Application of a Diagnostic Algorithm for Inherited Thrombocytopenia Patients in the Setting of a Developing Country

Abstract 1163

Inherited thrombocytopenias are rare genetic disorders which frequently represent a diagnostic challenge, even after extensive diagnostic work-up. This is due to the requirement of specialized laboratory tests and to the fact that the underlying disease-causing mutation remains in many cases still unknown. Increasing knowledge regarding the molecular etiology of inherited thrombocytopenias contributes both to patient diagnosis, prognosis and genetic counseling and may lead to the identification of novel regulators of platelet production. Recently, mutations in the ANKRD26 gene have been described, which underlie inherited thrombocytopenias with normal platelet size. We applied a previously proposed algorithm (Haematologica 2003; 88: 582–592) and developed a twining program in collaboration with a specialized center to improve the diagnostic yield in a cohort of patients in Argentina. Based on this algorithm, patients were classified according both to the presence or absence of clinical or laboratory features other than thrombocytopenia and to platelet size. Simple laboratory tests, such as platelet function studies, peripheral blood smear and flow cytometry were performed in all cases, while confirmatory tests were performed according to initial diagnostic suspicion, and included analysis of non-muscle myosin heavy chain IIA distribution in neutrophils by immunofluorescence and mutational screening of candidate genes, such as MYH9, RUNX1, MPL, WASP, GPIBA and ANKRD26. Studies unavailable at the local institution were performed in a specialized center in the setting of the twining program. Thirty-five patients, belonging to 14 pedigrees, were included, age 32 (4–72) years old, 20 were female. Platelet count was 83 (5–170)×10⁹/L, 49% were classified as macrothrombocytopenia, 46% had normal platelet size while 6% harboured small platelets. In the latter, a diagnosis of X-linked thrombocytopenia secondary to WASP mutation was made. Four pedigrees with macrothrombocytopenia had MYH9-related disease, one had Bernard-Soulier syndrome, while in four other, no molecular diagnosis was reached. Among pedigrees with normal platelet size, one had FPD/LMA due to RUNX1 mutation, whereas the c.-127A>G mutation in 5'UTR region of the ANKRD26 gene was detected in another family. In addition, a working diagnosis of familial ITP was made in a three-generation pedigree who had autosomal dominant thrombocytopenia and normal platelet size, based on the finding of shortened platelet life-span and complete response to splenectomy. Overall, a molecular diagnosis was established in 9 of 14 (64%) pedigrees, and MYH9-RD comprised the most frequent single disorder. Application of this diagnostic algorithm proved feasible in a resource-limited setting, as data provided by careful medical history and simple laboratory tests allowed to identify patients in whom further, specialized studies were justified. Collaboration of a specialized center was essential for molecular diagnosis and proved invaluable to build local capacities.