Constitutive skipping of alternatively spliced exon 10 in the ATP7A gene abolishes Golgi localization of the menkes protein and produces the occipital horn syndrome

The molecular and cellular basis of copper dysregulation and its relationship with human pathologies

Copper (Cu) is an essential micronutrient required for the activity of redox-active enzymes involved in critical metabolic reactions, signaling pathways, and biological functions. Transporters and chaperones control Cu ion levels and bioavailability to ensure proper subcellular and systemic Cu distribution. Intensive research has focused on understanding how mammalian cells maintain Cu homeostasis, and how molecular signals coordinate Cu acquisition and storage within organs. In humans, mutations of genes that regulate Cu homeostasis or facilitate interactions with Cu ions lead to numerous pathologic conditions. Malfunctions of the Cu⁺ -transporting ATPases ATP7A and ATP7B cause Menkes disease and Wilson disease, respectively. Additionally, defects in the mitochondrial and cellular distributions and homeostasis of Cu lead to severe neurodegenerative conditions, mitochondrial myopathies, and metabolic diseases. Cu has a dual nature in carcinogenesis as a promotor of tumor growth and an inducer of redox stress in cancer cells. Cu also plays role in cancer treatment as a component of drugs and a regulator of drug sensitivity and uptake. In this review, we provide an overview of the current knowledge of Cu metabolism and transport and its relation to various human pathologies.

Occipital Horn Syndrome as a Result of Splice Site Mutations in ATP7A. No Activity of ATP7A Splice Variants Missing Exon 10 or Exon 15

Disease-causing variants in ATP7A lead to two different phenotypes associated with copper deficiency; a lethal form called Menkes disease (MD), leading to early death, and a much milder form called occipital horn syndrome (OHS). Some investigators have proposed that an ATP7A transcript missing exon 10 leads to a partly active protein product resulting in the OHS phenotype. Here, we describe an individual with OHS, a biology professor, who survived until age 62 despite a splice site mutation, leading to skipping of exon 15. ATP7A transcripts missing exon 10, or exon 15 preserve the reading frame, but it is unknown if either of these alternative transcripts encode functional protein variants. We have investigated the molecular consequence of splice site mutations leading to skipping of exon 10 or exon 15 which have been identified in individuals with OHS, or MD. By comparing ATP7A expression in fibroblasts from three individuals with OHS (OHS-fibroblasts) to ATP7A expression in fibroblasts from two individuals with MD (MD-fibroblasts), we demonstrate that transcripts missing either exon 10 or exon 15 were present in similar amounts in OHS-fibroblasts and MD-fibroblasts. No ATP7A protein encoded from these transcripts could be detected in the OHS and MD fibroblast. These results, combined with the observation that constructs encoding ATP7A cDNA sequences missing either exon 10, or exon 15 were unable to complement the high iron requirement of the ccc2Δ yeast strain, provide evidence that neither a transcript missing exon 10 nor a transcript missing exon 15 results in functional ATP7A protein. In contrast, higher amounts of wild-type ATP7A transcript were present in the OHS-fibroblasts compared with the MD-fibroblasts. We found that the MD-fibroblasts contained between 0 and 0.5% of wild-type ATP7A transcript, whereas the OHS-fibroblasts contained between 3 and 5% wild-type transcripts compared with the control fibroblasts. In summary these results indicate that protein variants encoded by ATP7A transcripts missing either exon 10 or exon 15 are not functional and not responsible for the OHS phenotype. In contrast, expression of only 3-5% of wild-type transcript compared with the controls permits the OHS phenotype.

ATP7A-Regulated Enzyme Metalation and Trafficking in the Menkes Disease Puzzle

Copper is vital for numerous cellular functions affecting all tissues and organ systems in the body. The copper pump, ATP7A is critical for whole-body, cellular, and subcellular copper homeostasis, and dysfunction due to genetic defects results in Menkes disease. ATP7A dysfunction leads to copper deficiency in nervous tissue, liver, and blood but accumulation in other tissues. Site-specific cellular deficiencies of copper lead to loss of function of copper-dependent enzymes in all tissues, and the range of Menkes disease pathologies observed can now be explained in full by lack of specific copper enzymes. New pathways involving copper activated lysosomal and steroid sulfatases link patient symptoms usually related to other inborn errors of metabolism to Menkes disease. Additionally, new roles for lysyl oxidase in activation of molecules necessary for the innate immune system, and novel adapter molecules that play roles in ERGIC trafficking of brain receptors and other proteins, are emerging. We here summarize the current knowledge of the roles of copper enzyme function in Menkes disease, with a focus on ATP7A-mediated enzyme metalation in the secretory pathway. By establishing mechanistic relationships between copper-dependent cellular processes and Menkes disease symptoms in patients will not only increase understanding of copper biology but will also allow for the identification of an expanding range of copper-dependent enzymes and pathways. This will raise awareness of rare patient symptoms, and thus aid in early diagnosis of Menkes disease patients.

Defining the Clinical, Molecular and Ultrastructural Characteristics in Occipital Horn Syndrome: Two New Cases and Review of the Literature

Occipital horn syndrome (OHS) is a rare connective tissue disorder caused by pathogenic variants in ATP7A, encoding a copper transporter. The main clinical features, including cutis laxa, bony exostoses, and bladder diverticula are attributed to a decreased activity of lysyl oxidase (LOX), a cupro-enzyme involved in collagen crosslinking. The absence of large case series and natural history studies precludes efficient diagnosis and management of OHS patients. This study describes the clinical and molecular characteristics of two new patients and 32 patients previously reported in the literature. We report on the need for long-term specialized care and follow-up, in which MR angiography, echocardiography and spirometry should be incorporated into standard follow-up guidelines for OHS patients, next to neurodevelopmental, orthopedic and urological follow-up. Furthermore, we report on ultrastructural abnormalities including increased collagen diameter, mild elastic fiber abnormalities and multiple autophagolysosomes reflecting the role of lysyl oxidase and defective ATP7A trafficking as pathomechanisms of OHS.

Characterization of ATP7A missense mutants suggests a correlation between intracellular trafficking and severity of Menkes disease

Menkes disease (MD) is caused by mutations in ATP7A, encoding a copper-transporting P-type ATPase which exhibits copper-dependent trafficking. ATP7A is found in the Trans-Golgi Network (TGN) at low copper concentrations, and in the post-Golgi compartments and the plasma membrane at higher concentrations. Here we have analyzed the effect of 36 ATP7A missense mutations identified in phenotypically different MD patients. Nine mutations identified in patients with severe MD, virtually eliminated ATP7A synthesis, in most cases due to aberrant RNA splicing. A group of 21 predominantly severe mutations led to trapping of the protein in TGN and displayed essentially no activity in a yeast-based functional assay. These were predicted to inhibit the catalytic phosphorylation of the protein. Four mutants showed diffuse post-TGN localization, while two displayed copper dependent trafficking. These six variants were identified in patients with mild MD and typically displayed activity in the yeast assay. The four post-TGN located mutants were presumably affected in the catalytic dephosphorylation of the protein. Together these results indicate that the severity of MD correlate with cellular localization of ATP7A and support previous studies indicating that phosphorylation is crucial for the exit of ATP7A from TGN, while dephosphorylation is crucial for recycling back to TGN.

Mottled Mice and Non-Mammalian Models of Menkes Disease

Menkes disease is a multi-systemic copper metabolism disorder caused by mutations in the X-linked ATP7A gene and characterized by progressive neurodegeneration and severe connective tissue defects. The ATP7A protein is a copper (Cu)-transporting ATPase expressed in all tissues and plays a critical role in the maintenance of copper homeostasis in cells of the whole body. ATP7A participates in copper absorption in the small intestine and in copper transport to the central nervous system (CNS) across the blood-brain-barrier (BBB) and blood–cerebrospinal fluid barrier (BCSFB). Cu is essential for synaptogenesis and axonal development. In cells, ATP7A participates in the incorporation of copper into Cu-dependent enzymes during the course of its maturation in the secretory pathway. There is a high degree of homology (>80%) between the human ATP7A and murine Atp7a genes. Mice with mutations in the Atp7a gene, called mottled mutants, are well-established and excellent models of Menkes disease. Mottled mutants closely recapitulate the Menkes phenotype and are invaluable for studying Cu-metabolism. They provide useful models for exploring and testing new forms of therapy in Menkes disease. Recently, non-mammalian models of Menkes disease, Drosophila melanogaster and Danio rerio mutants were used in experiments which would be technically difficult to carry out in mammals.

Twenty-five novel mutations including duplications in the ATP7A gene

Twenty-five novel mutations including duplications in the ATP7A gene. Menkes disease (MD) and occipital horn syndrome (OHS) are allelic X-linked recessive copper deficiency disorders resulting from ATP7A gene mutations. MD is a severe condition leading to progressive neurological degeneration and death in early childhood, whereas OHS has a milder phenotype with mainly connective tissue abnormalities. Until now, molecular analyses have revealed only deletions and point mutations in both diseases. This study reports new molecular data in a series of 40 patients referred for either MD or OHS. We describe 23 point mutations (9 missense mutations, 7 splice site variants, 4 nonsense mutations, and 3 small insertions or deletions) and 7 intragenic deletions. Of these, 18 point mutations and 3 deletions are novel. Furthermore, our finding of four whole exon duplications enlarges the mutation spectrum in the ATP7A gene. ATP7A alterations were found in 85% of cases. Of these alterations, two thirds were point mutations and the remaining one third consisted of large rearrangements. We found that 66.6% of point mutations resulted in impaired ATP7A transcript splicing, a phenomenon more frequent than expected. This finding enabled us to confirm the pathogenic role of ATP7A mutations, particularly in missense and splice site variants.

Role of the P-Type ATPases, ATP7A and ATP7B in brain copper homeostasis

Over the past two decades there have been significant advances in our understanding of copper homeostasis and the pathological consequences of copper dysregulation. Cumulative evidence is revealing a complex regulatory network of proteins and pathways that maintain copper homeostasis. The recognition of copper dysregulation as a key pathological feature in prominent neurodegenerative disorders such as Alzheimer's, Parkinson's, and prion diseases has led to increased research focus on the mechanisms controlling copper homeostasis in the brain. The copper-transporting P-type ATPases (copper-ATPases), ATP7A and ATP7B, are critical components of the copper regulatory network. Our understanding of the biochemistry and cell biology of these complex proteins has grown significantly since their discovery in 1993. They are large polytopic transmembrane proteins with six copper-binding motifs within the cytoplasmic N-terminal domain, eight transmembrane domains, and highly conserved catalytic domains. These proteins catalyze ATP-dependent copper transport across cell membranes for the metallation of many essential cuproenzymes, as well as for the removal of excess cellular copper to prevent copper toxicity. A key functional aspect of these copper transporters is their copper-responsive trafficking between the trans-Golgi network and the cell periphery. ATP7A- and ATP7B-deficiency, due to genetic mutation, underlie the inherited copper transport disorders, Menkes and Wilson diseases, respectively. Their importance in maintaining brain copper homeostasis is underscored by the severe neuropathological deficits in these disorders. Herein we will review and update our current knowledge of these copper transporters in the brain and the central nervous system, their distribution and regulation, their role in normal brain copper homeostasis, and how their absence or dysfunction contributes to disturbances in copper homeostasis and neurodegeneration.

An overview and update of ATP7A mutations leading to Menkes disease and occipital horn syndrome

Menkes disease (MD) is a lethal multisystemic disorder of copper metabolism. Progressive neurodegeneration and connective tissue disturbances, together with the peculiar "kinky" hair, are the main manifestations. MD is inherited as an X-linked recessive trait, and as expected the vast majority of patients are males. MD occurs because of mutations in the ATP7A gene and the vast majority of ATP7A mutations are intragenic mutations or partial gene deletions. ATP7A is an energy-dependent transmembrane protein, which is involved in the delivery of copper to the secreted copper enzymes and in the export of surplus copper from cells. Severely affected MD patients die usually before the third year of life. A cure for the disease does not exist, but very early copper-histidine treatment may correct some of the neurological symptoms. This study reviews 274 published and 18 novel disease causing mutations identified in 370 unrelated MD patients, nonpathogenic variants of ATP7A, functional studies of the ATP7A mutations, and animal models of MD.

Involvement of the molecular chaperone Hspa5 in copper homeostasis in astrocytes

Copper (Cu) ion availability in tissues and cells must be closely regulated within safe limits by Cu transporters and chaperones. Astrocytes play key roles in metal homeostasis and distribution in the brain that are only partially understood. The purpose of this study was to define the role that the protein chaperone Hspa5, also known as Grp78, plays in Cu homeostasis in astrocytes. First passage cultures of primary astrocytes from neonatal rats and cultures of the C6 rat glioma cells were used as models. We found that the level of Cu accumulation in astrocyte cultures increased with Cu concentrations in the medium, and Cu treatment significantly reduced cellular levels of iron (Fe), manganese (Mn) and zinc (Zn). Cu accumulation specifically induced protein expression of Hspa5 but not metallothioneins (MTs) in astrocytes. In C6 cells, Hspa5 was identified as one component of a Cu-binding complex and shown to directly bind Cu. However, the level of Hspa5 expression was not proportional to Cu accumulation in astrocytes and C6 cells: astrocytes expressed low protein levels of Hspa5 compared to C6 cells but accumulated significantly more Cu than did C6 cells. Consistent with this finding, astrocytes expressed a lower level of the Cu-extruding protein Atp7a than did C6 cells, and depletion of Hspa5 by RNA interference resulted in significantly increased Cu accumulation and induction of MT1/2 expression. These data demonstrate that Hspa5 is involved in Cu homeostasis in astrocytes but not as a Cu storage protein.

ATP7A-related copper transport diseases-emerging concepts and future trends

This Review summarizes recent advances in understanding copper-transporting ATPase 1 (ATP7A), and examines the neurological phenotypes associated with dysfunction of this protein. Involvement of ATP7A in axonal outgrowth, synapse integrity and neuronal activation underscores the fundamental importance of copper metabolism to neurological function. Defects in ATP7A cause Menkes disease, an infantile-onset, lethal condition. Neonatal diagnosis and early treatment with copper injections enhance survival in patients with this disease, and can normalize clinical outcomes if mutant ATP7A molecules retain small amounts of residual activity. Gene replacement rescues a mouse model of Menkes disease, suggesting a potential therapeutic approach for patients with complete loss-of-function ATP7A mutations. Remarkably, a newly discovered ATP7A disorder-isolated distal motor neuropathy-has none of the characteristic clinical or biochemical abnormalities of Menkes disease or its milder allelic variant occipital horn syndrome (OHS), instead resembling Charcot-Marie-Tooth disease type 2. These findings indicate that ATP7A has a crucial but previously unappreciated role in motor neuron maintenance, and that the mechanism underlying ATP7A-related distal motor neuropathy is distinct from Menkes disease and OHS pathophysiology. Collectively, these insights refine our knowledge of the neurology of ATP7A-related copper transport diseases and pave the way for further progress in understanding ATP7A function.

Splice site mutations in the ATP7A gene

Menkes disease (MD) is caused by mutations in the ATP7A gene. We describe 33 novel splice site mutations detected in patients with MD or the milder phenotypic form, Occipital Horn Syndrome. We review these 33 mutations together with 28 previously published splice site mutations. We investigate 12 mutations for their effect on the mRNA transcript in vivo. Transcriptional data from another 16 mutations were collected from the literature. The theoretical consequences of splice site mutations, predicted with the bioinformatics tool Human Splice Finder, were investigated and evaluated in relation to in vivo results. Ninety-six percent of the mutations identified in 45 patients with classical MD were predicted to have a significant effect on splicing, which concurs with the absence of any detectable wild-type transcript in all 19 patients investigated in vivo. Sixty-seven percent of the mutations identified in 12 patients with milder phenotypes were predicted to have no significant effect on splicing, which concurs with the presence of wild-type transcript in 7 out of 9 patients investigated in vivo. Both the in silico predictions and the in vivo results support the hypothesis previously suggested by us and others, that the presence of some wild-type transcript is correlated to a milder phenotype.

Function and regulation of human copper-transporting ATPases

Copper-transporting ATPases (Cu-ATPases) ATP7A and ATP7B are evolutionarily conserved polytopic membrane proteins with essential roles in human physiology. The Cu-ATPases are expressed in most tissues, and their transport activity is crucial for central nervous system development, liver function, connective tissue formation, and many other physiological processes. The loss of ATP7A or ATP7B function is associated with severe metabolic disorders, Menkes disease, and Wilson disease. In cells, the Cu-ATPases maintain intracellular copper concentration by transporting copper from the cytosol across cellular membranes. They also contribute to protein biosynthesis by delivering copper into the lumen of the secretory pathway where metal ion is incorporated into copper-dependent enzymes. The biosynthetic and homeostatic functions of Cu-ATPases are performed in different cell compartments; targeting to these compartments and the functional activity of Cu-ATPase are both regulated by copper. In recent years, significant progress has been made in understanding the structure, function, and regulation of these essential transporters. These studies raised many new questions related to specific physiological roles of Cu-ATPases in various tissues and complex mechanisms that control the Cu-ATPase function. This review summarizes current data on the structural organization and functional properties of ATP7A and ATP7B as well as their localization and functions in various tissues, and discusses the current models of regulated trafficking of human Cu-ATPases.

Molecular pathogenesis of Wilson and Menkes disease: correlation of mutations with molecular defects and disease phenotypes

The trace metal copper is essential for a variety of biological processes, but extremely toxic when present in excessive amounts. Therefore, concentrations of this metal in the body are kept under tight control. Central regulators of cellular copper metabolism are the copper-transporting P-type ATPases ATP7A and ATP7B. Mutations in ATP7A or ATP7B disrupt the homeostatic copper balance, resulting in copper deficiency (Menkes disease) or copper overload (Wilson disease), respectively. ATP7A and ATP7B exert their functions in copper transport through a variety of interdependent mechanisms and regulatory events, including their catalytic ATPase activity, copper-induced trafficking, post-translational modifications and protein-protein interactions. This paper reviews the extensive efforts that have been undertaken over the past few years to dissect and characterise these mechanisms, and how these are affected in Menkes and Wilson disease. As both disorders are characterised by an extensive clinical heterogeneity, we will discus how the underlying genetic defects correlate with the molecular functions of ATP7A and ATP7B and with the clinical expression of these disorders.

Trafficking of the copper-ATPases, ATP7A and ATP7B: Role in copper homeostasis

Copper is essential for human health and copper imbalance is a key factor in the aetiology and pathology of several neurodegenerative diseases. The copper-transporting P-type ATPases, ATP7A and ATP7B are key molecules required for the regulation and maintenance of mammalian copper homeostasis. Their absence or malfunction leads to the genetically inherited disorders, Menkes and Wilson diseases, respectively. These proteins have a dual role in cells, namely to provide copper to essential cuproenzymes and to mediate the excretion of excess intracellular copper. A unique feature of ATP7A and ATP7B that is integral to these functions is their ability to sense and respond to intracellular copper levels, the latter manifested through their copper-regulated trafficking from the transGolgi network to the appropriate cellular membrane domain (basolateral or apical, respectively) to eliminate excess copper from the cell. Research over the last decade has yielded significant insight into the enzymatic properties and cell biology of the copper-ATPases. With recent advances in elucidating their localization and trafficking in human and animal tissues in response to physiological stimuli, we are progressing rapidly towards an integrated understanding of their physiological significance at the level of the whole animal. This knowledge in turn is helping to clarify the biochemical and cellular basis not only for the phenotypes conferred by individual Menkes and Wilson disease patient mutations, but also for the clinical variability of phenotypes associated with each of these diseases. Importantly, this information is also providing a rational basis for the applicability and appropriateness of certain diagnostic markers and therapeutic regimes. This overview will provide an update on the current state of our understanding of the localization and trafficking properties of the copper-ATPases in cells and tissues, the molecular signals and posttranslational interactions that govern their trafficking activities, and the cellular basis for the clinical phenotypes associated with disease-causing mutations.

Cell biology of membrane trafficking in human disease

Understanding the molecular and cellular mechanisms underlying membrane traffic pathways is crucial to the treatment and cure of human disease. Various human diseases caused by changes in cellular homeostasis arise through a single gene mutation(s) resulting in compromised membrane trafficking. Many pathogenic agents such as viruses, bacteria, or parasites have evolved mechanisms to subvert the host cell response to infection, or have hijacked cellular mechanisms to proliferate and ensure pathogen survival. Understanding the consequence of genetic mutations or pathogenic infection on membrane traffic has also enabled greater understanding of the interactions between organisms and the surrounding environment. This review focuses on human genetic defects and molecular mechanisms that underlie eukaryote exocytosis and endocytosis and current and future prospects for alleviation of a variety of human diseases.

Functional copper transport explains neurologic sparing in occipital horn syndrome

OBJECTIVE

A range of neurologic morbidity characterizes childhood-onset copper transport defects, including severe Menkes disease and milder occipital horn syndrome. Both phenotypes are caused by mutations in ATP7A, which encodes a copper-transporting adenosine triphosphatase, although defects causing occipital horn syndrome are rarely reported and nearly always involve exon-skipping (six of eight prior reports). Our objective was to characterize a novel occipital horn syndrome mutation (N1304S) not associated with aberrant splicing and to determine whether functional copper transport was associated with this allele.

METHODS

We studied two brothers with typical occipital horn syndrome and used yeast complementation and timed growth assays, exploiting a Saccharomyces cerevisiae mutant strain, to assess in vitro N1304S copper transport.

RESULTS

We documented that N1304S has approximately 33% residual copper transport, a result not inconsistent with a similar patient we reported with an exon-skipping mutation whose cells showed correctly spliced mRNA transcripts 36% of normal.

CONCLUSION

These patients' mild neurologic phenotypes, together with our yeast complementation and growth experiments, imply that N1304S does not completely block copper transport to the developing brain early in life. The findings suggest that neurologic sparing in untreated occipital horn syndrome is associated with approximately 30% residual functional activity of ATP7A.

Alternative Splice Variants Encoding Unstable Protein Domains Exist in the Human Brain

Alternative splicing has been recognized as a major mechanism by which protein diversity is increased without significantly increasing genome size in animals and has crucial medical implications, as many alternative splice variants are known to cause diseases. Despite the importance of knowing what structural changes alternative splicing introduces to the encoded proteins for the consideration of its significance, the problem has not been adequately explored. Therefore, we systematically examined the structures of the proteins encoded by the alternative splice variants in the HUGE protein database derived from long (>4 kb) human brain cDNAs. Limiting our analyses to reliable alternative splice junctions, we found alternative splice junctions to have a slight tendency to avoid the interior of SCOP domains and a strong statistically significant tendency to coincide with SCOP domain boundaries. These findings reflect the occurrence of some alternative splicing events that utilize protein structural units as a cassette. However, 50 cases were identified in which SCOP domains are disrupted in the middle by alternative splicing. In six of the cases, insertions are introduced at the molecular surface, presumably affecting protein functions, while in 11 of the cases alternatively spliced variants were found to encode pairs of stable and unstable proteins. The mRNAs encoding such unstable proteins are much less abundant than those encoding stable proteins and tend not to have corresponding mRNAs in non-primate species. We propose that most unstable proteins encoded by alternative splice variants lack normal functions and are an evolutionary dead-end.

A comparison of the mutation spectra of Menkes disease and Wilson disease

The genes for two copper-transporting ATPases, ATP7A and ATP7B, are defective in the heritable disorders of copper imbalance, Menkes disease (MNK) and Wilson disease (WND), respectively. A comparison of the two proteins shows extensive conservation in the signature domains, with amino acid identities outside of the conserved domains being limited. The mutation spectra of MNK and WND were compared to confirm and refine further regions critical for normal function. Mutations were found to be relatively widespread; however, the majority was concentrated within defined functional domains and membrane-spanning segments, reinforcing the importance of these regions for protein function. Of the total published point mutations in ATP7A, 23.0% are splice-site, 20.7% nonsense, 17.2% missense, and 39.1% small insertions/deletions. There is a high prevalence (58.2%) of missense mutations in ATP7B. For the other mutations in ATP7B, 7.4% are splice-site, 7.4% nonsense, and 27.0% small insertions/deletions. A region of possible importance is the intervening sequence between the last copper-binding domain and the first transmembrane helix, as this region has a high percentage of MNK mutations. Similarly, the region containing the ATP-binding domain has 24.6% of all WND mutations. The study of mutation locations is useful for defining critical regions or residues and for efficient molecular diagnosis.

Smac3, a novel Smac/DIABLO splicing variant, attenuates the stability and apoptosis-inhibiting activity of X-linked inhibitor of apoptosis protein

X-linked inhibitor of apoptosis protein (XIAP), the most potent member of the inhibitor of apoptosis protein (IAP) family, plays a crucial role in the regulation of apoptosis. XIAP is structurally characterized by three baculovirus IAP repeat (BIR) domains that mediate binding to and inhibition of caspases and a RING domain that confers ubiquitin ligase activity. The caspase inhibitory activity of XIAP can be eliminated by the second mitochondria-derived activator of caspases (Smac)/direct IAP-binding protein with low pI (DIABLO) during apoptosis. Here we report the identification and characterization of a novel isoform of Smac/DIABLO named Smac3, which is generated by alternative splicing of exon 4. Smac3 contains an NH2-terminal mitochondrial targeting sequence required for mitochondrial targeting of Smac3 and an IAP-binding motif essential for Smac3 binding to XIAP. Smac3 is released from mitochondria into the cytosol in response to apoptotic stimuli, where it interacts with the second and third BIR domains of XIAP. Smac3 disrupts processed caspase-9 binding to XIAP, promotes caspase-3 activation, and potentiates apoptosis. Strikingly, Smac3, but not Smac/DIABLO, accelerates XIAP auto-ubiquitination and destruction. Smac3-stimulated XIAP ubiquitination is contingent upon the physical association of XIAP with Smac3 and an intact RING domain of XIAP. Smac3-accelerated XIAP destabilization is, at least in part, attributed to its ability to enhance XIAP ubiquitination. Our study demonstrates that Smac3 is functionally additive to, but independent of, Smac/DIABLO.

A semiautomated approach to gene discovery through expressed sequence tag data mining: discovery of new human transporter genes

Identification and functional characterization of the genes in the human genome remain a major challenge. A principal source of publicly available information used for this purpose is the National Center for Biotechnology Information database of expressed sequence tags (dbEST), which contains over 4 million human ESTs. To extract the information buried in this data more effectively, we have developed a semiautomated method to mine dbEST for uncharacterized human genes. Starting with a single protein input sequence, a family of related proteins from all species is compiled. This entire family is then used to mine the human EST database for new gene candidates. Evaluation of putative new gene candidates in the context of a family of characterized proteins provides a framework for inference of the structure and function of the new genes. When applied to a test data set of 28 families within the major facilitator superfamily (MFS) of membrane transporters, our protocol found 73 previously characterized human MFS genes and 43 new MFS gene candidates. Development of this approach provided insights into the problems and pitfalls of automated data mining using public databases.

Abnormal mRNA splicing resulting from consensus sequence splicing mutations of ATP7B

More than 200 Wilson disease (WD) disease-causing mutations have been defined to date. Missense mutations are largely prevalent while splice-site mutations are limited in number. Most reside in the splice donor or acceptor sites and only a minority are detected in splicing consensus sequences. Furthermore, only a few splicing mutations have been studied at the RNA level to date. In this study, using the RT-PCR method we performed the molecular characterization of four consensus splice-site mutations identified by DNA analysis in patients with WD. One of them, previously described 1707+3insT, occurred at position 3 in the donor splice site of intron 4, while the other three, 2122-8T>G, 2866-6T>G, and 3061-12T>A, are novel and occurred in the acceptor splice sites of introns 7, 12, and 13, respectively. Analysis revealed a prevalently abnormal splicing in the samples carrying the mutations compared to the normal controls. Comparison of RNA splicing with normal controls in liver and lymphocytes further suggests that abnormal splicing of the WD gene is also present and differentially regulated in normal tissues. The results produced in this study strongly suggest that DNA mutations residing in the consensus sequence of WD gene splice sites result in the WD phenotype by interfering with the production of the normal WD protein. Further studies are necessary to better quantify the amount of different transcripts produced by these mutations, and establish their correlation with the disease phenotype.

Drug targets in Menkes disease - prospective developments

Menkes disease (MNK) is an X-linked recessive disorder characterised by a copper-transporting ATPase defect. In the affected cells, copper transport from the cytosol to the Golgi apparatus is disturbed, resulting in a reduction of copper efflux. Orally-administered copper, which accumulates in the intestine, cannot be absorbed and thus a copper deficiency arises. The characteristic features of MNK are progressive neurological degeneration, connective tissue disorders and hair abnormalities, which are caused by a reduction in the activity of several copper-dependent enzymes, due to concomitant copper deficiency. Subcutaneous injections of copper-histidine complex, which currently forms the accepted mode of treatment, prevent the neurological degeneration in some patients when the treatment is initiated soon after birth. However, when the treatment is started later, the neurological degenerative processes are not prevented. Moreover, the treatment does not improve the connective tissue disorders that are caused by the low activity of lysyl oxidase. In order to solve these problems, a form of the treatment aimed at delivering copper into the Golgi apparatus should be studied. An attempt is made in this review to present what is currently known about MNK and its variants, the efficacy and problems of currently accepted treatments and finally therapeutic targets in MNK.

Neuropathology of occipital horn syndrome

Occipital horn syndrome, formerly known as Ehlers-Danlos syndrome type IX or X-linked cutis laxa, is an allelic variant of Menkes' syndrome. Although the clinical symptomatology and systemic pathology findings have been well described in occipital horn syndrome, the neuropathology has not previously been reported. A kindred affected by the X-linked occipital horn syndrome is followed at the University of Alabama at Birmingham. A severely mentally retarded dysmorphic man, who died at the age of 26 years, never gained the ability to walk or talk. Among other findings at autopsy, the patient had the skeletal anomalies previously described with occipital horn syndrome. Neuropathologic findings included neovascularization and extreme reduplication of the cerebral arteries, in conjunction with cystic medial degeneration; bilateral cerebellar hypoplasia; focal cortical dysplasia, and cerebellar heterotopias. These findings are similar to those seen in the brains of patients with Menkes' syndrome, which is not surprising, given the known phenotypic overlap and the proven allelism of occipital horn syndrome with classic Menkes' syndrome.

A novel frameshift mutation in exon 23 of ATP7A (MNK) results in occipital horn syndrome and not in Menkes disease

Menkes disease and occipital horn syndrome (OHS) are allelic, X-linked recessive copper-deficiency disorders resulting from mutations in ATP7A, or MNK. Classic Menkes disease has a severe phenotype, with death in early childhood, whereas OHS has a milder phenotype, with, mainly, connective-tissue abnormalities. Data suggest that steady-state localization of ATP7A to the trans-Golgi network (TGN) is necessary for proper activity of lysyl oxidase, which is the predominant cuproenzyme whose activity is deficient in OHS and which is essential for maintenance of connective-tissue integrity. Recently, it was reported that ATP7A-transcript levels as low as 2%-5% of normal are sufficient to result in the milder phenotype, OHS, rather than the phenotype of Menkes disease. In contrast to previously reported cases of OHS, we describe a case of OHS in which, because of a frameshift mutation, no normal ATP7A is produced. Although abundant levels of mutant transcript are present, there are substantially reduced levels of the truncated protein, which lacks the key dileucine motif L1487L1488. It has been demonstrated that the dileucine motif L1487L1488 functions as an endocytic signal for ATP7A cycling between the TGN and the plasma membrane. The present report is the first to describe an ATP7A truncation that results in OHS rather than in Menkes disease. The data from the present report support the concepts that (1) OHS results from lower levels of functional ATP7A and (2) ATP7A does not require the dileucine motif to function in copper efflux.

Intragenic deletions at Atp7a in mouse models for Menkes disease

Mottled mice have mutations in the copper-transporting ATPase Atp7a. They are proven models for the human disorder Menkes disease (MD), which results from mutations in a homologous gene. Mottled mice can be divided into three classes: class 1, in which affected males die before birth; class 2, in which affected males die in the early postnatal period; and class 3, in which affected males survive to adulthood. In humans, it has been shown that mutations that lead to a complete absence of functional protein cause classical MD, which is characterized by death of boys in early childhood. We hypothesized that the most severely affected mottled alleles would be the most likely to carry mutations equivalent to those causing classical MD and therefore undertook mutational analysis of several class 1 mottled alleles to assess whether these were appropriate models for the disease at the molecular level. Two novel mutations, a deletion of exons 11-14 in mottled spot and an insertion in exon 10 leading to missplicing in mottled candy, were identified. However, these are both "in-frame" mutations, as are the other eight Atp7a mutations reported to date, and therefore no frameshift or nonsense mutations have yet been associated with the mottled phenotype. This contrasts with the mutation spectrum associated with MD, emphasizing the need for caution when mottled mice are used as models for the clinical disorder.

Cellular copper transport and metabolism

The transport and cellular metabolism of Cu depends on a series of membrane proteins and smaller soluble peptides that comprise a functionally integrated system for maintaining cellular Cu homeostasis. Inward transport across the plasma membrane appears to be a function of integral membrane proteins that form the channels that select Cu ions for passage. Two membrane-bound Cu-transporting ATPase enzymes, ATP7A and ATP7B, the products of the Menkes and Wilson disease genes, respectively, catalyze an ATP-dependent transfer of Cu to intracellular compartments or expel Cu from the cell. ATP7A and ATP7B work in concert with a series of smaller peptides, the copper chaperones, that exchange Cu at the ATPase sites or incorporate the Cu directly into the structure of Cu-dependent enzymes such as cytochrome c oxidase and Cu, Zn superoxide dismutase. These mechanisms come into play in response to a high influx of Cu or during the course of normal Cu metabolism.

EST comparison indicates 38% of human mRNAs contain possible alternative splice forms

Expressed sequence tag (EST) databases represent a large volume of information on expressed genes including tissue type, expression profile and exon structure. In this study we create an extensive data set of human alternative splicing. We report the analysis of 7867 non-redundant mRNAs, 3011 of which contained alternative splice forms (38% of all mRNAs analysed). From a total of 12572 ESTs 4560 different possible alternative splice forms were detected. Interestingly, 70% of the alternative splice forms correspond to exon deletion events with only 30% exonic insertions. We experimentally verified 19 different splice forms from 16 genes in a total subset of 20 studied; all of the respective genes are of medical relevance.

Similar splice-site mutations of the ATP7A gene lead to different phenotypes: classical Menkes disease or occipital horn syndrome

More than 150 point mutations have now been identified in the ATP7A gene. Most of these mutations lead to the classic form of Menkes disease (MD), and a few lead to the milder occipital horn syndrome (OHS). To get a better understanding of molecular changes leading to classic MD and OHS, we took advantage of the unique finding of three patients with similar mutations but different phenotypes. Although all three patients had mutations located in the splice-donor site of intron 6, only two of the patients had the MD phenotype; the third had the OHS phenotype. Fibroblast cultures from the three patients were analyzed by reverse transcriptase (RT)-PCR to try to find an explanation of the different phenotypes. In all three patients, exon 6 was deleted in the majority of the ATP7A transcripts. However, by RT-PCR amplification with an exon 6-specific primer, we were able to amplify exon 6-containing mRNA products from all three patients, even though they were in low abundance. Sequencing of these products indicated that only the patient with OHS had correctly spliced exon 6-containing transcripts. We used two different methods of quantitative RT-PCR analysis and found that the level of correctly spliced mRNA in this patient was 2%-5% of the level found in unaffected individuals. These findings indicate that the presence of barely detectable amounts of correctly spliced ATP7A transcript is sufficient to permit the development of the milder OHS phenotype, as opposed to classic MD.

Characterization of the Menkes protein copper-binding domains and their role in copper-induced protein relocalization

Menkes disease is a fatal X-linked disorder of copper metabolism. The gene defective in Menkes disease (ATP7A) encodes a copper transporting P-type ATPase (MNK or ATP7A) with six copper-binding domains at its N-terminus. MNK is normally localized to the trans -Golgi network in cultured cells, but relocates to the plasma membrane in the presence of elevated extracellular copper. In this study, the role of the six copper-binding domains on copper-induced redistribution is investigated. In a recombinant clone, when all the wild-type copper-binding motifs are mutated from GMXCXXC to GMXSXXS and the cells grown in medium containing elevated copper, relocalization of the recombinant protein to the plasma membrane was not observed. Using the same assay with any one of the six copper-binding domains intact, MNK moves to the plasma membrane in a way indistinguishable from the wild-type protein. Therefore, the copper-binding domains are vital for MNK trafficking and only a single domain is sufficient for this redistribution to occur.

Defective copper-induced trafficking and localization of the Menkes protein in patients with mild and copper-treated classical Menkes disease

Menkes disease is an X-linked disorder of copper metabolism. An overall copper deficiency reduces the activity of copper-dependent enzymes accounting for the clinical presentation of affected individuals. The Menkes gene product (MNK) is a P-type ATPase and is considered to be the main copper efflux protein in most cells. The protein is located primarily at the trans -Golgi network (TGN), but relocalizes to the plasma membrane in elevated copper conditions to expel the excess copper from the cell. Here we report the first missense mutation which causes mild Menkes disease, a mutation in a successfully copper-treated classical Menkes patient and the effect of each mutation on the localization of MNK within the cell. Using western blot analysis, MNK was detectable in cells from both patients, but appeared to be mislocalized in the treated case. In the mild Menkes patient, the protein appeared to be located in the TGN but failed to redistribute towards the cell periphery in response to copper. This is the first description of a mutation in a Menkes patient which affects the trafficking of MNK, and the loss of this process is consistent with the clinical phenotype.

Genetic disorders of membrane transport. IV. Wilson's disease and Menkes disease

Copper is an essential transition metal that permits the facile transfer of electrons in a series of critical biochemical pathways. Menkes disease and Wilson's disease are inherited disorders of copper metabolism resulting from the absence or dysfunction of homologous copper-transporting ATPases that reside in the trans-Golgi network of all cells. Despite striking differences in the clinical presentation of these two diseases, the respective ATPases function in precisely the same manner within the cell and the unique clinical features of each disease are entirely the result of the tissue-specific expression of each protein. Elucidation of the basic defect in these rare genetic disorders has provided a valuable heuristic paradigm for understanding the mechanisms of cellular copper homeostasis.