The Genetics of The Genetics of

IchthyosisIchthyosis

Sherri J. Bale, PhD, FACMGSherri J. Bale, PhD, FACMGClinical Director, GeneDxClinical Director, GeneDx

FIRST Family ConferenceFIRST Family Conference

Orlando, FL - June 27, 2010Orlando, FL - June 27, 2010

What we’re going to talk about

A primer on how ichthyosis genetically occurs, the chances of passing it along and what genetic tests are available and how they are administered

How many different How many different ichthyoses are there?ichthyoses are there?

> 20 disorders fit the definition of > 20 disorders fit the definition of ichthyosisichthyosis

> 10 other related disorders with > 10 other related disorders with more localized scaling/hyperkeratosismore localized scaling/hyperkeratosis

How are ichthyoses How are ichthyoses classified?classified?

Clinical featuresClinical features Non-syndromic ichthyosesNon-syndromic ichthyoses Syndromic ichthyosesSyndromic ichthyoses Related disordersRelated disorders

Inheritance patternInheritance pattern

Gene defectsGene defects

EtiologyEtiology Enzyme deficienciesEnzyme deficiencies Structural protein defectsStructural protein defects Regulatory protein defectsRegulatory protein defects OtherOther

Genetics 101Genetics 101 Chromosomes – structures inside the Chromosomes – structures inside the

nucleus (command center) of the cell.nucleus (command center) of the cell.

On the chromosomes, we carry genesOn the chromosomes, we carry genes

Genes are made up of a chemical called Genes are made up of a chemical called

DNADNA

Chromosomes, and thus genes, are passed Chromosomes, and thus genes, are passed

from parent to child following “rules of from parent to child following “rules of

inheritance” [Mendel’s lawsinheritance” [Mendel’s laws]]

Genetics 101Genetics 101

Human Chromosomes

Types of InheritanceTypes of Inheritance X-linkedX-linked

RecessiveRecessive Dominant (rare)Dominant (rare)

AutosomalAutosomal RecessiveRecessive DominantDominant

Steroid-Sulfatase Steroid-Sulfatase DeficiencyDeficiency

X-linked recessiveX-linked recessive Incidence 1:6000 malesIncidence 1:6000 males

Primary featuresPrimary features Onset between 1 and 3 weeks of ageOnset between 1 and 3 weeks of age Dark scale, tightly adherentDark scale, tightly adherent Most prominent on flexure surfacesMost prominent on flexure surfaces

(aka “Dirty neck” ichthyosis)(aka “Dirty neck” ichthyosis) Asymptomatic corneal opacities (10-50%)Asymptomatic corneal opacities (10-50%) Cryptorchism (12-25%), increased risk of Cryptorchism (12-25%), increased risk of

testicular cancertesticular cancer The disease does not improve with ageThe disease does not improve with age

• Diagnostic• plasma cholesterol sulfate levels• Assay to directly measure activity of steroid sulfatase is rarely done•Decreased placental sulfatase causes delayed onset/progression of labor in affected male fetuses

• Genetics•STS gene on chromosome Xp22.32• 90% of affected males have large intragenic deletions, or contiguous gene deletions

Steroid-Sulfatase DeficiencySteroid-Sulfatase DeficiencySteroid-Sulfatase DeficiencySteroid-Sulfatase Deficiency

Ichthyosis Gene Epidermolytic hyperkeratosis KRT1; KRT10 Epidermolysis Bullosa Siemens KRT2ePachyonychia congenita I,II KRT6a,b,

KRT16, KRT17

Epidermolytic PPK KRT9Non-epidermolytic PPK many genesKeratitis-Ichth-Deafness (KID) GJB2 (GJB6)Erythrokeratoderma variabilis GJB3, GJB4

Autosomal Dominant IchthyosesAutosomal Dominant IchthyosesAutosomal Dominant IchthyosesAutosomal Dominant Ichthyoses

Epidermolytic Epidermolytic HyperkeratosisHyperkeratosis

Autosomal DominantAutosomal Dominant (1/2 the cases are due to (1/2 the cases are due to newnew mutations) mutations)

Incidence 1:200,000-1:300,000Incidence 1:200,000-1:300,000 Primary FeaturesPrimary Features

Neonatal blistering, erosions and denuded skinNeonatal blistering, erosions and denuded skin Progressive Hyperkeratosis, esp. of the flexuresProgressive Hyperkeratosis, esp. of the flexures Variable palm/sole involvementVariable palm/sole involvement

• Genetics•Due to mutation in keratin-1 (KRT1) or keratin-10 (KRT10) gene

• >40 different mutations, most are missense changes

• >80% cluster at hot spots at the beginning or end of the gene

•In 30% of all EHK patients mutations occur at Arg156 in KRT10

Epidermolytic HyperkeratosisEpidermolytic HyperkeratosisEpidermolytic HyperkeratosisEpidermolytic Hyperkeratosis

?

How can you say its autosomal How can you say its autosomal dominant? I’m the only person in my dominant? I’m the only person in my

family with this disorder!family with this disorder!

How can you say its autosomal How can you say its autosomal dominant? I’m the only person in my dominant? I’m the only person in my

family with this disorder!family with this disorder!

Germline Mutation

Mutation

Ovaries

Testes

Sperm Egg cell

Mutation

Germline Mutation

Mutation

Conception

Disease

?

I have a been diagnosed with an I have a been diagnosed with an ‘Epidermal Nevus’. What is it and how ‘Epidermal Nevus’. What is it and how

does it come about?does it come about?

I have a been diagnosed with an I have a been diagnosed with an ‘Epidermal Nevus’. What is it and how ‘Epidermal Nevus’. What is it and how

does it come about?does it come about?

GametesZygote

Two cell lineages

Cell Migration

Mosaic

Post-zygoticmutation

Mutation

‘Epidermal Nevus’ = Skin Mosaicism for Mutation

Mosaicism• Due to DNA Mutation that occurs during mitosis

of a single cell at early stages of fetal development “post-zygotic mutation”

• All descendent cells will carry the mutation, other cells are normal

• Gives rise to two (or more) genetically distinct cell lines derived from a single zygote

• Mosaicism can affect somatic and/or germline tissues

• Generally only parts of the organism are affected

I have an ‘Epidermal Nevus’. Should I I have an ‘Epidermal Nevus’. Should I be worried about my children?be worried about my children?

I have an ‘Epidermal Nevus’. Should I I have an ‘Epidermal Nevus’. Should I be worried about my children?be worried about my children?

• If germline is affected, mutation can be transmitted to the offspring resulting in full-blown disease

?

What is my risk of having an affected What is my risk of having an affected child?child?

What is my risk of having an affected What is my risk of having an affected child?child?

• Greater than the population risk for a new mutation

• Depends on what percentage of germ cells harbor mutation

• Rule of thumb:• Small nevus--

small risk• Large nevus on

both sides of the body--high risk

?

Ichthyosis Gene

Harlequin ichthyosis ABCA12Lamellar ichthyosis TGM1, ABCA12CIE ALOXE3; ALOX12B

IchthyinCytochrome P450

Sjögren-Larsson syndrome ALDH3A2Neutral lipid storage disease CGI-58

(ABHD5)Netherton syndrome SPINK5Refsum disease PAHX, PEX7Trichothiodystrophy+Ichthyosis ERCC2; ERCC3

Autosomal Recessive Autosomal Recessive IchthyosesIchthyoses

Autosomal Recessive Autosomal Recessive IchthyosesIchthyoses

Lamellar IchthyosisLamellar Ichthyosis Autosomal RecessiveAutosomal Recessive Incidence 1:200,000Incidence 1:200,000

Primary Features:Primary Features: Collodion baby phenotypeCollodion baby phenotype Plate-like, large, dark scalePlate-like, large, dark scale Ectropion, EclabiumEctropion, Eclabium Scarring alopeciaScarring alopecia

Lamellar IchthyosisLamellar Ichthyosis Due to mutation in the TGM1 gene Due to mutation in the TGM1 gene

in the vast majority of cases, coding in the vast majority of cases, coding for Transglutaminase-1for Transglutaminase-1

A few common mutations exist (the A few common mutations exist (the “German” splice-site mutation) and “German” splice-site mutation) and R141 and R142 in exon 3.R141 and R142 in exon 3.

Few families with mutation in Few families with mutation in ABCA12, Ichthyin, and cytochrome ABCA12, Ichthyin, and cytochrome P450 genesP450 genes

Congenital Ichthyosiform Congenital Ichthyosiform ErythrodermaErythroderma

Autosomal RecessiveAutosomal Recessive Incidence 1:200,000-300,000Incidence 1:200,000-300,000 Primary featuresPrimary features

Collodion baby presentationCollodion baby presentation Bright red (erythrodermic) skinBright red (erythrodermic) skin Fine, white scaleFine, white scale

Due to mutation in many different Due to mutation in many different genes, 5 of which are knowngenes, 5 of which are known ALOX12B and ALOXE3 (in about ~10%)ALOX12B and ALOXE3 (in about ~10%) Ichthyin (in about ~10%)Ichthyin (in about ~10%) A new cytochrome P450 geneA new cytochrome P450 gene

Enzymes encoded by these genesEnzymes encoded by these genes are are involved in lipid metabolisminvolved in lipid metabolism

Operate in common membrane Operate in common membrane arachidonic acid pathway arachidonic acid pathway (lipoxygenase)(lipoxygenase)

Congenital Ichthyosiform Congenital Ichthyosiform ErythrodermaErythroderma

Congenital Ichthyosiform Congenital Ichthyosiform ErythrodermaErythroderma

Harlequin IchthyosisHarlequin Ichthyosis Autosomal recessiveAutosomal recessive Mutation in ABCA12 geneMutation in ABCA12 gene

(ATP-binding cassette transporter protein)(ATP-binding cassette transporter protein) Primary features:Primary features:

Thick, armor-like plates of scale that fissure Thick, armor-like plates of scale that fissure and crackand crack

Eclabium and EctropionEclabium and Ectropion Poor prognosis, although survivors have a Poor prognosis, although survivors have a

congenital ichthyosiform erythroderma congenital ichthyosiform erythroderma phenotypephenotype

So what is a mutation, So what is a mutation, anyway?anyway?

How do we detect a mutation?

• Chromosomes

• DNA

• Metabolic

KaryotypeKaryotype arrayCGHarrayCGH FISHFISH

Sequence analysisSequence analysis Mutation scanningMutation scanning Targeted mutation Targeted mutation

analysisanalysis

AnalytesAnalytes Enzyme assayEnzyme assay

What do we need for mutation What do we need for mutation testing?testing?

Material Material required for required for testing:testing: Buccal swabsBuccal swabs

BloodBlood

Skin punch Skin punch biopsybiopsy

How is DNA Sequencing Done?How is DNA Sequencing Done?Gly278Arg

What is the use of this What is the use of this mutation information ?mutation information ?

Identification of disease-causing Identification of disease-causing mutation(s) allows answers to the mutation(s) allows answers to the questions: questions:

What do I have?What do I have? Why do I have it or how did it happen?Why do I have it or how did it happen? What is the chance it will happen What is the chance it will happen

again?again? What’s wrong with my skin and how What’s wrong with my skin and how

best can it be treated?best can it be treated?