Reverse transcriptase

[Anton Scott Goustin]

Interesting hypothesis:  Loss of the Tropical Green Belt encourages
emergence of new viruses targeting humans.

Let me respond with evidence:
1)  HIV-1 and HIV-2 probably originated in sub-Saharan Africa.  HIV-2 is
   closely-related to the simian immunodeficiency virus (SIV), a family
   retroviruses which infects Old World monkeys.  The age of SIV is not
   known, so it is possible that disruption of the tropical rain forest
   since WWII may have accelerated the movement of SIV from one monkey
   species to another.  It is reasonable to hypothesize that the re-
   lationship of the SIV strain SIVagm to its natural host (African
   green monkey) may be old, since it does not cause disease in its
   native host.  This may be an example of a virus-host relationship
   which co-evolved to minimize morbidity-mortality of the host.  A
   prediction of your hypothesis would be that the pathogenic SIVs
   spread from hosts where they were benign into new hosts where they
   might be pathogenic.  Testable, but I don't know the answer.

2)  Not all retroviruses are nasty. For example, HTLV-I, the first human
   retrovirus, is very old and causes little or no pathology in popula-
   tions in which it is endemic.  For example, HTLV-I infection is en-
   demic in Japan, with a gradient from high to low going from the most
   southerly locations north. For example, HTLV-I seroprevalence is ~35%
   in Okinawa, and ~20% in Kyushu.  Its prevalence drops off rapidly as
   you move N in Japan.  It is virtually unknown in Hokkaido and among
   the Ainu.  Although there are ~2 million infected Japanese, only a
   small percentage suffer from the infection. About 1% of the 2 million
   develop an aggressive, often fatal, T lymphocyte tumor called adult
   T cell lymphoma (ATL).  The other 99% are generally unaffected, al-
   though another 1% develop a more mild T cell lymphoproliferative dis-
   ease.  Since HTLV-I also occurs in aboriginal populations along the
   western coasts of Canada and Alaska, it has been hypothesized that
   HTLV-I moved from Asia to America accross the Bering landbridge which
   closed more than 15,000 years ago.

   However, in support of your hypothesis, a new pathological manifesta-
   tions of HTLV-I is emerging.  It is called HTLV-associated myelopathy
   (HAM) when it occurs in Japan and other temperate regions, and it is
 called tropical spastic paraparesis (TSP) when it occurs in other areas
   such as in South America and in the Caribbean. There is some evidence
  to suggest that HTLV-I in people suffering from TSP/HAM is a divergent
   form of HTLV-I, molecularly divergent from the HTLV-I genome found in
   Japanese asymptomatics and ATL sufferers.  Thus, it is possible that
  the TSP/HAM variants of HTLV-I are rather new offshoots of the ancient
   HTLV-I which crossed Beringia over 15,000 years ago.

  Where did HTLV-I come from? The nearest relative of HTLV-I is a cattle
   virus called bovine leukemia virus (BLV), which is found in African
   cattle.  It is not too difficult to imagine a transfer of BLV to
   humans,
   due to the close relationships between man and cattle, for example
   among the Masai of E Africa.

   Again consistent with your hypothesis, both BLV and HTLV-I arerather
   benign in both man and cattle.  Your hypothesis would predict that
   fringe regions of HTLV-I or BLV endemic areas, in which the viruses
   are new, might feature increase morbidity-mortality of the 
   infections.

3) A third relationship is the relationship between HIV and bovine
   immuno-
   deficiency virus (BIV).  A new bovine lentivirus has recently been
   discovered, related to both BIV and HIV, recently published in J.Gen.
   Virol. (Chadwick et al., Vol. 76, 189 (1995)).  It is called Jembrana
   disease virus (JDV), and occurs in Indonesian cattle.  Its most close
   relative is BIV.  The pathogenesis of JDV is more severe than BIV.
   However, at this point, one cannot rule out the possibility that the     
   pathogenesis is a property of the host (Bali cattle) rather than a    
   property of the pathogen (JDV).  Perhaps you might try to read more
   in this area.  It is possible that JDV is a recent offshoot of BIV
   rather recently introduced into Bali cattle.  I know nothing more
   about the host, its history or ecology.  Perhaps the JDV-Bali cattle
   relationship might provide ripe territory for a test of your ideas.

    GOOD LUCK!

On Fri, 26 May 1995, chatski carl wrote: