The chromosome 22q11.2 deletion: from the unification of biomedical fields to a new kind of genetic condition.

How can genetics reshape nosology? This paper examines the way knowledge about a genetic mutation - the microdeletion at chromosomal locus 22q11.2 - transformed our understanding of several rare clinical syndromes and designated a qualitatively new population of patients. Taking the 1400 papers about the 22q11.2 deletion and the clinical conditions with which it was associated, we generate a network of papers tied by citations for each of the last 35 years. Using a modularity algorithm, we identify communities and evaluate their salience for the networks' overall structure. This analysis, supplemented by historical research and fieldwork with relevant experts and the advocates of affected children conducted during 2011-12, reveals that the 22q11.2 deletion acted as a 'boundary object' that unified clinical literatures and led to the emergence of a new kind of medical condition: 22q11.2 Deletion Syndrome (DS). The case of 22q11.2DS extends our understanding of 'genomic designation' - the delineation and diagnosis of clinically diffuse conditions according to characteristics of the genome - and demonstrates that observations from genetics can reconfigure existing categories of biomedical research and lead to the emergence of qualitatively new diagnostic categories.