDK4 and CDK6 are inhibited. Cell cycle arrest is induced.

An evidence for the induction of ECI by HMM-HA was established by adding HAse to naked mole rat fibroblasts leading to reverse the effect of early contact inhibition. Moreover a decrease of the HMM-HA concentration in naked mole rat cells caused by knockdown of Has2 or overexpression of hyaluronidase combined with the simultaneous expression of the viral oncoprotein SV40 LT and H-Ras V12 in xenograft transplantations in mice showed increased tumor formation (Tian et al. 2013).

At this point the effect of HMM-HA on human cells has not been tested since the HMM-HA is not commercially available yet. The biotechnological production of the HMM-HA with its anti carcinogenic properties offers a promising approach for the future fight against cancer. Therefore the HMM-HA production is the primary objective of our iGEM project.

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