Heterogeneity in the genetic alterations and in the clinical presentation of acrodermatitis enteropathic: Case report and review of the literature

Abstract

Acrodermatitis enteropathic (AE) is a rare autosomal recessive disorder due to a zinc deficiency and characterized by a classical triad of symptoms: dermatitis, alopecia, and diarrhea. The defective gene is SLC39A4, which encodes a zinc transporter. Nevertheless many abnormalities in SLC39A4 have been relieved, only 50% of patients show alterations. Here is reported the case of an infant with mild and incomplete manifestations of AE, for whom the SLC39A4 genetic test was performed. A novel mutation in SLC39A4 was identified. Zinc replacement improved rapidly the skin lesions. Our case highlights the importance of suspecting this rare condition and to perform the genetic test even in those patients who do not fulfil the classical triad of symptoms. Further efforts should be addressed to identify a more strength correlation between genotype and phenotype of this disorder.

Introduction

Acrodermatitis enteropathic (AE) is a rare autosomal recessive disorder characterized by a classical triad of symptoms: dermatitis involving symmetrically perioral, intertriginous, and acral areas, alopecia, and diarrhea. The first description of this entity was reported in 1942 when the term of AE was coined to postulate the relationship between skin and gastrointestinal symptoms; ¹ then in 1973 the link between AE and zinc was first reported and zinc supplementation became the therapy of choice for this condition. ² The pathogenesis of AE is linked to a severe zinc deficiency derived from a defect of zinc absorption. Zinc in foods is primarily absorbed in the small intestine and then eliminated through pancreatic and intestinal secretions.^3,4 Human ZIP4 (Zrt and Irt-like proteins) is the best characterized transporter of zinc with an extracellular N-terminus rich in histidine and cysteine suggesting possible residues for zinc binding. ⁵ ZIP4 is expressed in the duodenum and jejunum (the two main sites of zinc uptake) and in the kidney (involved in zinc re-absorption). ⁵ During zinc deficiency, ZIP4 expression is increased at the apical surfaces of enterocytes and endoderm cells, while when the cellular concentration is normal or high, ZIP4 undergoes degradation. ⁵ Another family of zinc transporters (ZnT family proteins) facilitate the zinc efflux into circulation, where zinc binds to albumin and it is distributed to all tissues. ³ In particular, ZnT4 is highly expressed in the mammary gland, sustaining the function of zinc transport in breast milk. ³ Despite the low concentration of zinc in human breast milk, it is usually enough for infants in the first months of life.

The causative gene for AE has been identified in SLC39A4, whose product is the ZIP4 zinc transporter.^6,7 AE is currently classified within the Mendelian Inheritance In Man Classification system as phenotype (MIM number 201100, gene locus MIM number 607059). ⁸ In the Human Gene Mutation Database professional 2013.2 a total of 42 mutations have been described including: missense, nonsense, and splicing site mutations; small insertions/deletions; and few gross rearrangements. ⁹ However, SLC39A4 mutations are observed in about 50% of the cases tested, and most of them are homozygotes or compound heterozygotes and very few heterozygotes only. ¹⁰

We reported an unusual clinical presentation of AE in a child carrying a novel mutation in SLC39A4 gene.

Case presentation

An 11-month-old boy from Pakistan was admitted to our clinic presenting acral lesions from the age of 9 months, especially at the feet and legs, with mild periorificial lesions in perineal and perioral area. He had been fully breastfed up to 6 months of age and his general conditions were good, with body weight and height in the 25th percentile. The dermatologic examinations revealed erythematous scaly plaque, erosion, and crusting symmetrically distributed on feet and fingers with onychodystrophy (Figure 1a). Similar lesions but less marked were present in perioral and in perineal regions. Since previous treatment with local antibiotics and antimycotics failed, further investigations were performed. Laboratory parameters were normal, except for low alkaline phosphatase level (61 U/L) and a near normal plasma zinc level (9.3 μmL/L; normal value, 9.8–16.8 μmL/L). A skin biopsy showed non-specific erosion of the epidermis with necrotic keratinocytes. Therefore, in the suspect of AE, SLC39A4 genetic test was performed. In the patient we identified a 5 bp homozygous deletion in the exon 9, spanning from the nucleotide 1461 to 1465 of the cDNA of the gene (NM_130849.2: c.1461_1465delGAGAC). His parents were heterozygous for this mutation. Zinc replacement (3 mg/kg/day of elemental zinc) improved rapidly the skin lesions with normalization within 1 month of both zinc (15.2 μmL/L) and alkaline phosphatise (192 U/L) levels; only post-inflammatory hyperpigmentary areas on fingers and legs are now observed (Fig. 1b–d)).

Figure 1.

(a) Erythematous scaly plaque, erosion, and crusting involving feet before treatment. (b) Marked improvement of the lesions on feet after 1 month of zinc supplementation (3 mg/kg/day). (c, d) Acral dermatitis localized on fingers and on knees after 1 month of zinc supplementation (3 mg/kg/day).

Discussion

Zinc is a transition metal ubiquitously present in the human organism and is a trace element essential in maintaining cell functions and homeostasis (e.g. enzyme catalysis, structural integrity of proteins, and cell regulation). ¹¹ Zinc is present in the whole human body (2–3 g in an adult) interacting with all classes of enzymes: this action defines the versatility and the importance in maintaining various cellular functions. ¹² Only a minimum part of the total body’s zinc (0.1%) is present in serum (80% bound to albumin and 20% to alfa2-macroglobulin), while the great part is distributed in skeletal muscle (60%), in bones (30%), and in the skin and liver (5%). The gastrointestinal system is central to maintain zinc homeostasis: however the mechanisms of gastrointestinal zinc uptake, intracellular transport, and export are partially unknown. To date, two classes of zinc transporters have been described: Zrt-, Irt-like proteins (ZIP, which belong to the Solute Carrier family 394 [SLC39A]) whose function is to mediate cytosolic zinc uptake, and Zn transporters (Znt, belonging to the SLC30A family) implicated in decreasing cytosolic zinc concentrations.^13,14 ZIP family comprises 14 members of proteins that can mobilize, in addition or association to zinc, other metals (e.g. copper and nickel). ⁵ Within ZIP family, human ZIP4 is the best characterized transporter with two major isoforms of 647 and 622 amino acids long; it has eight transmembrane domains and its topology model has been predicted based on the structure of the yeast homolog ¹⁵ and some mammalian ZIP transporters. ¹⁶ The extracellular N-terminus is constituted of a long sequence of 327 amino acids, rich in histidine and cysteine suggesting possible residues for zinc binding. ¹⁷ During zinc deficiency ZIP4 protein expression is increased at the apical surfaces of enterocytes and endoderm cells; ¹⁸ in contrast when the cellular concentration of zinc is normal or high, ZIP4 undergoes ubiquitination, rapid endocytosis, and successive degradation. ¹⁹ This function is probably coordinated within the intracelllular loop between three and four transmembrane domains; ZIP4 is expressed in the duodenum and jejunum (the two main sites of zinc uptake) and in the kidney (involved in zinc reabsorption). ²⁰

Otherwise, ZnT family comprises 10 members of proteins named ZnT1-ZnT10 involved in mobilizing cytosolic zinc into the extracellular space. Within the ZnT family, ZnT1 is essential during embryonic development and protect cells from excess of zinc influx during pathological conditions, while ZnT2 is essential for zinc mobilization to human breast milk and the mutation in the maternal gene is associated with a transient zinc deficiency in the newborn. In contrast, ZnT4 is highly expressed in the mice mammary gland, and associated with reduction of milk zinc levels in mice. Finally ZnT3 is essential to transport zinc into synaptic vesicles in neuronal cells and is thought to be implicated in prevention of the age-related cognitive loss. ¹²

When dietary concentrations of zinc are high, a smaller portion of zinc can be absorbed paracellularly. After engaged into enterocytes, ZnT family proteins facilitate efflux of zinc into circulation where zinc binds to albumin and is distributed to all tissues. ³

Zinc in human milk has a greater bioavailability since it is bound with proteins with lower molecular weight and more digestibility than those in cows’ milk; however in the first month of the infant’s life the zinc content in human milk has high concentration (>3 mg/L) with a progressive decline to <1 mg/L by 6 months. ²¹

Epidemiological studies on AE indicate an incidence of 1:500,000 births, ¹⁰ but the frequency seems more common in sub-Saharan Africa and South East Asia probably due to the founder effect of consanguinity.^3,22 AE usually starts in infancy, at weaning in breastfed infants or earlier in bottle-fed infants, because of the facilitate absorption of zinc in breast milk. The first manifestations are skin lesions spreading symmetrically to periorificial and retro-auricular sites and then reaching extremities. The lesions are erythematous scaly plaques, eczematous, or vescicolo-bullous involving the face in the periorificial sites (mouth, eyes, and nose), the scalp, the diaper area and perineum, and the extremities (fingers and toes). Onychodystrophy, paronychia, and oral and ocular manifestations (e.g. stomatitis, perlèche, blepharitis, conjunctivitis) may occur. Without treatment, AE has an intermittent but progressive course, often complicated by bacterial or fungal infections (especially Candida albicans) and also alopecia may appear. Diarrhea can be accompanied by anorexia and growth failure. At last, the patient may develop neuropsychiatric disorders (e.g. irritability, apathy, mental slowing, lethargy), as well as hypogeusia and hypogonadism. Hair shafts on polarized light microscopy show dark and bright bands with post-inflammatory pigmentary alterations.^1–3,22,23 However, the classical triad of symptoms is present only in about one-third of patients, ¹ while the first symptoms may also appear in the first months of life as intermittent crusting on posterior scalp evolving in complete alopecia. ²³

The histopathological features are non-specific, but share similar patterns with deficiency dermatitis. ³ Confluent parakeratosis, a thinner granular layer, focal spongiosis, and epidermal acanthosis may be visualized in the early stages, while chronic lesions show psoriasiform hyperplasia, necrolysis of the keratinocytes in stratum spinosum and stratum granulosum with cytoplasmic pallor, and ballooning degeneration; intraepidermal vesicles with necrotic keratinocytes are also described. In case of pustular lesions, histopathological examination can reveal pustule formation in the stratum spinosum containing numerous neutrophils and scattered eosinophils, and below the pustules hyalinization of superficial dermal collagen bundles. ³ The histological appearance of small intestinal mucosa is variable. Non-specific villous flattening and an inflammatory cell infiltrate have been described, but light microscopy may also be entirely normal. On electron microscopy, abnormal Paneth cell inclusions have been identified, probably due to zinc deficiency, and disappear after zinc replacement. ³ These findings strongly suggest that wasting and lethality in AE patients reflects the loss-of-function of the intestine zinc transporter ZIP4, which leads to abnormal Paneth cell gene expression, disruption of the intestinal stem cell niche, and diminished function of the intestinal mucosa. ²⁴

Plasma zinc level is the first diagnostic test to perform and should be checked in children with suspected AE. The reference values for zinc levels are not standardized: normal ranges are 60–120 μg/dL or 7.6–17 μmol/L, depending on the laboratory references.^2–4 Despite this, a plasma level <50 μg/dL is suggestive of AE.^2–4,22,23 However, plasma zinc does not always reflect the body zinc status, representing only 0.1% of the body’s total zinc stores, and in rare cases of AE can be in normal range. ³ Serum alkaline phosphatase can be useful as an adjunctive investigation, and is an indirect parameter of zinc deficiency. ³ A serum albumin should also be determined, because zinc is bound by this protein in the circulation. ³ Also the determination of erythrocyte metallothionein concentration, a zinc-regulated protein, has been studied as indirect measure of zinc level.^25,26

The differential diagnosis of AE includes acquired zinc deficiency syndrome (due to a low zinc level in breast milk), cutaneous diseases (e.g. atopic dermatitis, cutaneous candidiasis, and seborrheic dermatitis), or other conditions such as propionic acidemia and methilmalonic acidemia, diets rich in phytates and calcium, intestinal malabsorption syndromes, and pancreatic or hepatic insufficiencies. ⁴

Zinc replacement is the standard therapy also in patients with clinical characteristic of AE without laboratory determination: the rapid improve after zinc replacement therapy can validate the diagnosis. Standard dosing is 3–5 mg/kg/day of elemental zinc in two doses with a maintenance dose of 1–2 mg/kg/day.^3,4 The maintaining dose should be corrected through periodic control of plasma zinc level and alkaline phosphatase and should be increased during pregnancy and lactation. The treatment is generally well tolerated without significant side effects, and allows normal growth without intellectual delay. Occasionally, high-dose zinc supplementation can cause gastric irritation (nausea, vomiting, and gastric hemorrhage) and can affect copper metabolism. Zinc is found in many dairy products such as crustaceous (e.g. oysters, crabs), meet (beef, turkey, chicken, pork), tree nuts (cashews, almonds), legumes (beans), and peanuts and whole grain. ²⁹

Despite this, some cases partially resistant to zinc sulphate therapy have been described: in such patients high dose therapy with zinc gluconate and vitamin C has shown to improve the clinical and serological patterns. ²²

As previously argued, AE may show a wide variety of clinical presentations with different scores of severity, such as clinical forms partially resistant to therapy. The mutation in SLC39A4 identified in our patient has not been previously described before and causes the change of the Arginine (R) at the position 487 for a Serine (S), adding a tail of 154 new amino acids (excluding the S487) to the C-terminus of the protein, after which a new translation stop codon is reached (p.R487SfsX155). The predicted mutant protein holds the transmembrane domains I–III and might retain the ability to integrate to the membrane and to recognize the substrate. Given their sequence conservation and amphipathic nature, transmembrane domains IV and V missing in the mutant protein are predicted to be involved in the substrate transport. According to this hypothesis, conserved residues in these regions are essential for the channel function. ²⁷ A homozygous 14 bp deletion in exon 9 predicting to add the same tail to the mutant protein has been described in another patient presenting with an unusual form of AE. ²⁸

Conclusion

These recent findings suggest the wide complexity underlying the zinc metabolism process, in which many transporters in addition to ZIP4, may contribute in maintaining zinc homeostasis and playing a key role in the pathogenesis of AE.

Our case highlights the importance of suspecting AE even in those patients which do not fulfil the classical triad of manifestations and, in case of clinical suspect, to perform the SLC39A4 genetic test even in case of near-normal zinc levels. Further efforts should be addressed to register a wider number of AE clinical observations and of the respective genetic abnormalities to allow a more strength correlation between genotype and phenotype of this disorder.