the role of cftr mutations in causing …...•every person with cystic fibrosis inherited two...

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THE ROLE OF CFTRMUTATIONS IN CAUSING CYSTIC FIBROSIS (CF)


Overview

• CF occurs when a child inherits two copies of a CFTR gene mutation resulting in a CF genotype1,2

• CFTR protein activity is determined by CFTR protein quantity and function1,3

• In CF, there is a reduction in total CFTR protein activity leading to an imbalance of fluid and ions in organs throughout the body1,3,4

• Different CFTR mutations produce different effects on CFTR protein quantity and function1,3,4

• When patients and caregivers are educated about their genotype, they are empowered to be active partners in their own care5

CF is a complex, genetic disease

References: 1. Boyle MP, De Boeck K. Lancet Resp Med. 2013;1(2):158-163. 2. Zielenski J. Respiration. 2000;67(2):117-133. 3. Derichs N. Eur Resp Rev. 2013;22(127):58-65. 4. Elborn JS. Lancet. 2016;388(10059):2519-2531. 5. Cystic Fibrosis News Today. https://www.cysticfibrosisnewstoday.com/2015/02/06/cf-trust-will-pay-for-patients-to-know-their-specific-genotype. February 6, 2015. Accessed October 22, 2018.

CFTR, cystic fibrosis transmembrane conductance regulator.


CFTR proteins in normal cells


CFTR proteins: An important regulator of fluid and ion balance in organs throughout the body

• CFTR proteins are found on epithelial cell surfaces in organs throughout the body1-4

• Normally, CFTR protein channels transport ions, such as chloride and bicarbonate, through the epithelial cell surface in these organs1-4

• Maintaining water and salt balance at the epithelial cell surface requires an adequate quantity and function of CFTR proteins5

References: 1. Zielenski J. Respiration. 2000;67(2):117-133. 2. O’Sullivan BP, Freedman SD. Lancet. 2009;373(9678):1891-1904. 3. MacDonald KD et al. PediatrDrugs. 2007;9(1):1-10. 4. Derichs N. Eur Resp Rev. 2013;22(127):58-65. 5. Elborn JS. Lancet. 2016;388(10059):2519-2531. 6. Boyle MP, De Boeck K. Lancet Rep Med. 2013;1:158-163.

CFTRquantity X = Total

CFTR activityChannel-openprobability ConductanceX

CFTR function

• Total CFTR activity can be defined as total ion transport mediated by CFTR protein channels at the cell surface. It is dependent on CFTR protein quantity and function4,6


CFTR protein function is determined by channel-open probability and channel conductance

Reference: 1. Wang W, Linsdell P. Biochim Biophys Acta. 2012;1818(3):851-860. 2. Hwang TC, Kirk KL. Cold Spring Harb Perspect Med. 2013;3(1):a009498. 3. Bompadre SG et al. J Gen Physiol. 2007;129(4):285-298.

Channel-open probability: the fraction of time that a single CFTR protein channel is open and transporting ions.1

Channel conductance: rate at which ions move through open CFTR protein channels.2

Normal CFTR channel-open probability3 Normal CFTR channel conductance1

Status of a single channel over time3*

*Based on in vitro experimentation.


CFTR proteins reach the cell membrane through a multi-step process

References: 1. Boyle MP, De Boeck K. Lancet Resp Med. 2013;1:158-163. 2. Derichs N. Eur Resp Rev. 2013;22(127):58-65.

CFTR mutations may affect different steps of this process to reduce CFTR quantity and/or function and reduce total CFTR protein activity1,2

X

X

X

X

X X

CFTR gene expression

CFTR protein synthesis

CFTR protein function and breakdown

CFTR protein trafficking(delivery to the cell surface)


The importance of understanding CFTR mutations and their nomenclature


CFTR mutations are inherited genetically

Reference: 1. Boyle MP, De Boeck K. Lancet Rep Med. 2013;1:158-163.

If a person inherits 2 copies of a disease-causing CFTRmutation (one from each parent), CFTR protein dysfunction can occur1

• CFTR protein dysfunction impairs cellular chloride transport

• CF disease can be the result


Different naming conventions are used by researchers in publications, genetic clinics, and

diagnostic laboratories

CFTR mutations can be referred to by different names

References: 1. Cystic Fibrosis Foundation. Patient Registry Annual Data Report 2016. Bethesda, MD. © 2017 Cystic Fibrosis Foundation. 2. Ogino S, et al. The Journal

of molecular diagnostics:2007;9(1):1-6 3. NCBI GenBank. https://www.ncbi.nlm.nih.gov/genbank. Accessed October 25, 2018.

Names of CFTR Mutations1,2,3

Name Definition Utilization

Legacy name (F508del)

Commonly used colloquial nomenclature

Used primarily in genetic clinics and diagnostic laboratories

cDNA name (c.1521_1523delCTT)

DNA sequenceUsed to identify mutations as listed

in global sequence databases such as GenBank®

Protein name (p.Phe508del)

Amino acid sequence (three-letter code)

Used to identify pathogenic mechanism of mutation based on

altered protein synthesis


Certain CFTR mutations may affect CFTR quantity and/or function, reducing total CFTR activity

References: 1. Castellani C, Assael BM. Cell Mol Life Sci. 2017;74:129-140. 2. Derichs N. Eur Resp Rev. 2013;22(127):58-65.

Certain CFTR mutations result in some

CFTR activity1,2

Other CFTR mutations

result in little to no CFTR activity1,2

CBAVD, congenital bilateral absence of vas deferens.

Spectrum of phenotypes associated with total CFTR activity1

Adapted with permission from Castellani and Assael.

People with 2 CFTR mutations resulting in loss of CFTR activity generally have a CF phenotype, which may include1,2

• Elevated sweat chloride (>60 mmol/L)• Pancreatic insufficiency• CBAVD• Lung function decline over time• Pseudomonas aeruginosa colonization


The effects of CFTR mutations on total CFTR activity


Different CFTR mutations produce different effects on CFTR protein quantity and function

References: 1. Derichs N. Eur Respir Rev. 2013;22(127):58-65. 2. Boyle MP, De Boeck K. Lancet Rep Med. 2013;1:158-163. 3. Zielenski J. Respiration. 2000;67(2):117-133. 4. Cystic Fibrosis Foundation. Patient Registry Annual Data Report 2016. Bethesda, MD. © 2017 Cystic Fibrosis Foundation.

CFTR mutations are grouped into classes according to their effects on CFTR protein synthesis, trafficking, or function1-3

Defective synthesis (Class I)

Defective processing and trafficking (Class II)

Decreased splicing (Class V)

Decreased protein stability (Class VI)

Defective gating (Class III)

Defective conductance

(Class IV)

The top 25 mutations are comprised of those in classes I-V4

CFTRquantity X = Total

CFTR activityChannel-openprobability

ConductanceXCFTR function


An absence of CFTR protein…

…regardless of function since few to no CFTR proteins reach the surface...

…results in little to no total CFTR activity

Examples of CFTR mutations that result in defective biosynthesis of the CFTR protein

References: 1. Derichs N. Eur Resp Rev. 2013;22(127):58-65. 2. Boyle MP, De Boeck K. Lancet Rep Med. 2013;1:158-163. 3. Zielenski J. Respiration. 2000;67(2):117-133. 4. Ogino S et al. J Mol Diagn. 2007;9(1):1-6. 5. NCBI ClinVar https://www.ncbi.nlm.nih.gov/clinvar/RCV000029523. Accessed October 25, 2018. 6. Cystic Fibrosis Foundation. Patient Registry Annual Data Report 2016. Bethesda, MD. © 2017 Cystic Fibrosis Foundation.

Examples (Class I)

Legacy Name4,5 cDNA Name4,5 Protein Name4,5 Type4 US Prevalence (%)6

G542X c.1624G>T p.Gly542X Nonsense 4.7

1717-1G-->A c.1585-1G>A No Protein Name Splice defect 1.6

3659delC c.3528delC p.Lys1177SerfsX15 Frameshift 0.7

Defective synthesis yields no functional

CFTR protein1-3Little to none1-3

CFTR quantity CFTR function Total CFTR activity

XChannel-open

probability:Little to none

Conductance: Little to noneX =


Examples of CFTR mutations that result in defective processing and trafficking of the CFTR protein

References: 1. Derichs N. Eur Resp Rev. 2013;22(127):58-65. 2. Boyle MP, De Boeck K. Lancet Rep Med. 2013;1:158-163. 3. Zielenski J. Respiration. 2000;67(2):117‐133. 4. Ogino S et al. J Mol Diagn. 2007;9(1):1-6. 5. Cystic Fibrosis Foundation. Patient Registry Annual Data Report 2016. Bethesda, MD. © 2017 Cystic Fibrosis Foundation.

aHomozygous and heterozygous.

Examples (Class II)

Legacy Name cDNA Name Protein Name Type4 US Prevalence (%)5

F508del c.1521_1523delCTT p.Phe508del Amino acid deletion 86.4a

N1303K c.3909C>G p.Asn1303Lys Missense 2.4

Significantly reduced CFTR protein…

…regardless of function since few to no CFTR proteins reach the surface...


Defective processing and trafficking results in few CFTR proteins

at cell surface1-3

Little to none1-3


XChannel-open




Example of CFTR mutations that reduce channel-open probability in the CFTR protein


Examples (Class III)


G551D c.1652G>A p.Gly551Asp Missense 4.4

Although CFTR protein quantity may

be normal…

…function is severely reduced due to decreased channel-open probability...


Normal Little to none1-3


XDefective channel

gating at cell membrane1-3

Conductance: NormalX =


Example of CFTR mutations that result in a CFTR protein with defective conductance


Examples (Class IV)


R117H c.350G>A p.Arg117His Missense 3.0

Although CFTRprotein quantity may

be normal…

…function is reduced due todecreased conductance...

…and results in some total CFTR activity

Normal Some1-3


XChannel-open

probability: Normal

Conductance: Reduced1-3X =


Examples of CFTR mutations that result in some functional CFTR at the cell surface


Examples (Class V)

Legacy Name cDNA Name Protein Name Type4,5 US prevalence (%)5

3849+10kbC->T c.3717+12191C>T p.Phe508del Splice defect 1.5

A455E c.1364C>A p.Ala455Glu Missense 0.6

CFTR protein quantity is reduced…

…even though CFTR function is not impaired...

…and results in some total CFTR activity

Reduced1-3 Some1-3


XChannel-open




The importance of patients and caregivers knowing their CF genotype


Patients benefit from knowing their CF genotype• Patients/Caregivers who are educated about their CF genotypes may be empowered to be active partners in their own care

• Patients/Caregivers who know about their genotype may1:

�Better understand their CF symptoms and how CF may progress

�Seek access to treatment plans that are most appropriate for them

�Actively participate in making care decisions with their doctor or healthcare team

�Be informed about research studies that may be an option for them

Reference: 1. Cystic Fibrosis News Today. https://www.cysticfibrosisnewstoday.com/2015/02/06/cf-trust-will-pay-for-patients-to-know-their-specific-genotype. February

6, 2015. Accessed October 22, 2018.


Engaging patients and caregivers in conversations about CF

Communicating with patients and/or caregivers about how their CF genotype can impact care

and treatment decisions is important; but helping them understand the reasons can be

challenging. Below are some points that can be useful when talking with patients/caregivers

about their CF genotype:

Consider discussing how:

• Every person with cystic fibrosis inherited two mutated CF-causing genes which together form

a person’s CF genotype1

• Different mutations impair CFTR function in different ways leading to variations in the

symptoms, disease severity, and disease progression each person with CF experiences2

• Despite the variation in presentation, all genotypes experience disease progression, making

personalized disease management important throughout a patient’s lifetime2,3

• A care plan can also be personalized based on environment, physiology, patient and caregiver

preferences, and lifestyle4

References: 1. Orenstein D et al. Cystic Fibrosis: A Guide for Patient and Family. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2004. 2. Zielenski J.

Respiration. 2000;67(2):117‐133. 3. McKone EF. J Cyst Fibros. 2015;14(5):580-586. doi:10.1016/j.jcf.2015.01.005. Epub 2015 Feb 3. 4. Feng L et al. Health Aff(Millwood). 2018;37(5):773-779.

The most important thing for the patient/caregiver to remember is that when the genotypeis known, the care team can customize CF care and treatment plans to help optimize each patient’s situation


THE ROLE OF CFTR IN CFSummary


Summary

References: 1. Boyle MP, De Boeck K. Lancet Rep Med. 2013;1:158-163. 2. Derichs N. Eur Resp Rev. 2013;22(127):58-65. 3. Cystic Fibrosis Foundation. Patient Registry Annual Data Report 2016. Bethesda, MD. © 2017 Cystic Fibrosis Foundation. 4. Zielenski J. Respiration. 2000;67(2):117‐133. 5. Feng L et al. Health Aff(Millwood). 2018;37(5):773-779.

CFTR protein is an important cellular regulator in organs

throughout the body1,2

CFTR mutations result in a loss of total CFTR protein activity,

which is the underlying cause of CF1,2

Different types of CFTR mutations vary in their effects on

CFTR protein quantity and/or function2,3

Both CFTR mutations play a role in determining phenotype or

disease severity4

Discussing genotype with patients helps them understand their CF

disease presentation and treatment options5

the role of cftr mutations in causing …...•every person with cystic fibrosis inherited two...

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