Initially, Wilson disease (WD) was not regarded as a hereditary disorder. Its genetic origin with autosomal recessive inheritance was reported several decades after the Wilson’s first description of the disease [65]. In 1993, the causative gene ATP7B (OMIM 606882) was identified and cloned [21, 22]. ATP7B is a large gene spanning approximately 80 kb of genomic DNA on chromosome 13 (13q14). The gene consists of 21 exons, which encode a 1,465-amino acid protein, ATP7B.

ATP7B protein is a member of the P-type ATPase family of membrane proteins that pump ions and lipids across the cellular membranes and the key regulator of cellular Cu metabolism in human [66]. The protein contains several highly conserved functional domains: six metal-binding domains (MBDs), each with a metal-binding motif GMxCxxC; eight hydrophobic transmembrane domains (TMs) that form a path through cell membranes for Cu translocation; the phosphatase domain (A-domain); the phosphorylation domain (P-domain); and the ATP-binding domain (N-domain). Expression of ATP7B is predominant in the liver. Lower expression levels are detected in the brain, heart, lungs, kidney, and placenta [21, 22]. ATP7B has a dual function in cells. Biosynthetic role is represented by Cu delivery for incorporation into copper-dependent enzymes such as CP [67], and homeostatic role is fulfilled by removal of excess Cu from cells [68]. Symptoms of WD are consequences of (1) impaired Cu excretion from the liver to bile resulting in its accumulation in the liver and (2) dysfunctional incorporation of Cu into CP leading to hypoceruloplasminemia. When the capacity of the liver to store Cu is exceeded, free Cu is released into the bloodstream and deposited in various organs, predominantly the brain, cornea, and kidneys.

More than 700 mutations have been reported in ATP7B gene (HGMD Professional 2014.2) and most patients are compound-heterozygous. The mutations are scattered throughout the gene and are mostly missense (Table 14.2). They affect protein stability, intracellular localization, various activities depending on afflicted domain (catalytic, transport, phosphorylation), as well as protein expression levels.

Pathogenic mutations are found in up to 98 % of WD patients [69, 70], but most studies report lower detection rate, approximately 70–85 %. Failure to identify two ATP7B mutations in a given patient should not exclude the diagnosis of WD. Allele frequency of most mutations is very low. Only several mutations occur with a high frequency in certain populations due to a founder effect. For instance, p.His1069Gln is the most common ATP7B mutation in Europe, accounting for 30–70 % of all detected mutations [71–74]. Mutation p.Arg778Leu is the most common one among Chinese and Taiwanese WD patients (20–40 %) [75–78]. In Sardinian WD patients, mutation c.-441_-427del predominates with frequency 60.5 % of detected mutations [79], and in Saudi Arabia, p.Gln1399Argfs*6 mutation is the most common one [80, 81]. In Indian patients, mutation p.Cys271* is observed with the highest frequency (20–24 %) [69, 82]. Identification of population-specific prevalent mutations facilitates genetic testing in given populations, but the analysis of the entire coding region of ATP7B as well as the copy number analysis (to detect deletions of one or more exons) may still be desirable.

The most commonly reported numbers are 1:30,000 for worldwide WD prevalence and 1 % for population frequency of heterozygous mutation carriers [83]. Recent data from the United Kingdom however suggest that WD may be considerably more common with the prevalence of 1:7,000 and carrier frequency of 2.2–5 % [70].

The typical age of onset is between 5 and 35 years, but approximately 4 % of patients become symptomatic in the fifth decade or later [84]. The most common manifestations of WD are hepatic (50 %) and neuropsychiatric (50 %), but more than 60 % of neuropsychiatric patients have liver cirrhosis at the time of diagnosis [72]. The hepatic manifestation of WD occurs almost 10 years earlier than neurological symptoms with the mean age of onset 15 years [85]. The hepatic presentation of WD can be divided into several forms: asymptomatic elevation of liver enzymes, acute hepatitis mimicking viral hepatitis with jaundice, acute liver failure usually accompanied by hemolytic anemia, and chronic hepatitis with liver cirrhosis [83].

The mean age of onset in neurologically presenting patients is 20 years [85]; however, the earliest onset of neurological symptoms was 6 years and the latest was 72 years [83]. The neurological presentation is divided into four phenotypes with a significant overlap: parkinsonism, tremors, ataxia, and dystonia [83]. Tremor, occurring in almost 80 % of patients with the neurological manifestation, is the most frequent neurological symptom [86]. Classically, high-amplitude low-frequency coarse proximal “wing-beating” tremor has been described, but WD patients can manifest with any type and combination of tremors, resting, intentional, and/or postural. Dysarthria occurs in more than 70 % of WD patients and most frequently is of mixed type with varying spastic, cerebellar, hypokinetic, and dystonic components [87]. Parkinsonism, commonly manifesting as symmetric akinesia and rigidity, hypomimia, dysarthria, and drooling, occurs in almost 40 % of patients with neurological presentation. Parkinsonism can be also the manifestation of hepatic encephalopathy in severe cirrhosis. Dystonic phenotype, occurring in 10–30 % of cases, is usually most severe and resistant to treatment [86]. Dystonia can be focal, commonly in the orofacial region causing the typical facies with “vacuous smile” or in the hand region, but can be also segmental or generalized in some patients. Other movement disorders like chorea, balism, myoclonus as well as pyramidal signs and autonomic system impairment are less frequent.

Psychiatric symptoms affect more than 60 % of patients during the disease progression and can be the initial presentation in 20–40 % of patients [88–90]. The most common psychiatric symptoms include mood disorders, anxiety, and personality changes such as irritability, aggressiveness, and disinhibition, while psychotic symptoms occur less frequently. Cognitive disorders in patients with neuropsychiatric presentation are common but variable and rarely lead to dementia [91]. The most severely affected cognitive domain is executive functions, particularly attention [92].

Ophthalmologic manifestations include the Kayser-Fleischer ring (K-F ring) and sunflower cataract caused by copper deposition in the corneal Descemet’s membrane and anterior lens capsule, respectively. The K-F ring is detectable in 80–95 % of patients with neurological presentation as golden brownish coloration seen at the periphery of the cornea [93], while sunflower cataract is less common. Other manifestations of WD include hematologic changes (thrombocytopenia, leukopenia, hemolytic anemia), bone and joint involvement (pain, osteoporosis, spontaneous fractures), and renal involvement (hypercalciuria, hyperphosphaturia, nephrocalcinosis).

Genotype-phenotype correlation in WD is challenging due to the great clinical variability and high prevalence of patients carrying two different mutations. The correlation studies are hampered by a paucity of heterozygotes with the same ATP7B genotype as well as homozygotes for less frequent mutations. However, some associations have been proposed. The mutation p.His1069Gln tends to cause later-onset (the second to the third decade) and neurological phenotype, while the mutations p.Cys271* and p.E122fs, common in Indian population, more often lead to earlier-onset (the first to the second decade) and more severe phenotype [69, 94–96].

Age of onset and clinical presentations often vary even among the patients carrying the same ATP7B mutation or the patients with different mutations leading to a similar protein defect. Heterogeneity of clinical phenotype proposed the speculations that there might be other loci modulating the basic WD phenotype. The most perspective candidates are genes encoding other members of copper homeostasis pathway, such as ATOX1 and COMMD1. However, the results of association studies have been so far inconclusive [35, 97–99], suggesting that these genes do not play a major role as modifying factors in WD. The factors and mechanism underlining the clinical variability are probably more complex since even monozygotic twins can manifest different features and severity of WD [100, 101].

Timely diagnosis is important since early treatment with chelation drugs such as d-penicillamine or trientine or zinc supplementation may improve symptoms and prevent irreversible liver and brain damage [102]. WD diagnosis is based mainly on findings of abnormal Cu metabolism, that is, serum CP level, serum Cu level, 24-h urinary Cu excretion, and Cu concentration in liver biopsy. The serum CP level is decreased to <0.2 g/L in almost 95 % of WD patients. The total serum Cu level in WD patients is usually decreased since it mostly represents the Cu bound to CP. Free, non-ceruloplasmin-bound Cu calculated manually as difference between total and CP bound serum Cu is more relevant for the WD diagnosis; it is typically increased to >1.6 μmol/L in non-treated WD patients. Daily urinary copper excretion >1.6 μmol/24 h is the best single test for WD diagnosis with sensitivity and specificity both above 90 %. Hepatic parenchymal copper content in liver biopsy elevated to >250 μg/g of dry weight is highly suggestive of WD, while values <50 μg/g virtually exclude WD [102]. Typical magnetic resonance imaging (MRI) findings include symmetrical hyperintensity in T2-weighted (T2w) images in basal ganglia (BG) and brainstem along with generalized atrophy. Supposedly pathognomonic MRI findings including “the face of the giant panda” and the “bright claustrum” are present in less than 14 % of WD patients. Altogether, MRI abnormalities occur in almost 90–100 % of neuropsychiatric, 40–75 % of hepatic, and 20–30 % of asymptomatic patients [103, 104]. Genetic testing confirms the WD diagnosis but is not necessary for diagnosis in typical cases. It is particularly helpful in patients with inconclusive biochemical findings and in asymptomatic WD patients, mostly relatives of WD cases, who may not manifest typical biochemical abnormalities. A scoring system for the WD diagnosis includes results of all the abovementioned methods (Table 14.3). The resulting Ferenci aka Leipzig score is the gold standard for WD diagnosis and is currently recommended in the guidelines for WD diagnosis and treatment [105].

The ATP7A gene (OMIM 300011) is located on chromosome X (Xq21.1), spans about 140 kb of genomic region, and is organized in 23 exons (the first exon in non-coding). The gene also shows a considerable similarity to exonic structure of the ATP7B gene, especially in the 3′ two-thirds, suggesting both genes have a common evolutionary ancestor [106, 107]. The gene product is a transmembrane copper-transporting P-type ATPase, ATP7A [108–110], which is 1,500 amino acids long. The protein shares about 60 % of sequence identity with the ATP7B protein. The N-terminus of the ATP7A protein includes six homologous metal-binding domains (MBDs), each containing a copper-binding motif GMxCxxC [111]. Eight transmembrane domains (TMs) anchor the protein to a membrane and form a channel for Cu translocation. Other domains include the phosphatase domain (A-domain), the phosphorylation domain (P-domain), and the ATP-binding domain (N-domain), and they mediate the catalytic activity of ATP7A. Besides the structure, both proteins are also highly related in function. Unlike ATP7B, expression of ATP7A is ubiquitous, with a low level in liver, suggesting its housekeeping role [108, 110]. The protein has two main functions: (1) the biosynthetic function, which involves transporting Cu to cuproenzymes at the secretory pathway, and (2) the homeostatic function of exporting of excess Cu from cells and transporting Cu across the gut mucosal wall and the BBB [112]. ATP7A is required during neuronal development probably due to its important role in synaptogenesis [113]. Apart from that, it has a role in NMDA glutamatergic signaling in hippocampal neurons [114]. Clinical symptoms in ATP7A mutations result from generalized hypocupremia and subsequent dysfunction of cuproenzymes. Relative Cu accumulation occurs in enterocytes, endothelia of the BBB, and kidney epithelia because its influx into barrier cells is intact, while its efflux is blocked due to ATP7A dysfunction [23].

Mutations in the ATP7A gene are inherited in X-linked recessive manner; therefore, affected patients are almost exclusively males, while heterozygous females are mostly asymptomatic carriers. To date above 300 different ATP7A mutations have been reported, and they range from single nucleotide changes to gross deletions or duplications [115] (HGMD Professional 2014.2). The most common mutation types, accounting for almost two-thirds of all mutations, are missense mutations, splice-site mutations, and gross deletions ranging from one to several exons (Table 14.4). The mutations are usually unique to each affected family, and the most recurrent mutation is c.2179G>A (p.Gly727Arg) with the estimated frequency of 2.7 %. The donor splice site of intron 8 also seems to be a mutation hot spot of ATP7A, since nine different mutations occur in this location. Interestingly, no missense mutations were observed in exons 2–7, which encode MBDs of ATP7A, suggesting that variations in these regions are more acceptable regarding the normal protein function [115].

The mutations may affect various features and functions of normal ATP7A, such as protein stability, intracellular localization, trafficking, Cu transport, and catalytic activity as reviewed by Tumer [115]. Incidentally, splice-site mutations do not necessarily lead to a complete disruption of splicing, but a small amount of normal transcript and subsequently normal protein can be produced [116–118].

Mutations in the ATP7A gene are associated with three clinical entities: Menkes disease (MD) (OMIM 309400), occipital horn syndrome (OHS) (OMIM 304150), and X-linked distal motor neuropathy (OMIM 300489) [119].

The estimated incidence of MD ranges between 1:40,000 and 1:360,000 live births [120]. MD clinically manifests as infantile-onset cerebral and cerebellar neurodegeneration, failure to thrive, coarse hair (kinky hair or pili torti), and connective tissue abnormalities. Affected infants may present with prolonged jaundice, hypothermia, hypoglycemia, and feeding difficulties in the early neonatal period. They develop often intractable epileptic seizures, hypotonia, vomiting, diarrhea, and developmental regression in the 2nd or 3rd month of life and usually die within first 3 years of life [121, 122]. Approximately 6 % of patients manifest slightly milder MD phenotype [120]. Biochemical findings include low Cu and CP levels and increased ratio of dopamine metabolite dihydroxyphenylacetic acid (DOPAC) and the norepinephrine metabolite dihydroxyphenylglycol (DHPG) in the blood and cerebrospinal fluid (CSF) [123, 124]. Connective tissue disorders manifest as loose skin (cutis laxa) particularly in the neck and axillar regions, arterial aneurysms, fragile bones, and other structural bone abnormalities particularly in the rib cage (pectus excavatum). Brain MRI abnormalities become evident several months after birth and include diffuse atrophy, ventriculomegaly, tortuosity of cerebral blood vessels, delayed myelination, signal abnormalities in BG, and high incidence of subdural hematomas [125, 126]. Subcutaneous Cu replacement in the first 2 postnatal weeks improves survival and may even normalize developmental outcomes in some patients with residual ATP7A activity [127–131]. During the critical period for treatment, MD cannot be diagnosed clinically, and biochemical or genetic screening of newborns at risk is necessary.

OHS has a milder clinical presentation with a usual symptoms onset between 3 and 10 years and less severe neurological deficit including slight muscle weakness, clumsiness, dysautonomia (orthostatic hypotension, chronic diarrhea, and heart conduction disorders), and variably subnormal cognitive function. The name refers to the wedge-shaped calcifications that form bilaterally within the occipital attachments of trapezius and sternocleidomastoid muscles [122]. Connective tissue abnormalities in OHS are similar to MD, but since these patients survive longer, various bone and joint deformities become apparent during development. Bladder diverticula lead to chronic urinary infections. Biochemical findings include low to normal levels of Cu and CP in the blood and abnormal plasma and CSF catecholamine levels. Life expectancy is variable and some patients may survive until the sixth decade [122, 132]. It has been estimated that approximately 3 % of ATP7A mutations manifest as OHS [120].

ATP7A-related distal motor neuropathy is the mildest phenotype manifesting as a monosymptomatic progressive peripheral neuropathy that has been classified within the group of distal hereditary motor neuropathies. It presents between 10 and 35 years of age in majority of patients. No overt Cu metabolic abnormalities can be detected in this phenotype [133, 134].

No obvious correlation between ATP7A mutations and the clinical severity of MD has been described, but in general, the severe classic form of MD with early death is caused mostly by nonsense mutations, early truncating mutations, and gross deletions. Patients with the mild form of MD and OHS often carry late truncating mutations [135, 136], mutations leading to synthesis of partially functional protein or reduced amount of normal protein [116–118, 137]. Skipping of exons with mutations has also been observed in mildly affected patients [138, 139]. Several missense mutations causing amino acids substitutions within or near TMs are associated with the distal motor neuropathy phenotype [133, 134]. These mutations supposedly do not affect Cu-transporting function but rather cause aberrant intracellular localization of ATP7A [140]. Nevertheless, the phenotypic variability is often observed even in the family members with the same ATP7A mutation illustrating that other factors, genetic and non-genetic, may underlie phenotypic variability of ATP7A-related diseases [141–143].

A few females with the phenotype of MD have been identified, most of them carrying chromosomal aberrations, especially an X-autosome translocations, which disrupt the ATP7A gene [144–147]. The classical severe phenotype observed in some cases may be attributed to a preferential inactivation of the normal X chromosome, but clinical features of female MD patients are usually milder with much longer life expectancy than in males [148].

Several acquired causes of hypermanganesemia have been described, but the first inherited inborn error of Mn metabolism was identified only recently. This autosomal recessive disorder, which is caused by mutations in the SLC30A10 gene (OMIM 611146), is characterized by hypermanganesemia with dystonia, polycythemia, and cirrhosis (OMIM 613280) [60, 61]. The gene is located on chromosome 1 (1q41) and codes for a SLC family 30, member 10. SLC30A10 is highly expressed in liver and brain and, due to the sequence homology with other members of the same family, was initially presumed to be a Zn transporter [155]. However, recent studies confirm that it plays a key role in Mn transport and cell protection from Mn toxicity [61]. SLC30A10 mutations are predicted to give rise to a truncated protein or affect its normal function due to the disruption of a highly conserved area or functional domain. Nevertheless, no apparent genotype-phenotype correlation could be outlined since only 12 homozygous SLC30A10 mutations in 20 patients have been reported so far (Table 14.7) [60, 61].

Clinical symptoms include a childhood-onset chronic liver disease and a movement disorder. Neurological symptoms typically develop in the first decade and manifest as dystonia with a characteristic high-stepping (cock-walk) gait variably accompanied by dysarthria, spastic paraparesis, parkinsonism, psychiatric symptoms, and motor neuropathy [61]. Rare cases with adult-onset parkinsonism were also reported [60]. Severity of liver involvement is highly variable, ranging from mildly elevated liver transaminases to liver failure due to cirrhosis.

MRI is specific for Mn deposits showing typical hyperintensities predominantly in GP, extending also to striatum, cerebellum, pituitary, and white matter in T1w images, while only mild GP hypointensities are seen in T2w images. In an autopsy case, 16-fold increase of Mn concentration was documented in BG along with neuronal loss, reactive astrocytosis, activated microglia, myelin loss, spongiosis, and rare axonal spheroids, predominantly in GP [156]. Neuropathological findings are similar to those found in neurodegeneration with brain iron accumulation (NBIA) and WD [157, 158] suggesting that mechanisms of brain damage may be similar for various metal species. Increased Mn levels can be detected in blood, urine, and liver biopsy. Other laboratory findings are polycythemia, low plasma ferritin, and Fe levels [61]. This disorder is treatable; clinical improvement can be achieved by Fe supplementation and chelation treatment with disodium calcium edetate [159, 160].

Mutations in the ATP13A2 gene (PARK9, OMIM 610513) have been identified in patients with Kufor-Rakeb syndrome (OMIM 606693), a rare autosomal recessive form of juvenile-onset parkinsonism [161]. The gene was mapped to chromosome 1 (1p36.13) and encodes a lysosomal P5-type ATPase. The protein is involved in protection from Mn-induced cell death [62, 162], Zn homeostasis, and accumulation of α-synuclein [163–165], all of which are also implicated in the pathogenesis of PD. To date, 23 homozygous or compound-heterozygous mutations have been identified in ATP13A2 (Table 14.8), and they lead to mRNA degradation, protein misfolding, truncation, and/or degradation [178, 180, 181].