Michael D. Tharp

Human mastocytosis is characterized by increased mast cells. It usually occurs as a sporadic disease that is often transient and limited in children and persistent or progressive in adults. The c-KIT protooncogene encodes KIT, a tyrosine kinase that is the receptor for mast cell growth factor. Because mutated KIT can transform cells, we examined c-KIT in skin lesions of 22 patients with sporadic mastocytosis and 3 patients with familial mastocytosis. All patients with adult sporadic mastocytosis had somatic c-KIT mutations in codon 816 causing substitution of valine for aspartate and spontaneous activation of mast cell growth factor receptor (P = 0.0001). A subset of four pediatric onset cases with clinically unusual disease also had codon 816 activating mutations substituting valine, tyrosine, or phenylalanine for aspartate. Typical pediatric patients lacked 816 mutations, but limited sequencing showed three of six had a novel dominant inactivating mutation substituting lysine for glutamic acid in position 839, the site of a potential salt bridge that is highly conserved in receptor tyrosine kinases. No c-KIT mutations were found in the entire coding region of three patients with familial mastocytosis. We conclude that c-KIT somatic mutations substituting valine in position 816 of KIT are characteristic of sporadic adult mastocytosis and may cause this disease. Similar mutations causing activation of the mast cell growth factor receptor are found in children apparently at risk for extensive or persistent disease. In contrast, typical pediatric mastocytosis patients lack these mutations and may express inactivating c-KIT mutations. Familial mastocytosis, however, may occur in the absence of c-KIT coding mutations.

The glycoprotein, avidin, conjugated either to the enzyme horseradish peroxidase, or to the fluorochrome dyes, fluorescein or rhodamine, identifies the granules of mast cells in both tissues and cell suspensions. In the absence of prior fixation, mast cells were not identified with conjugated avidin; however, granules released from these cells were stained with this labeled glycoprotein. The specificity of avidin for mast cells was confirmed by the absence of conjugated avidin-positive cells in the skin of mice (S1/S1d) deficient in mature dermal mast cells. Electron microscopic studies confirmed that avidin binds specifically to individual mast cell granules rather than to other cellular structures. Rodent and human mast cells were readily stained with avidin conjugated to horseradish peroxidase or to either of the fluorochrome dyes. The conjugated avidin staining technique is a reliable and simple method for identifying rodent and human mast cells, one that is useful as both an investigative and a clinical tool.

Importance: Omalizumab is indicated for the management of chronic idiopathic urticaria (CIU) (also known as chronic spontaneous urticaria) in adolescents and adults with persistent hives not controlled with antihistamines. The effectiveness of omalizumab in the real-world management of CIU is largely unknown. Objective: To quantitatively synthesize what is known about the benefits and harms of omalizumab in the real-world clinical management of CIU regarding urticaria activity, treatment response, and adverse events. Data Sources: Published observational studies (January 1, 2006, to January 1, 2018) and scientific abstracts on the effectiveness of omalizumab in CIU were identified using PubMed, Embase, Web of Science, and Cochrane search engines; references were searched to identify additional studies. Study Selection: Included studies were observational in design and included at least 1 outcome in common with other studies and at a concurrent time point of exposure to omalizumab. A total of 67 articles (35.2% of those screened) were included in the analysis. Data Extraction and Synthesis: PRISMA and MOOSE guidelines were followed; independent selection and data extraction were completed by 2 observers. Random-effects meta-analyses were performed. Main Outcomes and Measures: Main outcomes were change in weekly Urticaria Activity Score (UAS7; range, 0-42), change in Urticaria Activity Score (UAS; range 0-6) (higher score indicating worse outcome in both scales), complete and partial response rates (percentages), and adverse event rate (percentage). Results: Omalizumab therapy was associated with an improvement in UAS7 scores (-25.6 points, 95% CI, -28.2 to -23.0; P < .001; 15 studies, 294 patients), an improvement in UAS scores (-4.7 points, 95% CI, -5.0 to -4.4, P < .001; 10 studies, 1158 patients), an average complete response rate of 72.2% (95% CI, 66.1%-78.3%; P < .001; 45 studies, 1158 patients) with an additional average partial response rate of 17.8% (95% CI, 11.7%-23.9%; P < .001; 37 studies, 908 patients), and an average adverse event rate of 4.0% (95% CI, 1.0%-7.0%; P < .001; any level of severity, 47 studies, 1314 patients). Conclusions and Relevance: Benefits and safety of omalizumab in the real-world treatment of CIU meet or exceed results gleaned from clinical trials. These real-world data on omalizumab in CIU may help inform both clinical treatment expectations and policy decision making.