Comprehensive clinical and molecular analysis of 12 families with type 1 recessive cutis laxa

Fibulin-4 and fibulin-5 in elastogenesis and beyond: Insights from mouse and human studies

The fibulin family of extracellular matrix/matricellular proteins is composed of long fibulins (fibulin-1, -2, -6) and short fibulins (fibulin-3, -4, -5, -7) and is involved in protein-protein interaction with the components of basement membrane and extracellular matrix proteins. Fibulin-1, -2, -3, -4, and -5 bind the monomeric form of elastin (tropoelastin) in vitro and fibulin-2, -3, -4, and -5 are shown to be involved in various aspects of elastic fiber development in vivo. In particular, fibulin-4 and -5 are critical molecules for elastic fiber assembly and play a non-redundant role during elastic fiber formation. Despite manifestation of systemic elastic fiber defects in all elastogenic tissues, fibulin-5 null (Fbln5(-/-)) mice have a normal lifespan. In contrast, fibulin-4 null (Fbln4(-/-)) mice die during the perinatal period due to rupture of aortic aneurysms, indicating differential functions of fibulin-4 and fibulin-5 in normal development. In this review, we will update biochemical characterization of fibulin-4 and fibulin-5 and discuss their roles in elastogenesis and outside of elastogenesis based on knowledge obtained from loss-of-function studies in mouse and in human patients with FBLN4 or FBLN5 mutations. Finally, we will evaluate therapeutic options for matrix-related diseases.

The microfibril hypothesis of glaucoma: implications for treatment of elevated intraocular pressure

Microfibrils are macromolecular aggregates located in the extracellular matrix of both elastic and nonelastic tissues that have essential functions in formation of elastic fibers and control of signaling through the transforming growth factor beta (TGFβ) family of cytokines. Elevation of systemic TGFβ and chronic activation of TGFβ signal transduction are associated with diseases caused by mutations in microfibril-associated genes, including FBN1. A role for microfibrils in glaucoma is suggested by identification of risk alleles in LOXL1 for exfoliation glaucoma and mutations in LTBP2 for primary congenital glaucoma, both of which are microfibril-associated genes. Recent identification of a mutation in another microfibril-associated gene, ADAMTS10, in a dog model of primary open-angle glaucoma led us to form the microfibril hypothesis of glaucoma, which in general states that defective microfibrils may be an underlying cause of glaucoma. Microfibril defects could contribute to glaucoma through alterations in biomechanical properties of tissue and/or through effects on signaling through TGFβ, which is well established to be elevated in the aqueous humor of glaucoma patients. Recent work has shown that diseases caused by microfibril defects are associated with increased concentrations of TGFβ protein and chronic activation of TGFβ-mediated signal transduction. In analogy with other microfibril-related diseases, defective microfibrils could provide a mechanism for the elevation of TGFβ2 in glaucomatous aqueous humor. If glaucoma shares mechanisms with other diseases caused by defective microfibrils, such as Marfan syndrome, therapeutic interventions to inhibit chronic activation of TGFβ signaling used in those diseases may be applied to glaucoma.

Nonsense-mediated mRNA decay: inter-individual variability and human disease

Nonsense-mediated mRNA decay (NMD) is a regulatory pathway that functions to degrade transcripts containing premature termination codons (PTCs) and to maintain normal transcriptome homeostasis. Nonsense and frameshift mutations that generate PTCs cause approximately one-third of all known human genetic diseases and thus NMD has a potentially important role in human disease. In genetic disorders in which the affected genes carry PTC-generating mutations, NMD acts as a double-edge sword. While it can benefit the patient by degrading PTC-containing mRNAs that encode detrimental, dominant-negative truncated proteins, it can also make the disease worse when a PTC-containing mRNA is degraded that encodes a mutant but still functional protein. There is evidence that the magnitude of NMD varies between individuals, which, in turn, has been shown to correlate with both clinical presentations and the patients' responses to drugs that promote read-through of PTCs. In this review, we examine the evidence supporting the existence of inter-individual variability in NMD efficiency and discuss the genetic factors that underlie this variability. We propose that inter-individual variability in NMD efficiency is a common phenomenon in human populations and that an individual's NMD efficiency should be taken into consideration when testing, developing, and making therapeutic decisions for diseases caused by genes harboring PTCs.

Genotype-phenotype spectrum of PYCR1-related autosomal recessive cutis laxa

Autosomal recessive cutis laxa type 2B (ARCL2B; OMIM # 612940) is a segmental progeroid disorder caused by mutations in PYCR1 encoding pyrroline-5-carboxylate reductase 1, which is part of the conserved proline de novo synthesis pathway. Here we describe 33 patients with PYCR1-related ARCL from 27 families with initial diagnoses varying between wrinkly skin syndrome, gerodermia osteodysplastica, De Barsy syndrome or more severe progeria syndromes. Given the difficult differential diagnosis of ARCL syndromes we performed a systematic comparison of clinical features of PYCR1-related ARCL. Intrauterine growth retardation, a characteristic triangular facial gestalt, psychomotor retardation, and hypotonia were the most relevant distinctive hallmarks of ARCL due to proline de novo synthesis defects. Corneal clouding or cataracts, athetoid movements, and finger contractures were rather rare features, but had a high predictive value. In our cohort we identified 20 different PYCR1 mutations of which seven were novel. Most of the mutations accumulated in exons 4 to 6. Missense alterations of highly conserved residues were most frequent followed by splice site changes and a single nonsense mutation. Analysis of genotype-phenotype correlation revealed that patients with mutations in the first two exons had lower average clinical scores and absent or only mild intellectual disability. Structural analyses predicted interference with PYCR1 multimerization for a subset of missense mutations. These findings have implications for the clinics as well as the pathomechanism of PYCR1-related ARCL.

Cutis laxa: intersection of elastic fiber biogenesis, TGFβ signaling, the secretory pathway and metabolism

Cutis laxa (CL), a disease characterized by redundant and inelastic skin, displays extensive locus heterogeneity. Together with geroderma osteodysplasticum and arterial tortuosity syndrome, which show phenotypic overlap with CL, eleven CL-related genes have been identified to date, which encode proteins within 3 groups. Elastin, fibulin-4, fibulin-5 and latent transforming growth factor-β-binding protein 4 are secreted proteins which form elastic fibers and are involved in the sequestration and subsequent activation of transforming growth factor-β (TGFβ). Proteins within the second group, localized to the secretory pathway, perform transport and membrane trafficking functions necessary for the modification and secretion of elastic fiber components. Key proteins include a subunit of the vacuolar-type proton pump, which ensures the efficient secretion of tropoelastin, the precursor or elastin. A copper transporter is required for the activity of lysyl oxidases, which crosslink collagen and elastin. A Rab6-interacting goglin recruits kinesin motors to Golgi-vesicles facilitating the transport from the Golgi to the plasma membrane. The Rab and Ras interactor 2 regulates the activity of Rab5, a small guanosine triphosphatase essential for the endocytosis of various cell surface receptors, including integrins. Proteins of the third group related to CL perform metabolic functions within the mitochondria, inhibiting the accumulation of reactive oxygen species. Two of these proteins catalyze subsequent steps in the conversion of glutamate to proline. The third transports dehydroascorbate into mitochondria. Recent studies on CL-related proteins highlight the intricate connections among membrane trafficking, metabolism, extracellular matrix assembly, and TGFβ signaling.

Twenty patients including 7 probands with autosomal dominant cutis laxa confirm clinical and molecular homogeneity

Background

Elastin gene mutations have been associated with a variety of phenotypes. Autosomal dominant cutis laxa (ADCL) is a rare disorder that presents with lax skin, typical facial characteristics, inguinal hernias, aortic root dilatation and pulmonary emphysema. In most patients, frameshift mutations are found in the 3’ region of the elastin gene (exons 30-34) which result in a C-terminally extended protein, though exceptions have been reported.

Methods

We clinically and molecularly characterized the thus far largest cohort of ADCL patients, consisting of 19 patients from six families and one sporadic patient.

Results

Molecular analysis showed C-terminal frameshift mutations in exon 30, 32, and 34 of the elastin gene and identified a mutational hotspot in exon 32 (c.2262delA). This cohort confirms the previously reported clinical constellation of skin laxity (100%), inguinal hernias (51%), aortic root dilatation (55%) and emphysema (37%).

Conclusion

ADCL is a clinically and molecularly homogeneous disorder, but intra- and interfamilial variability in the severity of organ involvement needs to be taken into account. Regular cardiovascular and pulmonary evaluations are imperative in the clinical follow-up of these patients.