Exome sequencing identifies MPL as a causative gene in familial aplastic anemia

Abstract

The primary cause of aplastic anemia remains unknown in many patients. The aim of this study was to clarify the genetic cause of familial aplastic anemia. Genomic DNA of an affected individual from a multiplex consanguineous family was hybridized to a Nimblegen exome library before being sequenced on a GAIIx genome analyzer. Once the disease causing homozygous mutation had been confirmed in the consanguineous family, this gene was then analyzed for mutation in 33 uncharacterized index cases of aplastic anemia (<13 years) using denaturing HPLC. Abnormal traces were confirmed by direct sequencing. Exome sequencing identified a novel homozygous nonsense mutation in the thrombopoietin receptor gene MPL. An additional novel homozygous MPL mutation was identified in the screen of 33 aplastic anemia patients. This study shows for the first time a link between homozygous MPL mutations and familial aplastic anemia. It also highlights the important role of MPL in trilineage hematopoiesis.

Figure 1.

Genetic evaluation of MPL mutations in aplastic anemia. (A) Segregation of the c.1248G>A MPL mutation in Family 1 as identified by exome sequencing. Filled shapes: affected individuals; arrow: affected base. (B) The homozygous missense mutation c.1180C>T p.Pro394Ser segregates with disease in Family 2. Filled shapes: affected individuals; arrow: affected base. (C) Relative locations of the mutations identified in MPL from this study (marked by arrows) and previously published studies. The exon structure and derived protein structure with the functional domains are shown in the lower panel. Filled diamonds: nonsense mutations; open diamond: missense mutations; filled triangles: frame shift mutations; open triangle: 7bp deletion; S: splice site mutations; *: recurrent mutation with <5 mutated alleles reported; †: recurrent mutation with 5–10 mutated alleles reported; ‡: recurrent mutation with >10 mutated alleles reported; SP; signal peptide; TM; transmembrane domain. Adapted from Ballamaier et al.

Figure 2.

Telomeres are not significantly short in patients with aplastic anemia associated with biallelic MPL mutations compared with controls. Telomere lengths are short in patients with TERC mutations compared to controls.