The Mystery of the Gray Platelets: Solved

In 1971, Dr. Giovanni Raccuglia published a curious case of an 11-year-old boy with a bleeding tendency out of proportion to his thrombocytopenia.¹  His platelets appeared large, gray, and agranular upon Wright staining. The plot thickened over the next 40 years, as several dozen patients were found to be affected by the syndrome. The tell-tale signs of the disorder were large platelets with a distinct deficiency of α-granules. In contrast to other known platelet granule abnormalities, such as Hermansky-Pudlak syndrome and Chédiak-Higashi syndrome, the dense granules in victims of gray platelet syndrome remained unaffected. These findings led investigators to ask what gene was responsible for this syndrome and how the defect impaired α-granule formation without affecting the formation of dense granules. The mystery spawned a multi-national search for the defective gene. Approximately one year ago, the search intensified as a team of Israeli and American investigators narrowed the location of the causative gene to a small piece of chromosome 3p, a region that contained 165 gene suspects.²  A second team confirmed this location and narrowed the lineup of suspects. The precise identity of the culprit gene, however, remained unknown. Now, after decades of searching, the gene responsible for gray platelet syndrome (the BEACH domain-encoding gene, NBEAL2) has been finally apprehended.

In an unusual twist to a genetic caper, the identity of NBEAL2 as the causative gene in gray platelet syndrome was identified by three investigative teams using three different experimental strategies and published simultaneously. The U.S.-Israeli group performed genome-wide linkage analysis and homozygosity mapping of 25 individuals with gray platelet syndrome. Sequencing of exons in 15 unrelated affected individuals demonstrated mutations in NBEAL2. In a British-French collaboration, whole-exome sequencing of four unrelated patients was performed. Previously observed variants and those not likely to affect platelet function were filtered. All four subjects demonstrated two mutations in NBEAL2. A third team, including investigators from the United States and Canada, used next-generation RNA sequence analysis of platelets from an individual with gray platelet syndrome to identify atypical transcripts derived from genes located in a 2.7 MB region previously identified by this group to be linked to gray platelet syndrome. This analysis demonstrated intron retention in transcripts of NBEAL2 in affected subjects. Sequencing of genomic NBEAL2 in the affected individual and two other families with gray platelet syndrome confirmed that NBEAL2 was the affected gene. Together, these studies provide irrefutable evidence of the role of NBEAL2 in gray platelet syndrome.