Abstracts from previous ASHG Annual Meetings are available online through the past ASHG meetings archive. In your abstract citation, use the appropriate URL for the meeting in which the abstract was presented. The following is a suggested style for citing abstracts.
Simpson R.S., Barnes P., Disruption of the microRNA pathway; (Abstract/Program #XX). Presented at the 60th Annual Meeting of The American Society of Human Genetics, Date, Location (e.g., November 6, 2010, Washington, DC).