NEW YORK (GenomeWeb) – A team from the UK has successfully sequenced the genome of a Zika virus (ZIKV) isolate obtained from a semen sample, offering an avenue for more careful exploration of genetic variations in sexually transmitted forms of the virus.
Using an Aedes albopictus mosquito cell line known as C6/36, the researchers propagated viral RNA from a semen sample collected from a man diagnosed with a Zika virus infection after a trip to the Caribbean. With supernatant from the mosquito cells and African green monkey cells exposed to the virus, they went on to do metagenomic RNA sequencing, producing ZIKV reads that mapped to an existing genome assembly for the virus.
"Sequence data for the virus isolated from a clinical semen sample align well with other sequences of ZIKV from the outbreak, including those from the Caribbean region, where the patient acquired the infection," the authors wrote in their Genome Announcements paper, published online today. "The characterization of ZIKV isolated from semen samples will help improve our understanding of possible viral polymorphisms resulting from infection in different cellular environments."
Although mosquitos remain the main mode of ZIKV infection in humans, the team noted that there is growing evidence that the virus can be passed from one human to another, including sexual transmission. Nevertheless, only short stretches of ZIKV sequence have been analyzed directly from semen samples of men infected with the virus.
"We have many unanswered questions about how Zika virus is able to be transmitted sexually, whereas similar viruses are not," the study's co-first author Barry Atkinson, a researcher with Public Health England's National Infection Service, said in a statement. "It is possible that the answers to these questions lie in the viral genome, but many more sequences from semen are required before scientists can see if there are any changes that shed light on this topic."
In an effort to take a closer look at ZIKV isolates from semen, the team took a crack at using different cell line types to isolate and grow the virus. Though the group was unable to directly transfer ZIKV from a semen sample to African green monkey Vero cells, it found that the virus could be grown in the C6/36 cell line.
Since supernatant from the mosquito cells caused cytopathic changes in the Vero cells, however, the researchers opted to sequence ZIKV from the mosquito cell line supernatant and from the supernatant of Vero cell lines exposed to that mosquito cell line culture liquid.
They used the Illumina MiSeq instrument to sequence RNA from the supernatant of mosquito cells infected with ZIKV for up to a week and Vero cells treated with mosquito cell supernatant for three days, mapping the resulting reads to the ZIKV PF13/251013-18 genome assembly.
When the team analyzed the new genome sequence data, it found that the ZIKV from the semen sample resembled Caribbean Zika viruses described in the past. The researchers suggested that additional ZIKV sequencing from semen samples may help in understanding viral adaptation to different tissue types, while providing clues about sexual transmission of the virus.