Genetics – What it can (and can’t) tell us about stutteringDennis Drayna, PhDNIDCD/National Institutes of HealthUS Public Health Service, DHHS

Developmental StutteringRecoveredPersistentGeneticUnknown

What causes stuttering?Many suggested factorsNo reproducible evidence for any except genetic causesEvidence for genetic causes comes from:Twin studiesAdoption studiesStudies of large families

Evidence for genetic factors in stuttering Twin studies9 independent twin studies published over 40 yearsEnglish, Dutch, Japanese, Finnish, Italian, DanishHeritability estimates have risen as studies have become larger and more sophisticatedMost recent heritability estimates greater than 80%Adoption studiesTwo independent studiesNeither large enough to achieve strong statistical significanceNo evidence stuttering is learned

Large familiesFamily X - IowaThe Utah FamilyThe Cameroon FamilyInitial explanations based on shared family environment became less likely as more distant family members were found who were affected and raised elsewhere

How do we identify a gene that causes a disorder?

The traditional paradigm1. Identify stable variation – a trait or a disorder2. Demonstrate that variation is due to genetic factors 3. Perform studies in families with many cases of the disorder to identify the location of the causative gene(s) – a linkage study4. Search the location to identify a genetic variant carried by the affected family members and not by unaffected family members

Linkage studies of stutteringInitial studies not very successfulWeak support for the findingsFailures to find the same location across different studiesNo clear identification of disease genes

Solution?Specialized populationsTake advantage of unusual population structurePakistan

Pakistani stuttering familiesPKST 72

Gene identification strategyFocus on this region on chromosome 12 The problem - 87 genes lie within this interval

The first stuttering geneFound an apparent mutation in a gene called GNPTABIn affected members of PKST72In affected members of other Pakistani stuttering familiesMutations were subsequently found in two related genes called GNPTG and NAGPA

What do these genes do?All cells contain a “recycling bin” called the lysosomeThe lysosome contains some 60 different enzymes that break down things the cell no longer needs to their basic building blocks, which can be re-usedThese 60 enzymes get to the lysosome by a targeting signal, called mannose-6-phosphateWhen an enzyme has a mannose-6-phosphate, the cell knows that enzyme belongs in the lysosome, and sends it thereThe products of the GNPTAB, GNPTG, and NAGPA genes put the mannose-6-phosphate on these 60 different enzymes

What happens when this targeting system goes wrong?Mutations in GNPTAB and GNPTG are associated with a disease called mucolipidosisRare inherited medical disorder

ML IIML IIINone of the symptoms of ML were found in stuttering individuals carrying mutations in GNPTAB, GNPTG, or NAGPA

Do the mutations identified thus far affect enzyme function?Mucolipidosis (ML) is associated with severe mutations of GNPTAB/GMLII typically displays 0-3% activityMLIII typically displays 3-15% activityMutations in NAGPA encoding the UCE have not been associated with human disease

Cellular effects of stuttering mutations on enzyme activityOur studies done in a test tube and in cells in culture show reliable decreases in GNPATB/G and NAGPA activity in the enzymes that contain mutations found in stutteringDecreases in activity are modest, about 50%

Where we are nowMutations in these 3 genes appear to account for ~ 13% of individuals with persistent stutteringMost of the mutations are rare, but several are seen repeatedly

Can we identify genes that underlie more cases of stuttering?44 Pakistani families – chromosome 121 Pakistani family – chromosome 31 Pakistani family – chromosome 161 extended Cameroonian family

Cameroonian Stuttering FamilyRaza et al 2013

The causative gene?Contrary to our expectations, there is not one gene that causes stuttering in this familyDifferent branches of the family have different causative genes, on different chromosomesChromosomes 2, 3, 14, and 15

Chromosome 15 geneActs in a similar process in the cell as that produced by the GNPTAB,GNPTG and NAGPA genesMutations in this gene appear to be responsible for about 6% of persistent stuttering cases

Stuttering Loci and GenesJune 2012

Deafness Loci and GenesJune 2012

Working hypothesisA specific group of nerve cells in the brain are unique to speech production and also uniquely sensitive to this metabolic deficitGoal – Identify these cells, discover what they do, determine what they’re connected to, and understand how this inherited deficit uniquely affects them

State of the science I. After adjustments for the murky genetics of stuttering, it’s now clear that some of the traditional human genetic methods can be used to find causative genesThere are now numerous causative genes at different locations, and there are likely to be moreIdentification of these causative genes will remain challenging

State of the science II. Mutations found in human stuttering exert deleterious effects on the intracellular trafficking and function of the GNPTAB/G and NAGPA enzymesOverall cellular activity reduced ~50%A major goal is to identify the site of neuropathology caused by these relatively mild mutations

Implications for SLPDifferent therapy outcomes for genetic vs. non-genetic stuttering?Different therapy outcomes for different genetic causesMedicalization of stuttering

AcknowledgmentsNIDCDChangsoo Kang M. Hashim RazaTae-Un HanCarlos Dominguez Eduardo SainzEmily ParisNHGRI/NISCAlice YoungJim MullikanDonna KrasnewichNIH Clinical CenterPenelope FriedmanNCBIAlejandro SchafferHollins Communications Research Institute - Jennifer MundorffCEMB/University of the PunjabJamil Ahmad Shahid Khan S. RiazuddinWashington University St. LouisWang Sik Lee, Stuart KornfeldTerra Barnes, Tim HolyStuttering Foundation of AmericaBritish Stammering AssociationNational Stuttering AssociationSpeak Clear Association of CameroonJoseph LukongStuttering research subjects worldwide