Expanding the genotypic and phenotypic spectrum of severe serine biosynthesis disorders.

Serine biosynthesis disorders comprise a spectrum of very rare autosomal recessive inborn errors of metabolism with wide phenotypic variability. Neu-Laxova syndrome represents the most severe expression and is characterized by multiple congenital anomalies and pre- or perinatal lethality. Here, we present the mutation spectrum and a detailed phenotypic analysis in 15 unrelated families with severe types of serine biosynthesis disorders. We identified likely disease-causing variants in the PHGDH and PSAT1 genes, several of which have not been reported previously. Phenotype analysis and a comprehensive review of the literature corroborates the evidence that serine biosynthesis disorders represent a continuum with varying degrees of phenotypic expression and suggest that even gradual differences at the severe end of the spectrum may be correlated with particular genotypes. We postulate that the individual residual enzyme activity of mutant proteins is the major determinant of the phenotypic variability, but further functional studies are needed to explore effects at the enzyme protein level.

Retinitis pigmentosa, cutis laxa, and pseudoxanthoma elasticum-like skin manifestations associated with GGCX mutations.

Gamma-glutamyl carboxylase (GGCX) mutations have been reported in patients with a pseudoxanthoma elasticum (PXE)-like phenotype, loose redundant skin, and multiple vitamin K-dependent coagulation factor deficiencies. We report on the clinical findings and molecular results in 13 affected members of two families who had a uniform phenotype consisting of (PXE)-like skin manifestations in the neck and trunk, loose sagging skin of the trunk and upper limbs, and retinitis pigmentosa confirmed by electroretinographies in 10 affected individuals. There were no coagulation abnormalities. Molecular investigations of the ATP-binding cassette subfamily C member 6 did not yield causative mutations. All 13 affected family members were found to be homozygous for the splice-site mutation c.373+3G>T in the GGCX gene. All tested parents were heterozygous for the mutation, and healthy siblings were either heterozygous or had the wild type. We suggest that the present patients represent a hitherto unreported phenotype associated with GGCX mutations. Digenic inheritance has been suggested to explain the variability in phenotype in GGCX mutation carriers. Consequently, the present phenotype may not be explained only by the GGCX mutations only but may be influenced by variants in other genes or epigenetic and environmental factors.

Double de novo mutations of ELA2 in cyclic and severe congenital neutropenia.

Heterozygous mutations of ELA2, encoding the protease neutrophil elastase (NE), cause either autosomal dominant cyclic neutropenia or severe congenital neutropenia (SCN). Three hypotheses have been proposed for how allelic mutations produce these different disorders: 1) disruption of proteolytic activity; 2) mislocalization of the protein; or 3) destabilization of the protein resulting in induction of the unfolded protein response. As with other dominant diseases with reduced reproductive fitness, sporadic cases can result from new mutations not inherited from either parent. Here we report an exceptional genetic phenomenon in which both a cyclic neutropenia patient and an SCN patient each possess two new ELA2 mutations. Because of the rarity of the phenomenon, we investigated the origins of the mutations and found that both arise nonmosaically and in cis from the paternally-inherited allele. Moreover, these cases offer a unique opportunity to investigate molecular pathways distinguishing these two forms of hereditary neutropenia. We have characterized the mutants separately and in combination, with respect to their effects on proteolysis, subcellular trafficking, and induction of the unfolded protein response. Each pair of mutations acts more or less additively to produce equivalent net effects on reducing proteolytic activity and induction of the unfolded protein response, yet each has different and somewhat opposing effects on disturbing subcellular localization, thus offering support for a role for protein mistrafficking as a disease mechanism.