A post glycosylphosphatidylinositol (GPI) attachment to proteins, type 2 (PGAP2) variant identified in Mabry syndrome index cases: Molecular genetics of the prototypical inherited GPI disorder.

We report that recessive inheritance of a post-GPI attachment to proteins 2 (PGAP2) gene variant results in the hyperphosphatasia with neurologic deficit (HPMRS) phenotype described by Mabry et al., in 1970. HPMRS, or Mabry syndrome, is now known to be one of 21 inherited glycosylphosphatidylinositol (GPI) deficiencies (IGDs), or GPI biosynthesis defects (GPIBDs). Bi-allelic mutations in at least six genes result in HPMRS phenotypes. Disruption of four phosphatidylinositol glycan (PIG) biosynthesis genes, PIGV, PIGO, PIGW and PIGY, expressed in the endoplasmic reticulum, result in HPMRS 1, 2, 5 and 6; disruption of the PGAP2 and PGAP3 genes, necessary for stabilizing the association of GPI anchored proteins (AP) with the Golgi membrane, result in HPMRS 3 and 4. We used exome sequencing to identify a novel homozygous missense PGAP2 variant NM_014489.3:c.881C > T, p.Thr294Met in two index patients and targeted sequencing to identify this variant in an unrelated patient. Rescue assays were conducted in two PGAP2 deficient cell lines, PGAP2 KO cells generated by CRISPR/Cas9 and PGAP2 deficient CHO cells, in order to examine the pathogenicity of the PGAP2 variant. First, we used the CHO rescue assay to establish that the wild type PGAP2 isoform 1, translated from transcript 1, is less active than the wild type PGAP2 isoform 8, translated from transcript 12 (alternatively spliced to omit exon 3). As a result, in our variant rescue assays, we used the more active NM_001256240.2:c.698C > T, p.Thr233Met isoform 8 instead of NM_014489.3:c.881C > T, p.Thr294Met isoform 1. Flow cytometric analysis showed that restoration of cell surface CD59 and CD55 with variant PGAP2 isoform 8, driven by the weak (pTA FLAG) promoter, was less efficient than wild type isoform 8. Therefore, we conclude that recessive inheritance of c.881C > T PGAP2, expressed as the hypomorphic PGAP2 c.698C > T, p.Thr233Met isoform 8, results in prototypical Mabry phenotype, HPMRS3 (GPIBD 8 [MIM: 614207]). This study highlights the need for long-term follow up of individuals with rare diseases in order to ensure that they benefit from innovations in diagnosis and treatment.

Mutation of perinatal myosin heavy chain associated with a Carney complex variant.

BACKGROUND: Familial cardiac myxomas occur in the hereditary syndrome Carney complex. Although PRKAR1A mutations can cause the Carney complex, the disorder is genetically heterogeneous. To identify the cause of a Carney complex variant associated with distal arthrogryposis (the trismus-pseudocamptodactyly syndrome), we performed clinical and genetic studies. METHODS: A large family with familial cardiac myxomas and the trismus-pseudocamptodactyly syndrome (Family 1) was identified and clinically evaluated along with two families with trismus and pseudocamptodactyly. Genetic linkage analyses were performed with the use of microsatellite polymorphisms to determine a locus for this Carney complex variant. Positional cloning and mutational analyses of candidate genes were performed to identify the genetic cause of disease in the family with the Carney complex as well as in the families with the trismus-pseudocamptodactyly syndrome. RESULTS: Clinical evaluations demonstrated that the Carney complex cosegregated with the trismus-pseudocamptodactyly syndrome in Family 1, and genetic analyses demonstrated linkage of the disease to chromosome 17p12-p13.1 (maximum multipoint lod score, 4.39). Sequence analysis revealed a missense mutation (Arg674Gln) in the perinatal myosin heavy-chain gene (MYH8). The same mutation was also found in the two families with the trismus-pseudocamptodactyly syndrome. Arg674 is highly conserved evolutionarily, localizes to the actin-binding domain of the perinatal myosin head, and is close to the ATP-binding site. We identified nonsynonymous MYH8 polymorphisms in patients with cardiac myxoma syndromes but without arthrogryposis. CONCLUSIONS: We describe a novel heart-hand syndrome involving familial cardiac myxomas and distal arthrogryposis and demonstrate that these disorders are caused by a founder mutation in the MYH8 gene. Our findings demonstrate novel roles for perinatal myosin in both the development of skeletal muscle and cardiac tumorigenesis.