Mostly it seems that the XMRV virus has some pretty unique and mutated and restrictive factors that make it truly no longer pathogenic in mice but can be carried by some mice (MUS types) and can be carried by all mammals including humans. The PMLV's have some restrictions within both "other" mammals as well as constraints within mouse species. If I'm reading it correctly and I haven't seen the entire thing just the abstract I think they are indicating the house mice are not a potential carrier for any of the strains detected so far.
From the abstract:
We describe the geographic and species distribution of the Mus Xpr1 variants, but failed to find the X-MLV-restrictive laboratory mouse allele in any wild mouse.
That means they found most mouse species can potentially be infected by "X-MLVs", let alone XMRV, and therefore potentially could be carriers. According to the paper, that would include most house mice. (Of course, no one has yet reported finding XMRV in wild mice, though I think Coffin once said his group was looking..)
Also, the reason XMRV is not infective in (most) lab mouse strains is due less to virus mutation than to mutations in the Xpr1 receptor of those mouse strains, which basically 'rendered' certain MLVs xenotropic. XMRV appears to have gotten its envelope sequences from one of these X-MLVs, and thus has the same inability to infect those lab mouse strains.
[Btw Alex, that is why lab mice can't be infected; they don't have 'immunity' but have a mutation in their Xpr1 receptors that denies X-MLVs access to their cells.]
There are three different variants of a key part of the Xpr1 gene in house mice and one variant in lab mice; they are denoted by superscript letters after "Xpr1" in the paper, but here I'll have to settle for a dash...
Xpr1-sxv is found in Mus domesticus
(as well as most non-house wild mice) and is permissive of all X and P-MLVs.
Xpr1-c is found in house mouse species M. castaneus
. It restricts P-MLV but not X-MLV.
Xpr1-m is found in two house mouse species, including M. musculus
(eastern Europe to the Pacific), is restrictive for all P-MLV and restrictive for many, but not all, X-MLV.
Xpr1-n is restrictive for all X-MLV. They found it only in lab mouse strains (which were derived from a few of the house mouse strains).
A very interesting bit of info from Yan et al: the 4 house mouse species (incl. M. domesticus
) are the only wild mouse species that carry endogenous X-MLV and P-MLV env sequences. Not surprisingly, they are the ones with the most variants of the Xpr1 receptor! It's likely that these viruses evolved in those species. Interestingly, they entered the germ line of mice around the same time that restrictive variants "c" and "m" evolved, i.e. more recently than 500,000 years ago.
Another interesting observation in the Yan study is that the general geographic areas with the highest incidence of prostate cancer (the U.S. and Western Europe) correspond roughly to the areas inhabited by house mouse species with the most permissive variant of the Xpr1 receptor, while the reverse is true for the places with the lowest prostate cancer incidence (East Asia and Eastern Europe).
And a fun fact:
..the fact that studies on human/hamster Xpr1 chimeras implicate as yet unidentified ECL4 residues in P-MLV entry.. (44)
They have got
to stop crossing humans and hamsters! Last time they did that we wound up with that kid from "The Wonder Years".