Diagnosis

The diagnosis of harlequin Ichthyosis (HI) relies on physical examination and certain laboratory examination including “Physical Assessment” at birth is very essential for the initial diagnosis of HI. The following findings may be noted at physical examination.

ü   Skin: Severely thickened skin with large, skinny plates of hyperkeratotic scale is present at birth. Deep, ‘erythematous fissure’ separate the scale.

ü   Eyes: Severe ‘ectropion’ is present. The free edges of the upper and lower eyelids are everted, leaving the conjunctivae at risk for desiccation and trauma.

ü   Ears: The pinnae may be small and rudimentary or absent.

ü   Lips: Severe traction on the lips causes ‘eclabium’ and a fixed, open mouth. This may result in feeding difficulties.

ü   Nose: Nasal hypoplasia and eroded nasal alae may occur.

ü   Extremities: The limbs are encased in the thick, hyperkeratotic skin, resulting in flexion contractures of the arms, the legs, and the digits. Limb motility is poor to absent. Circumferential constriction of a limb can occur, leading to distal swelling or even gangrene. ‘Hypoplasia’ of the fingers, toes, and fingernails is reported. ‘Polydactyly’ may also occur.

ü   Temperature Dysregulation: Thickened skin prevents normal sweat gland function and heat loss. The infants are heat intolerant and can become ‘hyperthermic’.

ü   Respiratory Status: Restriction of chest-wall expansion can result in respiratory distress, hypoventilation, and respiratory failure.

ü   Hydration Status: Dehydration from excess water loss can cause ‘tachycardia’ and poor urine output.

ü   Central Nervous System: Metabolic abnormalities can cause seizures. CNS depression can be a sign of sepsis or hypoxia. Hyperkeratosis may restrict spontaneous movements, making neurologic assessment difficult.

Biological Significance of Ichthyosis

In Harlequin Ichthyosis (HI) affected epidermis, several morphologic abnormalities including abnormal lamellar granules in the keratinocyte granular layer and a lack of extracellular lipid lamellae within the stratum corneum had been reported. Lack of ABCA12 function subsequently leads to disruption of lamellar granule lipid transport in the upper keratinizing epidermal cells resulting in malformation of the intercellular lipid layers of the stratum corneum in HI. Cultured epidermal keratinocytes from an HI patient carrying ABCA12 mutations demonstrated defective glucosylceramide transport, and this phenotype was recoverable by in vitro ABCA12 corrective gene transfer. To date, intracytoplasmic glucosylceramide transport has been studied using cultured keratinocytes from a total of three patients harboring ABCA12 mutations. One patient was a homozygote for a splice site mutation c.3295_2A4G and another patient was a compound heterozygote for p.Ser387Asn and p.Thr1387del. Only one heterozygous mutation p.Ile1494Thr was identified in the other patient. Cultured keratinocytes from all the three patients showed apparently disturbed glucosylceramide transport, although this assay is not quantitative.

Genotype — Phenotype correlation in ABCA12 Mutations

In Harlequin Ichthyosis, 44 ABCA12 mutations were reported to date. Among them, most mutations are truncation mutations including nonsense mutations, frameshift mutations (deletion/ insertion mutations), and splice site mutations. Other mutations reported in HI families are missense mutations, exon deletions, and single amino acid deletions.

Most truncation or deletion mutations underlying HI are thought to lead to severe loss of ABCA12 protein function affecting important nucleotide-binding fold domains and/or transmembrane domains. Thus far, in HI patients, at least one mutation on each allele must be a truncation or deletion mutation within a conserved region to cause serious loss of ABCA12. Complete loss of ABCA12 function due to homozygous or compound heterozygous truncation mutations always results in the HI patient phenotype.

Genotype	Phenotype
[truncation]+[truncation]	HI
[truncation]+[exon or conserved amino acid deletion]	HI
[exon or conserved amino acid deletion]+[exon or conserved amino acid deletion]	HI, CIE
[truncation]+[missense]	HI, CIE
[missense]+[missense]	LI, CIE

Phenotype	Genotype
HI	[truncation]+[truncation]
	[truncation]+[exon or conserved amino acid deletion]
	[exon or conserved amino acid deletion]+[exon or conserved amino acid deletion]
	[truncation]+[missense mutation]
	[exon or conserved amino acid deletion]+[missense mutation]
LI	[missense]+[missense]
CIE	[missense]+[missense]
	[missense]+[truncation]
	[missense mutation]+[exon or conserved amino acid deletion]

Clinical Significance

Among the 48 Harlequin Ichthyosis (HI) in whom ABCA12 mutation analysis has been reported, ABCA12 mutation have been identified in all HI families; the ABCA12 mutation detection rate is 100% (48/48) in HI families. Kelsell et al reported that one HI patient in whom ABCA12 mutation was not detected by directed sequencing, however, multiplex PCR (Polymerase Chain Reaction) and oligonucleotide array analysis subsequently revealed the deletion of ‘exon8’ in that patient. In this context harlequin Ichthyosis is thought to be genetically homogeneous for all causal ABCA12 mutation.

ABCA12 Mutation

Till Sep’2010, 56 ABCA12 mutations have been described (online database: http://www.derm-hokudai.jp/ABCA12/) in 66 unrelated families including 48 Harlequin Ichthyosis (HI), 10 Lamellar Ichthyosis (LI) and 8 Congenital Ichthyosiform Erythroderma (CIE) families. Mutations have been reported among ARCI (Autosomal Recessive Congenital Ichthyosis) patients with African, European, Pakistani/Indian, and Japanese backgrounds, from almost all over the world. Of the 56 mutations, 36% (20) are nonsense, 25% (14) are missense, 20% (11) comprise small deletions, 11% (6) are splice site, 5% (3) are large deletions, and 4% (2) are insertion mutations. At least, 62.5% (35) of the total reported mutations are predicted to result in truncated proteins. There is no apparent mutation hot spot in ABCA12, although mutations underlying LI phenotype are clustered in the region of the first ATP-binding cassette.

The most common reported mutation in ABCA12 is c.7322delC (p.Val2442SerfsTer28) in exon 49, which has been reported in 7 HI families with Pakistani background. This mutation has been identified only in the Pakistani population. Thomas et al. (2008) reported that 80% of HI patients and parents (10 screened) originated from the Pakistani/Indian area had the mutation 7322delC.

The second most common reported ABCA12 mutation is a missense mutation p.Asn1380Ser in Walker A motif of the first ATP-binding cassette, which is essential for the transporter function of ABCA12. This missense mutation p.Asn1380Ser has been identified in 5 LI families from Africa (3 families from Morocco and 2 families from Algeria).

Out of further 10 different ABCA12 mutations; each mutation has been identified in two unrelated families from certain geographic regions. Among these 10 mutations, 5 ABCA12 mutations — c.2021_2022del2, c.3295_2A4G, p.Thr1387del, p.Arg1950Ter, and p.Arg2482Ter — were found in 2 independent patients from Japan.

As for the other five mutations — p.Trp1294Ter, p.Gly1651Ser, p.Tyr1090Ter, c.2025delG, and p.Trp1744Ter — were found in two independent families with Pakistani, Algeria, Albanian/Bosnian, Anglo-Saxon and native origins, respectively.

ABCA12 Gene

As we discuss earlier that harlequin Ichthyosis (HI) is caused by the mutation in ABCA1 gene. Let’s try to understand the genetics behind this mutation and its co-relation with the rare genetic disorder HI.

What is the official name of ABCA12 gene?

The official name of ABCA12 gene is ATP-binding cassette, subfamily A (ABC1), member 12. 'ABCA12' 'is the gene’s official symbol.

Gene Family……

ABCA12 is a member of the “ATP-binding cassette (ABC) transporters”. It also belongs to a family of genes called “ATPase Superfamily (ATP)”.

[A gene family is a group of genes that share important characteristics. In many cases, gene in a family share a similar sequence of DNA building blocks ­ neucleotide. These genes provides instruction for making products like proteins that have a similar structure of function]

Location……

The ABCA 12 gene is located on the Long (q) arm of Chromosome 2 at position 34.

Cytogenetic Location: 2q34

Molecular Location in Chromosome 2: base pairs (bp) 215,796,265 to 216,003,150.

Chromosome 2: Human has 23 pairs of chromosome i. e. 23 x 2 = 46 number of chromosome (each copy inherited from each biological parent). Chromosome 2 is the 2^nd largest human chromosome, spanning more than 243 million building blocks of DNA base-pare and representing almost 8% of Total DNA in cells. Chromosome 2 is likely to contain 1300 to 1400 genes among the estimated 20,000-25,000 total genes in human genome.

Function…….

The ATP-binding cassette (ABC) gene represents the largest family of ‘Transmembrane’ (TI) proteins. The ABC super-family are grouped into 7 major sub-families depending on the alignment of the amino acid (ã ã) sequence in the nucleotide-binding folds (NBF) and phylogenetic analysis……

1.    ABCA (ABC1)

2.    ABCB (MDR/ TAP)

3.    ABCC (CFTR/ MRP)

4.    ABCD (ALD)

5.    ABCE (OABP)

6.    ABCF (GCN20)

7.    ABCG (White)

The human ABCA subfamily comprises 12 full transporters that are further divided into 2 subgroups based on the phylogenetic analysis and intron structure. The 1^st group includes 7 genes dispersed on 6 different chromosomes (ABCA1, ABCA2, ABCA3, ABCA4, ABCA7, ABCA12 and ABCA13) where as the 2^nd group contains 5 genes (ABCA5, ABCA6, ABCA8, ABCA9 and ABCA10) arranged in a cluster on chromosome 17q24.

ABC transporters bind and hydrolyze ATP to transport various molecules across a limiting membrane or into a vesicle. The ABCA subfamily membranes play a major role in transporting lipids (fats) in cells which make up the outer most layer of skin (the Epidermis). ABCA12 was recognized as a key molecule in keratinocyte lipid transport. ABCA12 is a keratinocyte transmembrane lipid transporter protein associated with lipid transport in lamellar granules to the apical surface of granular layer keratinocytes.

Cause

Cause....

Harlequine Ichthyosis is caused by the mutation in a gene known as ABCA12 (Adenosine Triphosphate [ATP]-binding Cassette Transporter, subfamily A, member 12) in chromosome region 2q34. Patients with Harlequin Ichthyosis are usually homozygous for this mutation consistent with autosomal recessive inheritance.

The ABCA12 gene provides instruction for making a protein known as an ATP-binding Cassette (ABC) Transporter. The ABC transporter proteins carry many types of molecules across the cell membranes. In particular, the ABCA12 protein plays a major role in transporting fats (lipids) from the cytosol of the corneocyte into lamellar granules. Lamellar granules are intracellular granules that originate from the Golgi apparatus of keratinocyte in the stratum corneum. These granules are responsible for secreting lipids that maintain the skin barrier at the interface between the granular cell layer and cornified layers. The extruded lipids are arranged into lamellae in the intracellular space with the help of concomitantly released hydrolytic enzymes. The lamellae from the skin’s hydrophobic sphingolipid seal.

In HI the ABCA12-mediated transfer of lipid to lamellar granules is absent. The lamellar granules themselves are morphologically abnormal or absent. Normal extrusion of lipid from these granules into the intracellular space cannot occur, and lipid lamellae are not formed. This defective lipid ‘mortar’ between corneocyte ‘bricks’ results in aberrant skin permeability and lack of normal corneocyte desquamation.

The exact mechanism of this transport abnormality has yet to be elucidated. One hypothesis involves abnormal calcium-mediated signalling by means of calpains. Calpains are calcium-activated neutral proteases that are essential to normal epidermal differentiation. Calpains are consistently underexpressed in patients with harlequin ichthyosis compared with the general population.

The pivotal role of ABCA12 in harlequin ichthyosis is supported by in vitro data. Studies have demonstrated normalization of lipid transport when the wild-typeABCA12 gene is transferred to keratinocytes of patients with harlequin ichthyosis.

A milder form of ichthyosis, lamellar ichthyosis type 2, also involves mutations in the ABCA12 gene. The phenotypic difference between these two disorders has been explained on the basis of differing genotypic variants. Nonsense mutations inABCA12 are seen in harlequin ichthyosis, whereas missense mutations underlie lamellar ichthyosis type 2.

Histopathologic, Ultrastructural & Biochemical Factors

Histopathologic, ultrastructural and biochemical studies have identified several characteristic abnormalities in the skin of patients with harlequin ichthyosis (HI).

The 2 main abnormalities involve lamellar granules and the structural proteins of the cell cytoskeleton. The pathophysiology of the other abnormalities is yet to be elucidated.

Abnormal Lamellar Granule Structure and Function

All patients with harlequin ichthyosis have absent or defective lamellar granules and no intercellular lipid lamellae. The lipid abnormality is believed to allow excessive transepidermal water loss. Lack of released hydrolases prevents desquamation, resulting in a severe retention hyperkeratosis.

Abnormal conversion of Profilaggrin to Filaggrin

Profilaggrin is a phosphorylated poly-protein residing in keratohyalin granules in keratinocytes in the granular cell layer. During the evolution to the corneal layer, profilaggrin converts to filaggrin by means of dephosphorylation. Filaggrin allows dense packing of keratin filaments. Its subsequent breakdown into amino acids occurs prior to desquamation of the stratum corneum.

Abnormal Expression of Keratin

Keratinocyte cell cultures have yielded interesting and heterogeneous findings among patients with harlequin ichthyosis.

Keratin filament density is low in most patients. Expression of certain keratins is abnormal in some patients and normal in others. How this altered expression of structural proteins influences desquamation is uncertain.

Abnormal Keratohyalin Granules

Keratohyalin granules are identified by antifilaggrin antibodies and can be abnormal in some patients with harlequin ichthyosis. They can be large and stellate, small and rounded or absent.

Inheritance

How common is Harlequin Ichthyosis?

Harlequin Ichthyosis (HI) is extremely rare, there are currently 10 people living with it in United States.

It is estimated that the occurrence of HI is 1 out of every 3 million births.

How do people inherit Harlequin Ichthyosis?

People inherit genes from their mother and father. When a disorder is autosomal recessive, the affected new-born’s cells will have mutated gene copies contributed from both biological parents. When someone has only one mutated gene copy in each cell, they are said to ‘carry’ the disorder and can pass it to their children.

HI is inherited in an autosomal recessive pattern, when both parents carry the mutated gene copy; the chances of the child being affected are the following for each pregnancy…

1 in 4 (25%) chance of having a child affected with Harlequin Ichtyosis

1 in 2 (50%) chance of having a child who carries Harlequin Ichthyosis

1 in 4 (25%) chance of having a child who is completely unaffected from HI

History, Sign & Symptoms

History….

The disease has been known since 1750 and was 1^st described in the diary of a cleric from Charleston, South Carolina, Rev. Oliver Hart (The diary is printed in the Year Book of the City of Charleston, S. C., for 1896.)

“On Thursday, April ye 5^th, 1750, I went to see a most deplorable object of a child, born the night before, of one Mary Evans, in Chas: town. It was surprising to all who beheld it, and I scarcely know how to describe it. The skin was dry and hard, and seemed to be cracked in many places, somewhat resembling the Scales of a Fish. The Mouth was large and round, and wide open. It had no external nose, but two Holes where the Nose would have been. The Eyes appeared to be lumps of coagulated Blood, turned out, about the Bigness of a Plumb, ghastly to behold. It had no external Ears, but holes where the Ears should be. The Hands and Feet Appeared to be swoln, were crumpt up, & felt quite hard. The back part of its Head was much open. It made a strange kind of a noise, very low, which I cannot describe. It lived about eight and forty hours, and was alive when I saw it.” (source: http://archpedi.jamanetwork.com/article.aspx?volume=43&page=442)

Sign & Symptoms....

Pathophysiology

The features of the sufferers are severe cranial and facial deformities. The ears may be very poorly developed or absent entirely. The nose is also deformed, showing a flattened structure and reduce nostrils. The eyelids are severely everted (turn outward) —‘ectropion’—which leaves the eyes and the area around them very susceptible to infection. They often bleed upon birth. The lips pulled by the dry skin, are fixed into a wide grimace — ‘eclabium’. Arms, feet and fingers are almost always deformed in such a way that they cannot bend properly and may be below normal size.

They present hypoplasia in fingers, therefore they cannot grab things properly or they can barely touch them. Polydactyly, a condition in which one has more than the usual number of toes or fingers, has also been found in these infants.

They are extremely susceptible to changes in temperature due to their armour-like cracked skin, which prevent the normal heat loss. This can result in hyperthermia. Their respiration is also restricted by the skin, which impedes the chest wall from expanding and drawing in enough air. This can lead to hypoventilation and respiratory failure. Harlequins are often dehydrated as their plated skin is not well suited to keep the water in.

Epidemiology

Frequency: More than 100 cases of Harlequin Ichthyosis have been reported internationally.

Mortality/Morbidity: the mortality far harlequin ichthyosis rate is high. With neonatal intensive care and the advent of retinoid therapy, some babies have survived the new-born period. They are still at high risk of dying from systemic infection, which is the most common cause of death. A review of 45 cases by Rajpopat et. Al, found 25 survivors (56%), ranging age from 10months to 25 years, 20 deaths (44%) occurred from day 1 to day 52 and were as likely to be caused by respiratory failure as fulminant sepsis (75%) [Source: http://reference.medscape.com/medline/abstract/21339420]

Rece: No racial prediction is known for harlequins.

Sex: No increased risks of harlequin ichthyosis based on sex in known.

Harlequin Ichthyosis - What is it?

Harlequin Ichthyosis

What is it? ....

Harlequin Ichthyosis (HI) is a rare autosomal recessive genetic disorder of skin and the most severe form of congenital ichthyosis— characterized by a thickening of 'Keratin Layer' of fatal human skin. The skin forms large, diamond-shaped plates that are separated by deep cracks (fissures). These skin abnormalities affect the shape of the eyelids, nose, mouth and ears as well as limit the movements of the arms and legs. Restricted movement of the chest can lead to breathing difficulties and respiratory failure.

The Name came from....

The term 'Harlequin' derives from the newborn's facial expression and the triangular & diamond-shaped pattern (resembling the costume of Arlecchino) of 'hyperkeratosis' . The newborn's mouth is pulled wide open, mimicking a 'crown's smile'.

Babies with harlequin ichthyosis are born prematurely, covered with thick, hard, armor-like plates of cornified skin separated by deep fissures. The thick skin plates can pull at and distort the infant's facial features. The tightness of the skin pulls around the eyes and mouth, forcing the eyelids and lips to turn inside out, revealing the red inner skin. The chest and abdomen of the infant may be severely restricted by the tightness of the skin, making eating and breathing difficult. The hands and feet may be small, swollen and partially flexed. The ears may appear to be misshapen or missing but are really fused to the head by the thick skin.

In harlequin infants premature birth is typical, leaving the infants at risk for additional complications from early delivery. These infants are also at high risk for difficulty of breathing, infection, low body temperature and dehydration. Constriction and swelling of the mouth may interfere with the suck response and infants may need tube feeding. Medical monitoring is difficult because of the abnormal skin; electrodes cannot be placed effectively and blood vessels cannot be seen under the skin. Placing lines in the artery and vein of the umbical cord can aid in monitoring the infant and delivering fluids and nutrition. These infants may have problems maintaining normal levels of electrolytes especially sodium in their blood. They are particularly prone to develop hypernatremia (high sodium levels in blood). The baby's corneas need to be lubricated and protected if the eyelids are forced open by the tightness of the skin. A high humidity environment is a heated incubator is necessary to help maintain body temperature and to prevent the skin from cracking.

The thick plate like skin will gradually split and peel off. Antibiotic treatment may necessary to prevent infection at this point of time. Administration of retinoid such as oral etretinate (1mg/kg body wetght) may accelerate sheding of the thick scales. But a hurdle also exists here— it takes a week or two for etretinate to work loosening the scales.  Because most of the fatalities from this condition occur in the first few days of life, many of the successes attributed to etretinate use in the medical literature may be equally due to the high quality of care in the immediate new-born period and to a less severely affected new-born.  Unfortunately, because of the severity of their condition, some new-borns with harlequin ichthyosis will not survive, even with the best of care.