Detecting 22q11.2 Deletion Syndrome Using Flow Cytometry
Authors: Markham Stephen J , Bevilaqua Lisa , Zarrin Haley , McDonald-McGinn Donna , Zackai Elaine , Lambert Michele P 
Published: October 2019

Abstract
The 22q11.2 deletion syndrome (DS) is a multisystem chromosomal deletion disorder with pleotropic phenotypic presentations that may be subtle. We examine retrospectively whether we can support the diagnosis of 22q11.2DS by flow cytometric analysis of platelets for surface CD42 (GPIb/V/IX) as ~89% of these patients (with the classical deletion) should have a deficiency of GPIbbeta due to loss of one copy of the GPIBB gene. Loss of one copy of GPIBB may result in macrothrombocytopenia and a mild bleeding diathesis. We characterized the bleeding manifestations and the level of expression of the CD42 complex in patients with known 22q11.2 deletion to determine if CD42 levels could be used to assess bleeding risk. We evaluated 56 patients with 22q11.2 deletion and compared them to age-matched controls with normal platelet counts and no bleeding as well as with controls evaluated for concern for platelet disorder (platelet disorder subjects – PDS), mostly other thrombocytopenias. Comparisons were also made for other platelet parameters. Expression of the CD42b (GP1bb) varied in individuals with the 22q11.2 deletion. Although only 30/56 (54%) subjects had low CD42 expression (<70% control), CD42 expression overall in the 22q11.2 deleted patients was significantly lower than in PDS (69±5% vs 93±5% of PDS, p<0.002) or in normal controls (69±5% vs 100±5% of WT, p<0.001). For the total population (22q11.2 DS and PDS subjects), MPV and platelet count were inversely related (R2 0.18, p<0.001), but there was no correlation with either CD41 or CD42 expression. There was no significant difference in MPV or platelet count between PDS and 22q11.2DS subjects. When compared to normal control subjects, MPV was significantly higher in 22q11.2DS subjects compared to normal controls (9.5±0.3 vs 7.8±0.3, p< 0.001) and the platelet count was significantly lower (187±12 vs 295±16, p<0.001). We also examined the expression of CD41 (GPIIb) in subjects with 22q11.2DS as it would be expected that in macrothrombocytes expression levels of CD41 might be increased compared to controls since the platelets are larger than control platelets. However, there was no overall increase in CD41 expression compared to PDS subjects. However, the ratio of normalized CD41 expression to CD42 expression in subjects with 22q11.2 deletion was inversely correlated with MPV (-0.31, p=.02) and was significantly lower than in other platelet disorders (5.2±0.6 vs 4.3±0.6, p<0.05). Bleeding manifestations were reported in 19/56 patients with 22qDS and of these 9/19 (47%) had low expression of CD42b and did not correlate with level of expression of CD42 (p NS). Taken together these data demonstrate that macro-thrombocytopenia (MPV >9, plt <150) with low expression of CD42 (<70% control) and high normalized CD41:CD42 ratio (>7), the diagnosis of 22q11.2 deletion syndrome results in a specificty of 85% to detect 22q11.2DS with a positive predictive value of 67% when at least 3 of the 4 criteria are present. We suggest that if subjects evaluated for thrombocytopenia manifest this constellation, appropriate genetic testing should be considered and sent, especially given that the manifestations of the disorder can be subtle, but the consequences for management are significant. Future studies will focus on prospectively evaluating all subjects with the combination of decreased CD42 expression, macrothrombocytopenia and low normalized CD42:CD41 ratio to see if this combination can predict the presence of 22q11.2 deletion.

Disclosures
No relevant conflicts of interest to declare.