Inherited Peripheral Neuropathies Bristol Genetics Laboratory Kenneth Smith

1. Inherited Peripheral NeuropathiesBristol Genetics LaboratoryKenneth Smith SCOBEC A-Grade Training DayCardiff 15th October 08

2. Inherited Peripheral Neuropathies • Peripheral Neuropathies can be inherited or acquired. • The hereditary neuropathies are a clinically and genetically heterogeneous group of disorders with overall prevalence of 1 in 2,500. • This disease is characterised clinically by distal muscle wasting and weakness, distal sensory loss, reduced tendon reflexes, hypoflexia, variable amount of foot deformity and neurophysiological changes. • There are in excess of 40 causative genes associated with inherited peripheral neuropathies. • Age of onset and severity vary extensively. • AD, AR and XL forms • Broad overlap between genotype-phenotype correlation complicates testing strategy. “Mutations in a single gene can cause different phenotypes, and the same phenotype can be caused by mutations in different genes” Quote from Reilly MM. Sorting out the inherited neuropathies 2.

3. Inherited Peripheral Neuropathies: Classification • Clinical classification of the Inherited Peripheral Neuropathies (IPN) is primarily based on the motor/ sensory involvement: • Hereditary Motor and Sensory Neuropathy (HMSN or CMT). • Hereditary Motor Neuropathy (HMN). • Hereditary Sensory Neuropathy (HSN). • Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) is genetically related to HMSN and is also considered under the inherited neuropathies. • Neurophysiological classification of the Inherited Peripheral Neuropathies (IPN) is primarily based on Nerve Conduction Studies (NCS): • type 1 (median NCV<38m/s) demyelinating. • type 2 (median NCV>38m/s) axonal. • an intermediate form is increasingly recognised. • Further classification can be made based on inheritance pattern and nerve biopsy. • Even after considering these factors may still be left with any one of a number of candidate genes or the genetic basis may remain unknown.

4. CMT1 CMT2 Intermediate CMT HNPP abnormal myelin MCV<38m/s axonopathy MCV>38m/s combination of myelinopathy/ axonopathy MCV 25-45m/s AD or X-linked or isolated AD or isolated AD or isolated or X-linked 17p11.2 dosage No male to male transmission male to male transmission Classical CMT2 CMT2 + sensory especially with pain Positive FH or strong clinical indication GJB1 MPZ PMP22 MFN2 MPZ SPTLC1 PMP22 GJB1 AD or isolated - AD or sporadic - CMT1 or severe early onset NEFL - AR CMT1/2 early onset/ severe with vocal cord and diaphragm paralysis - rare AD CMT2 - DSD or CHN - AR CMT2 with proximal involvement and rapid progression GDAP1 (ION lab) NEFL MPZ GDAP1 NEFL EGR2 LMNA (Exeter lab) - AR or sporadic - CMT1 or severe early onset or DSD - often prominent sensory component with ataxia and severe axonal loss PRX Peripheral Neuropathies: Testing Strategy

5. Case 1: MPZ Family I. 1 2 3 4 5 Ref: aged 88 yrs (02) Clinically HMSN1 MPZ p.Lys108X Ref: aged 72yrs (02) Clinically unaffected MPZ normal Ref: aged 64yrs (02) Clinically unaffected MPZ p.Lys108X Ref: aged 88 yrs (02) Clinically unaffected MPZ normal II. 1 2 3 4 5 6 Ref: aged 58 yrs (01) Clinically HMSN1 MPZ p.Lys108X 2 Ref: aged 65 yrs (05) Clinically unaffected MPZ p.Lys108X III. 1 2 Ref: aged 28 yrs (01) NCV <22m/s Clinically HMSN 1 GJB1 mutation screen -ve MPZ p.Lys108X UV assumed pathogenic Ref: aged 37 yrs (07) MPZ p.Lys108X

6. Case 1: MPZ Family • 3 generation family with 3 affected individuals and 3 unaffected individuals all of which have the same UV in MPZ. • p.Lys108X had not previously been reported on the CMT database. • p.Lys108X reported as pathogenic based as nonsense and tracking with disease at the time. • Explanations: • Variable expressivity or reduced penetrance. • Clinical information is inaccurate. • UV is not pathogenic and the genetic basis of the peripheral neuropathy remain undetermined.

7. I. 1 2 3 4 5 Ref: aged 88 yrs (02) Clinically unaffected MPZ normal Ref: aged 88 yrs (02) Clinically HMSN1 MPZ p.Lys108X Ref: aged 72yrs (02) Clinically unaffected MPZ normal Ref: aged 64yrs (02) Unaffected MPZ p.Lys108X II. 1 2 3 4 5 6 Ref: aged 58 yrs (01) Clinically HMSN1 MPZ p.Lys108X 2 Ref: aged 65 yrs (05) Unaffected MPZ p.Lys108X Ref: aged 28 yrs (01) NCV <22m/s Clinically HMSN 1 MPZ p.Lys108X UV assumed pathogenic III. 1 2 Ref: aged 37 yrs (07) MPZ p.Lys108X Case 1: MPZ Family • Current in silico analysis predicts the variant is pathogenic. • Clinical information was reviewed in September 08. • Individuals I.5 and II.1 mild signs of peripheral neuropathy and considered clinically affected. • III.2 is 37 years of age and is likely presymptomatic.

8. MPZ gene MPZ gene PTC PTC p.Lys108X (Exon 3) Exons 1 2 3 4 5 6 Exons 1 2 3 4 5 6 7Kb 7Kb Escapes to NMD Dominant negative function More severe phenotype Subject to NMD Haploinsufficiency Less severe phenotype Case 1: MPZ Family • Why is the age of onset so late? • Previous examinations had not detected mild signs of peripheral neuropathy. • Recently published paper1 proposing that Nonsense Mediated Decay (NMD) may influence disease severity depending on position of PTC in MPZ gene. • Highlights need for accurate and detailed clinical information and the need to review clinical data in order to interpret unclassified variants.

9. I. 1 2 II. 1 Ref: aged 13 yrs (00) Dublin Lab Severe DSS phenotype involving upper and lower limbs. Onset: 3 yrs, wheelchair bound at 6 yrs. Bilateral hearing loss. Absent NCV. Dublin lab found PMP22 duplication. Genotype not consistent with DSS phenotype. Referred to Bristol for extended testing. Testing revealed: MPZ p.Arg67His Variant not reported in CMT database. Case 2: MPZ/PMP22 Family • Were the PMP22 duplication and the MPZ UV inherited or have they arisen de novo? • Was the variant a SNP in the Irish population? • Could p.Arg67His be eliciting a mild effect in a parent which is not evident on examination? • Could the combination of duplication in PMP22 and the variant in MPZ explain the phenotype or does a second contributing factor remain unidentified?

10. Case 2: MPZ/PMP22 Family II. 1 2 I. 1 Ref: aged 13 yrs (00) Dublin Lab Severe DSS phenotype involving upper and lower limbs. Onset: 6 years, now wheelchair bound. Dublin lab found PMP22 duplication. Genotype not consistent with DSS phenotype. Referred to Bristol for extended testing. Testing revealed: MPZ p.Arg67His Variant not reported in CMT database. In silico analysis support variant not being pathogenic. Pathogenic variants reported involving codon p.Arg67Pro Mother Unaffected carrier of UV p.Arg67His Ref: aged 45 yrs Father deceased but family recount information indicating he may have had mild symptoms of PN. • Mother was clinically unaffected and mutation analysis revealed she also had p.Arg67His variant. • Father was deceased. Duplication may have come from the father or may have arisen de novo. • The MPZ variant was not found in 50 population matched individuals or in our cohort of patients >100. • Published mutations involving this codon (but not this variant) with very variable phenotype. • In silico analysis supports this variant being benign.

11. Peripheral Neuropathies • These cases highlight: • The difficulty in ascertaining the effect of UV without functional studies particularly when there are so many different genes involved. • The need for accurate and up to date clinical information and a strategic reviewing process for all unexplained cases. • That until new technologies facilitate the testing of all known peripheral neuropathy genes an effective testing strategy is required. • Bristol Genetics Laboratory offers PMP22 duplication/deletion and MFN2 point mutation analysis to SCOBEC laboratories and mutation analysis of PMP22, MPZ, GJB1, NEFL, PRX, SPTLC1, EGR2 genes to all labs via the UKGTN. • References • 1. Reilly M. Sorting out the inherited Neuropathies. Pract Neurol. 2007 Apr;7(2):93-105 • 2. Inoue K et al. Molecular mechanism for distinct neurological phenotypes conveyed by allelic truncating mutations. Nature Genetics 2004; 36 (4):361-369. • CMT Mutation Database; http://www.molgen.ua.ac.be/CMTMutations/