Metabolic cutis laxa syndromes

In 1962 Menkes described an X-linked recessive disorder later to be known as Menkes disease (MD; MIM 309400), characterized by kinky hair, growth retardation and progressive neurological dysfunction with a fatal course. The characteristic abnormality of the scalp hair is caused by a combination of broken (trichorrhexis nodosa) and twisted (pili torti) hair of varying diameter of the hairshaft (Menkes et al. 1962) (Fig. 3c).

Subsequently, the classical phenotype was broadened by reporting hypothermia, skeletal anomalies and vascular defects due to a generalized connective tissue disorder in these patients. The physiopathology is entrapment of copper in the intestinal mucosa due to a defective transport mechanism, leading to systemic copper deficiency in multiple organs, especially in the brain (Danks et al. 1972).

MD is caused by a defect in the ATP7A gene (MIM 300011) leading to dysfunction of the ATP7A protein, a copper-transporting ATPase expressed in almost all tissues except the liver (Vulpe et al. 1993; Chelly et al. 1993; Mercer et al. 1993). Normally localized in the trans-Golgi network, it supplies the excreted copper-dependent enzymes with copper, facilitates the ATP fueled efflux in cells and the uptake of copper in the small intestine (Yamaguchi et al. 1996; Camakaris et al. 1995). In conditions where copper is increased it translocates to the plasma membrane and this copper efflux restores homeostasis (Petris et al. 1996). Dysfunction of this mechanism leads to decreased activity of the copper-dependent enzymes and deficiency of copper in the brain due to lack of transport across the blood-brain barrier. Failure of these copper-transport mechanisms has severe consequences. The classical Menkes phenotype leads to death in the first years. Since the initial description many other symptoms have been associated with MD (Table 1) and besides the classical, lethal phenotype milder forms have been described. Occipital Horn Syndrome (OHS MIM 304150 or X-linked cutis laxa), the mildest Menkes phenotype, manifests with cutis laxa (Fig. 3a), hernias, diverticula of the gastrointestinal tract and bladder, tortuosity of the arteries, hyperlaxity of the joints and skeletal anomalies. The neurological symptoms are less severe and life expectancy is longer than in classical Menkes disease. The most characteristic feature of OHS is the formation of occipital exostoses due to calcification of the attachments of the trapezius and sternocleidomastoid muscles to the occipital bone (Proud et al. 1996).

To date more than 357 mutations (not all published) have been discovered in MD and OHS with great clinical heterogeneity, inter- and intrafamilial (Møller et al. 2009).

Diagnosis is based on clinical features, low serum copper and ceruloplasmin. Sequencing of the ATP7A gene is however the diagnostic gold standard. Unfortunately, treatment options are very limited and mostly symptomatic. Recent studies suggest that parenteral or subcutaneous copper administration could prolong survival in patients with partially active ATP7A function, when initiated soon after birth (Kaler et al. 2008).

Congenital disorders of glycosylation (CDG) form a heterogeneous group of inborn errors of metabolism characterized by a defective biosynthesis of glycans. These disorders are often multisystemic. The protein N-glycosylation disorders can be subdivided in type I and II disorders based on the transferrin isoelectric focusing (TIEF) pattern. There are two main types of protein glycosylation of clinical significance: the N-glycosylation and the O-glycosylation. N-glycosylation starts in the cytoplasm and endoplasmatic reticulum (ER) where an oligosaccharide is assembled on a dolichol carrier and transferred to the nascent protein, which is then processed in the Golgi. O-glycosylation is limited to the Golgi. Defects in the N-glycosylation pathway generally show abnormal plasma TIEF profiles, whereas mucin type O-glycosylation defects lead to abnormal isofocusing of plasma apolipoprotein C-III (apoC-III) (Wopereis et al. 2006) .

Defects in the Conserved Oligomeric Golgi complex (COG) are mostly responsible for the recent expansion of novel CDG subtypes. COG7-CDG (CDG type IIe, MIM 608779) is caused by a mutation in subunit seven of the COG complex existing of eight subunits in total (Morava et al. 2007; Ng et al. 2007; Spaapen et al. 2005; Wu et al. 2004). The defect localized in the Golgi results in a combined N- and O-glycosylation defect.

To date six patients with COG7-CDG, all with a fatal outcome, have been described. All affected patients were from consanguineous descent. The clinical characteristics are growth retardation, microcephaly (Fig. 3b), hypotonia, cutis laxa or wrinkled skin, hepato-splenomegaly, skeletal dysplasia, cardiac anomalies, recurrent infections, feeding problems, hyperthermia, adducted thumbs, mental retardation, seizures and structural brain abnormalities. Liver function is disturbed leading to elevated serum transaminases and bilirubin (Morava et al. 2007; Ng et al. 2007; Spaapen et al. 2005; Wu et al. 2004).

With the discovery of COG7-CDG, the first association between cutis laxa and glycosylation was discovered (Wu et al. 2004). So far no histological evaluation is available proving abnormal elastin structure underlying the skin anomalies in COG7-CDG patients. It is hypothesized that hypoglycosylation of extracellular matrix proteins, elastin and/or collagen, could be causative.

Autosomal recessive cutis laxa type IIA (ARCL2A, Debré type MIM 219200) and wrinkly skin syndrome (WSS; MIM 278250) are characterized by excessive skin wrinkling or congenital cutis laxa, intrauterine growth retardation, failure to thrive, microcephaly, multiple dysmorphic features (Fig. 3d) and a large and late closing fontanel (Morava et al. 2005). Additionally, hypotonia, feeding problems, hyperlaxity of joints and an abnormal fat distribution might occur (Morava et al. 2008). The most common ocular anomalies are strabismus and myopia. Congenital hip dislocations and decreased bone mineral density are frequent skeletal findings. Developmental delay is observed in the majority of patients (mainly due to muscle hypotonia and joint laxity) but brain anomalies are rare. Neurological symptoms include psychomotor retardation, seizures, hearing loss and cobble-stone like brain dysgenesis (Van Maldergem et al. 2008; Morava et al. 2009b). In this unique form of cutis laxa syndrome the skin features seem to improve over time.

The initial finding of a combined N- and O-linked glycosylation defect in children diagnosed with ARCL2 made the first link between CDG and extracellular matrix anomalies in cutis laxa patients (Morava et al. 2006). All affected patients showed a type 2 pattern on transferrin isoelectrofocusing (TIEF), abnormal isoelectrofocusing of apolipoprotein C-III (apoC-III) and abnormal mass spectrometry of glycans of total serum proteins (Morava et al. 2008; Kornak et al. 2008).

In the other group of children demonstrating wrinkled skin, a Golgi transport defect causing COG7-CDG, supplied further pathogenetic evidence for the role of glycosylation in the normal maturation of extracellular matrix proteins. Consequently, CDG screening was performed in a large group of consanguineous patients with ARCL2. Genetic analysis led to the discovery of different loss of function mutations in the vesicular H⁺-ATPase subunit a2, ATP6V0A2 (Kornak et al. 2008). Since then several different mutations have been detected in WSS and ARCL2 without a clear genotype-phenotype correlation (Hucthagowder et al. 2009). The mechanism, leading to cutis laxa in ATP6V0A2-CDG patients has not yet been fully unraveled. Possible theories include an abnormal transport of extracellular matrix (ECM) proteins due to the v-ATPase related proton pump defect (Kornak et al. 2008; Guillard et al. 2009), delayed secretion of elastin and excessive storage of immature compounds like tropoelastin in the Golgi (Hucthagowder et al. 2009) and abnormal function and transport of glycosyltransferases leading to a hypoglycosylated state of diverse extracellular matrix components (Kornak et al. 2008; Hucthagowder et al. 2009).

Δ¹-pyrroline-5-carboxylate synthase (P5CS) (Fig. 2) catalyses the reduction of glutamate to Δ¹-pyrroline-5-carboxylate (P5C), a precursor of proline and ornithine. Deficiency of P5CS leads to decreased levels of proline and ornithine in some patients. Proline, apart from its role in protein synthesis, is hypothesized to play a role in neurotransmission. Ornithine is an important intermediate in the urea cycle. Two isoforms of P5CS, generated by alternative splicing, are known: the short isoform mainly participates in arginine production in the gut whereas the long isoform is important for proline synthesis in various tissues (Phang et al. 1995; Valle and Simell 1995).

Deficiency of P5CS was first described in two Algerian siblings with progressive neurodegeneration, joint laxity, skin hyperlaxity and bilateral subcapsular cataracts.

A missense mutation, R84Q, altering a conserved residue in the P5CS γ-glutamyl kinase domain and thereby reducing the activity of both isoforms was found in the two siblings (Kamoun et al. 1998; Baumgartner et al. 2000).

The clinical phenotype in P5CS deficiency (MIM 138250) is characterized by cutis laxa, microcephaly, bilateral subcapsular cataract, severe mental retardation, joint laxity and hypotonia, structural brain abnormalities, progressive neurodegeneration, seizures, peripheral neuropathy and dystonic movements in hands and feet (Baumgartner et al. 2005). Biochemically, P5CS deficiency leads to a very typical pattern of paradoxical hyperammonemia as well as hypoprolinaemia and deficiency of urea cycle intermediates (ornithine, citrulline and arginine). Two pathophysiological explanations have been suggested for the clinical findings in P5CS deficiency. First, protein synthesis could be impaired in tissues depending highly on the endogenous production of proline, for example the production of collagen in connective tissues leading to cutis laxa (Smith and Phang 1978). But in this case rather an Ehlers-Danlos-like phenotype should be expected. The presence of a proline transporter in synaptic vesicles and axon terminals of glutamatergic neurons suggest a role of proline in neurotransmission. This might explain the severe mental retardation in the patients (Shafqat et al. 1995). Alternatively, proline has been suggested to play a role in protecting cells from oxidative stress (Phang et al. 2008). Errors in proline synthesis could therefore lead to developmental defects through increased apoptosis. Low levels of ornithine impair the urea cycle and thus lead to low serum levels of citrulline and arginine, and hyperammonemia. Following a meal, dietary arginine will temporarily restore the urea cycle, explaining the paradoxical character of the hyperammonemia.

Bicknell et al. described a family with four members affected by autosomal recessive cutis laxa due to a C-terminal P5CS missense mutation (Bicknell et al. 2008). Detailed biochemical serum analysis only revealed mildly low ornithine levels under fasting conditions, but no significantly altered proline concentrations. Furthermore, the rate of proline biosynthesis from glutamate was normal in patient fibroblasts. In contrast, the clinical presentation was highly similar to the previously reported cases casting doubt on the role of proline synthesis itself in the disease mechanism.

Only a few patients have been reported, and evidence-based treatment options are thus very limited. Trials with ornithine treatment showed poor results (Baumgartner et al. 2005).

Pyrroline-5-carboxylate reductase 1 (PYCR1) catalyses the reduction of Δ¹-pyrroline-5-carboxylate (P5C) to proline, the final step in the synthesis of proline from glutamate (Fig. 2) (Adams and Frank 1980). Like P5CS, it plays a role in the biosynthesis of proline. PYCR1 is most highly expressed in bone and skin and is localized in mitochondria (Fig. 1).

Clinical features in PYCR1 deficient individuals (MIM 179035) include cutis laxa (most pronounced on the dorsum of hands and feet), dysmorphic features, osteopenia, distal arthrogryposis, joint laxity, hernias, mild mental retardation, athetoid movements and aplasia or hypoplasia of the corpus callosum. Although hypoprolinaemia would be expected, based on PYCR1 function, serum proline levels in PYCR1 deficient patients were within the normal range (Reversade et al. 2009). In urine, low-normal levels were found.

Guernsey et al. (2009) described a homozygous mutation in the PYCR1 gene, localized on chromosome 17, in five members of two Canadian families. This missense mutation, R266Q, leads to a premature termination of the PYCR1 gene product leading to loss of function. Eight additional mutations have been found in 35 members of 22 families. PYCR1 mutations have been found in various cutis laxa syndrome phenotypes, including De Barsy syndrome, Wrinkly Skin syndrome and also in ARCL2 patients, showing a similar clinical presentation as those with ATP6V0A2 defect (Leao-Teles et al. 2010).

Upon oxidative stress PYCR1-deficient fibroblasts showed low mitochondrial membrane potential, a disruption of the mitochondrial network, and increased apoptosis rates. Likewise, knockdown of PYCR1 in Xenopus and zebrafish entailed epidermal hypoplasia and blistering, accompanied by a massive increase in apoptosis (Reversade et al. 2009). Addition of exogenous proline did not influence these effects observed in vitro and in vivo.

As discussed for P5CS, it is not sure that proline synthesis plays the main role in the disease process. No treatment is available.

Basel-Vanagaite et al. recently described a new elastic tissue disorder in three patients from two related consanguineous Israeli-Arab families (Basel-Vanagaite et al. 2009). Clinical features included soft, redundant facial skin, macrocephaly, downslanting palpebral fissures, puffy eyelids, sagging cheeks, everted lower lip, micrognathia and abnormal skull morphology, gingival hypertrophy, irregular dentition, sparse scalp hair and severe joint hyperlaxity and scoliosis. Psychomotor development was normal. The syndrome was termed MACS: macrocephaly, alopecia, cutis laxa and scoliosis (MIM 613075). Genetic analysis revealed a single base pair deletion in the RIN2 gene on chromosome 20. Afterwards a very similar phenotype was described in three siblings from a consanguineous Algerian family (Syx et al. 2010). A deletion in the RIN2 gene was found in these patients.

The gene product, the Ras and Rab interactor 2 protein, has been proven to interact with the Rab5 protein, which is required for endosomal trafficking (Saito et al. 2002). Endosomal trafficking was not impaired in RIN2-deficient patients, but it has been suggested that RIN2-deficiency (MIM 610222) is responsible for the phenotype through impairment of other trafficking pathways, eg ER-to-Golgi or Golgi-to-plasma membrane (Syx et al. 2010). Fibroblasts of the patients reported so far showed abnormal morphology of the Golgi apparatus with swollen cisternae and vacuole accumulation. Biochemical investigations in one of the patients showed normal plasma N-glycans and variable alterations of the mucin type O-glycan Apolipoprotein C-III₁, similar to the profile seen in patients with abnormal Golgi-related O-glycosylation (Albrecht et al. 2010; Wopereis et al. 2003, 2006). Besides their role for endosome function RIN proteins have also been described as a regulator of the Ras-pathway. This function could explain the weak overlap between MACS syndrome and Costello syndrome (sparse hair, prominent lips, cutis laxa)(see below).

Transaldolase defiency, caused by a mutation in the TALDO1 gene (MIM 602063), presents with aortic coarctation, bleeding tendency, hepatosplenomegaly, telangiectasias of the skin and enlarged clitoris. Biochemical investigations showed thrombocytopenia and elevated concentrations of ribitol, D-arabitol and erythritol in urine and plasma. Recently erythronic acid was described as a novel biomarker for this disease (Engelke et al. 2010). One of the patients with this rare condition also showed severe cutis laxa phenotype. So far, there is no evidence, whether the association of TALDO1 defect and cutis laxa is a coincidence (Valayannopoulos et al. 2006).

Heterozygous germline mutations in genes encoding enzymes and regualators of the Ras-MAPK pathway cause Noonan syndrome (MIM 163950) and the related disorders Cardio-Facio-Cutaneous (MIM 115150), LEOPARD (MIM 151100 and 611554) and Costello syndrome (MIM 218040) (Gelb and Tartaglia 2006). These are clinically characterized by distinctive facial appearance, heart defects, ectodermal abnormalities, variable cognitive defects, motor delay and susceptibility to certain malignancies. Mutations in several genes (PTPN11, RAF, NF, HRAS, BRAF, MEK1/2, KRAS, SOS, SHOC2, NRAS and RAF1) have been identified in patients, all regulating the Ras-MAPK cascade (Sarkozy et al. 2009). Several patients have been reported with wrinkled skin and cutis laxa, especially in the inguinal, axillary regions, and affecting the hands and feet, caused by diverse mutations associated with Noonan, Cardio-Facio-Cutaneous or Costello syndromes (Kleefstra et al. 2011).

Gerodermia osteodysplastica (MIM 231070) is a clinical syndrome first described in 1950 by Bamatter et al. (Bamatter et al. 1950). It is characterized by lax, wrinkled skin, joint hyperlaxity, progeroid facial appearance, severe osteoporosis, malar and mandibular hypoplasia and growth retardation. The disorder is also known as ‘Walt Disney dwarfism’ due to the typical facial features (Bamatter et al. 1950). In a recent study by Hennies et al., genetic analysis of affected individuals from 13 families revealed nine pathogenic mutations in the GORAB (SCYL1BP1) gene (Hennies et al. 2008). The gene product GORAB interacts with Rab6, a protein involved in intracellular trafficking (Grigoriev et al. 2007). Interestingly, Rab6 has been demonstrated to interact with the conserved oligomeric Golgi complex (COG) (Sun et al. 2007). However, glycosylation defects have not been observed in GORAB deficiency (MIM 607983) (Hennies et al. 2008).