Thursday, July 23, 2020

Zheng-Li Shi..., Gurrl We Coming For Your Dome-Piece

counterpunch |  Our proposal is consistent with all the principal undisputed facts concerning SARS-CoV-2 and its origin. The MMP proposal has the additional benefit of reconciling many observations concerning SARS-CoV-2 that have proven difficult to reconcile with any natural zoonotic hypothesis.

For instance, using different approaches, numerous researchers have concluded that the SARS-CoV-2 spike protein has a very high affinity for the human ACE2 receptor (Walls et al., 2020; Piplani et al., 2020; Shang and Ye et al., 2020; Wrapp et al., 2020). Such exceptional affinities, ten to twenty times as great as that of the original SARS virus, do not arise at random, making it very hard to explain in any other way than for the virus to have been strongly selected in the presence of a human ACE2 receptor (Piplani et al., 2020).

In addition to this, a recent report found that the spike of RaTG13 binds the human ACE2 receptor (Shang and Ye et al., 2020). We proposed above that the virus in the mine directly infected humans lung cells. The main determinant of cell infection and species specificity of coronaviruses is initial receptor binding (Perlman and Netland, 2009). Thus RaTG13, unlike most bat coronaviruses, probably can enter and infect human cells, providing biological plausibility to the idea that the miners became infected with a coronavirus resembling RaTG13.

Moreover, the receptor binding domain (RBD) of SARS-CoV-2, which is the region of the spike that physically contacts the human ACE2 receptor, has recently been crystallised to reveal its spatial structure (Shang and Ye et al., 2020). These authors found close structural similarities between the spikes of SARS-CoV-2 and RaTG13 in how they bound the human ACE2 receptor:

“Second, as with SARS-CoV-2, bat RaTG13 RBM [a region of the RBD] contains a similar four-residue motif in the ACE2 binding ridge, supporting the notion that SARS-CoV-2 may have evolved from RaTG13 or a RaTG13-related bat coronavirus (Extended Data Table 3 and Extended Data Fig. 7). Third, the L486F, Y493Q and D501N residue changes from RaTG13 to SARS CoV-2 enhance ACE2 recognition and may have facilitated the bat-to-human transmission of SARS-CoV-2 (Extended Data Table 3 and Extended Data Fig. 7). A lysine-to-asparagine mutation at the 479 position in the SARS-CoV-2 RBD (corresponding to the 493 position in the SARS-CoV-2 RBD) enabled SARS-CoV to infect humans. Fourth, Leu455 contributes favourably to ACE2 recognition, and it is conserved between RaTG13 and SARS CoV-2; its presence in the SARS CoV-2 RBM may be important for the bat-to-human transmission of SARS-CoV-2″ (Shang and Ye et al., 2020). (italics added)

The significance of this molecular similarity is very great. Coronaviruses have evolved a diverse set of molecular solutions to solve the problem of binding ACE2 (Perlman and Netland, 2009; Forni et al., 2017). The fact that RaTG13 and SARS CoV-2 share the same solution makes RaTG13 a highly likely direct ancestor of Sars-CoV-2.

A further widely noted feature of SARS-CoV-2 is its furin site (Coutard et al., 2020). This site is absent from RaTG13 and other closely related coronaviruses. The most closely related virus with such a site is the highly lethal MERS (which broke out in 2012). Possession of a furin site enables SARS-CoV-2 (like MERS) to infect lungs and many other body tissues (such as the gastrointestinal tract and neurons), explaining much of its lethality (Hoffman et al., 2020; Lamers et al., 2020). However, no convincing explanation for how SARS-CoV-2 acquired this site has yet been offered. Our suggestion is that it arose due to the high selection pressure which existed in the miner’s lungs and which in general worked to ensure that the virus became highly adapted to the lungs. This explanation, which encompasses how SARS-CoV-2 came to target lung tissues in general, is an important aspect of our proposal.

The implication is therefore that the furin site was not acquired by recombination with another coronavirus and simply represents convergent evolution (as suggested by Andersen et al., 2020).
An intriguing alternative possibility is that SARS-CoV-2 acquired its furin site directly from the miner’s lungs. Humans possess an epithelial sodium channel protein called ENaC-a whose furin cleavage site is identical over eight amino acids to SARS-CoV-2 (Anand et al., 2020). ENaC-a protein is present in the same airway epithelial and lung tissues infected by SARS-CoV-2. It is known from plants that positive-stranded RNA viruses recombine readily with host mRNAs (Greene and Allison, 1994; Greene and Allison, 1996; Lommel and Xiong, 1991; Borja et al., 2007). The same evidence base is not available for positive-stranded animal RNA viruses, (though see Gorbalenya, 1992) but if plant viruses are a guide then acquisition of its furin site via recombination with the mRNA which encodes ENaC-a by SARS-CoV-2 is a strong possibility.

A further feature of SARS-CoV-2 has been the very limited adaptive evolution of its genome since the pandemic began (Zhan et al., 2020; van Dorp et al., 2020; Starr et al., 2020). It is a well-established principle that viruses that jump species undergo accelerated evolutionary change in their new host (e.g. Baric et al., 1997). Thus, SARS and MERS (both coronaviruses) underwent rapid and readily detectable adaptation to their new human hosts (Forni et al., 2017; Dudas and Rambaut, 2016). Such an adaptation period has not been observed for SARS-CoV-2 even though it has now infected many more individuals than SARS or MERS did. This has even led to suggestions that the SARS-CoV-2 virus had a period of cryptic circulation in humans infections that predated the pandemic (Chaw et al., 2020). The sole mutation consistently observed to accumulate across multiple studies is a D614G substitution in the spike protein (e.g. Korber et al., 2020). The numerically largest analysis of SARS-CoV-2 genomes, however, found no evidence at all for adaptive evolution, even for D614G (van Dorp et al., 2020).

The general observation is therefore that Sars-CoV-2 has remained functionally unchanged or virtually so (except for inconsequential genetic changes) since the pandemic began. This is a very important observation. It implies that SARS-CoV-2 is highly adapted across its whole set of component proteins and not just at the spike (Zhan et al., 2020). That is to say, its evolutionary leap to humans was completed before the 2019 pandemic began.

It is hard to imagine an explanation for this high adaptiveness other than some kind of passaging in a human body (Zhan et al., 2020). Not even passaging in human cells could have achieved such an outcome.

Two examples illustrate this point. In a follow up to Shang and Ye et al., (2020), a similar group of Minnesota researchers identified a distinct strategy by which the spike (S) protein (which contains the receptor bind domain; RBD) of SARS-CoV-2 evades the human immune system (Shang and Wan et al., 2020). This strategy involves more effective hiding of its RBD, but it implies again that the spike and the RBD evolved in tandem and in the presence of the human immune system (i.e. in a human body and not in tissue culture).

The Andersen authors, in their critique of a possible engineered origin for SARS-CoV-2, also stress the need for passaging in whole humans:

“Finally, the generation of the predicted O-linked glycans is also unlikely to have occurred during cell-culture passage, as such features suggest the involvement of an immune system” (Andersen et al., 2020).

The final point that we would like to make is that the principal zoonotic origin thesis is the one proposed by Andersen et al. Apart from being poorly supported this thesis is very complex. It requires two species jumps, at least two recombination events between quite distantly related coronaviruses and the physical transfer of a pangolin (having a coronavirus infection) from outside China (Andersen et al., 2020). Even then it provides no logical explanation of the adaptedness of SARS-CoV-2 across its whole genome or why the virus emerged in Wuhan.

By contrast, our MMP proposal requires only the one species jump, which is documented in the Master’s thesis. Although we do not rule out a possible role for mixed infections in the lungs of the miners, nor the possibility of recombination between closely related variants in those lungs, nor the potential acquisition of the furin site from a host mRNA, only mutation was needed to derive SARS-CoV-2 from RaTG13. Hence our attention earlier to the figure from P. Zhou et al., 2020showing that RaTG13 is the most closely related virus to SARS-CoV-2 over its entire length. This extended similarity is perfectly consistent with a mutational origin of SARS-CoV-2 from RaTG13.

In short, the MMP theory is a plausible and parsimonious explanation of all the key features of the COVID-19 pandemic and its origin. It accounts for the propensity of SARS-CoV-2 infections to target the lungs; the apparent preadapted nature of the virus; and its transmission from bats in Yunnan to humans in Wuhan.