The gray platelet syndrome is a rare inherited bleeding disorder characterized 
by macrothrombocytopenia and deficiency of alpha (α)-granules in platelets. The 
genetic defect responsible for gray platelet syndrome was recently identified in 
biallelic mutations in the NBEAL2 gene. We studied 11 consecutive families with 
inherited macrothrombocytopenia of unknown origin and α-granule deficiency. All 
of them underwent NBEAL2 DNA sequencing and evaluation of the platelet 
phenotype, including a systematic assessment of the α-granule content by 
immunofluorescence analysis for α-granule secretory proteins. We identified 9 
novel mutations hitting the two alleles of NBEAL2 in 4 probands. They included 
missense, nonsense and frameshift mutations, as well as nucleotide substitutions 
that altered the splicing mechanisms as determined at the RNA level. All the 
individuals with NBEAL2 biallelic mutations showed almost complete absence of 
platelet α-granules. Interestingly, the 13 individuals assumed to be 
asymptomatic because carriers of a mutated allele had platelet macrocytosis and 
significant reduction of the α-granule content. However, they were not 
thrombocytopenic. In the remaining 7 probands, we did not identify any NBEAL2 
alterations, suggesting that other genetic defect(s) are responsible for their 
platelet phenotype. Of note, these patients were characterized by a lower 
severity of the α-granule deficiency than individuals with two NBEAL2 mutated 
alleles. Our data extend the spectrum of mutations responsible for gray platelet 
syndrome and demonstrate that macrothrombocytopenia with α-granule deficiency is 
a genetic heterogeneous trait. In terms of practical applications, the screening 
of NBEAL2 is worthwhile only in patients with macrothrombocytopenia and severe 
reduction of the α-granules. Finally, individuals carrying one NBEAL2 mutated 
allele have mild laboratory abnormalities, suggesting that even 
haploinsufficiency has an effect on platelet phenotype.