Which molecular mechanism do you consider explaining the cytotoxicity of 1,4-D? do you think that TRPV1 activation with opening of a calcium-channel may represent a possible mechanism? what's your view when cytotoxicity in the nasal mucosa is 'explained' mechanistically as 'irritation’? is the statement 'no indication for cytotoxicity' made in USEPA documents at certain occasions misleading?

The molecular mechanism for the cytotoxicity of 1,4-D is unclear.  I'm not seen the data that addresses the role of TRPV1 activation in 1,4 - dioxane toxicity.  I attempted to locate the Mo et al. (2022) reference cited in the Round 1 responses by Expert 6, but was unsuccessful, based on the incomplete information provided.

With respect to "irritation" and "cytotoxicity", both are rather generic terms for which the nature of specific effects needs to be described.

Based on a search of US EPA (2013), rather than 'no indication of cytoxicity', it was concluded that liver and nasal tumours occurred in some studies in the absence of cytotoxicity. Missing, however, is a robust discussion of the significance (or not) of these observations in specific studies relative to the overall weight of evidence for the hypothesized mode of action based on predefined considerations for ranking of confidence (e.g., low, moderate or high, based, e.g., on comparison with other datasets assessed by the Agency). Outliers in individual studies are often a function of experimental variables and may not detract from the overall weight of evidence.   

The cancer-initiating or supporting mutagenic mechanisms in this setting should not likely involve a single gene.  The tumor suppressor mechanisms could potentially involve TRPV1 but alternative candidates, e.g., the p53 axis, besides other pathways, could be significant.      

The hypothesized TRPV1 hypothesis seems a very low probability MoA.  Although the effects reported in the cited Mo etal study are likely reliable, they are likely of very limited if any relevance to informing oral or inhalation systemic DX dosimetry in that the protocol used local injection of 10 ul of up to a 5% (50 mg/ml) DX solution directly into the footpad of mice.   Such systemic doses would likely not be attainable beyond the localized footpad injection site.  
It remains unclear what is the underlying MoA accounting for DX cytotoxicity other than such events importantly are absent when the DX systemic dose is insufficient to saturate either intrinsically expressed of cyp2e1. 
Cytotoxicity may be mediated by unidentified MoAs associated with high-dose specific and dose-disproportionately-high tissue concentrations of parent DX and/or high-dose specific secondary responses associated with metabolic saturation (induction of cyp2e1 and 2e1-associated oxidative stress). 
The overall nasal data (histopath, MoA, dosimetry) do not appear sufficient to rule in or out a cytotoxic MoA.  However, the high-dose specific nature of the response is consistent with a systemically-driven metabolic saturation-induction MoA hypothesis that may be further augmented by direct site of contact irritation (unlikely be relevant to human drinking water exposures or to occupational inhalation exposures in that inhalation nasal tumor exposures are unlikely to be tolerated by humans). 

The suggestion that TRPV1 may play a critical role is just an option which should be explored further. It does not provide a reliable 'explanation' at this point. The 'irritation' in the nasal mucosa, a combination of oxidative stress triggered by the chemical (DX) and a possible inflammatory response, both leading to cytotoxicity and subsequent proliferation appears very plausible. Oxidative DNA damage is an additional factor to be taken into consideration. Further work is needed to explain the irritating/cytotoxic effects of DX on a molecular level, although I do not believe that more insight into this is pivotal for the risk assessment of DX.    

If we consider high dose 1,4-DX and ROS as a mode-of-action I am uncertain on the mechanism of action that causes cytotoxicity - ROS induces pleiotropic effects that lead to cytotoxicity including DNA (genotoxic and epigenetic events), Protein oxidation, mitochondrial disruption and lipid peroxidation. Although TRPV1 activation like capsaicin may occur but this could be difficult to sort out given the many targets of ROS in a cell. Calcium channel disruption may be the final blow to a cell leading to death but how to go about pin pointing that among the background of noise from ROS will be challenging and take some time. The given data with an assumption of threshold will likely provide a RfD with adequate protection of human health.

The role of TRPV1 and other MOA work could be addressed in hepatocytes from humans to go to the species of concern.

The effects on nasal mucosa "irritation" and "no indication of cytotoxicity" are based typically on H&E staining so not surprising, there is only so much you can see in a H&E.