TCF4, The Gene Behind Pitt-Hopkins Syndrome

Pitt-Hopkins syndrome is a rare, genetic, neurodevelopmental disorder caused by haploinsufficiency of TCF4 that leads to several different physiological complications including intellectual disabilities and developmental delays.

Introduction

Pitt-Hopkins syndrome (PTHS) is an ultra-rare genetic disorder affecting at least 1,286 people worldwide. PTHS is caused by a mutation in the TCF4 gene, which impairs TCF4 protein function, causing a number of negative downstream effects. Patients with this condition will often experience developmental delays, GI issues, intellectual disabilities, seizures, breathing problems, problems with their motor skills, distinctive facial features, and may also struggle with social and communication issues associated with autism spectrum disorder [1]. Currently, there is no cure for PTHS, however, some symptoms can be managed through specialized health care and personalized education plans [2]. In this article, we will dig into the clinical and genetic details of PTHS, as well as the history and future of PTHS and PTHS patient care.

Tell Me About This Rare Disease

Genetic basis: PTHS occurs when a mutation takes place on the TCF4 gene located on chromosome 18, locus 18q21.2 [3]. PTHS typically occurs due to de novo mutations, but in very rare cases can also be inherited in an autosomal dominant pattern [3]. Causative mutations may include missense, nonsense, frameshift, splice-site mutations, translocations, and/or large deletions, some of which are shown in Figure 1 [4]. Although much remains a mystery surrounding the TCF4 gene, here is what we do know.

Figure 1: Schematic of variants identified in the TCF4 gene causing intellectual disability and possibility of PTHS [5].

The TCF4 gene, or Transcription Factor 4, is a complex gene encoding multiple mRNA isoforms with greater variability at the 5’ end [3] [6]. This TCF4 protein is a vital transcription factor that plays an important role in the activation of gene expression of many genes, including maturation, differentiation, and apoptotic cellular programs [1]. TCF4 forms a homodimer and heterodimer with other basic helix-loop-helix proteins and together bind with promoter regions of proteins that are involved in pathways that are associated with breathing phenotypes and promoter regions for neurodevelopmental genes [7]. For example, TCF4 forms a heterodimer with ASCL1 (as shown in Figure 2) which is involved in pathways that if disrupted are thought to be associated with breathing issues in patients with PTHS [7].

Figure 2: TCF4 has numerous co-factors for activating different transcriptional programs [7].

TCF4 is expressed in a variety of different tissues, however, it is most abundantly expressed in the central nervous system (CNS). TCF4 is also developmentally regulated, with the highest amount of expression occurring during fetal development [8]. When this protein is unable to function properly it leads to brain development and neuronal differentiation issues seen in patients with PTHS [9].

Clinical Presentation: PTHS is a neurodevelopmental disorder that causes developmental delays and varying intellectual disabilities. On top of this, chronic GI issues affect about 75% of all patients affected by PTHS [10]. Additional symptoms often include breathing problems such as hyperventilation and apnea, epilepsy, delayed fine motor skills, inability to speak or impaired communication skills, hypotonia, ataxia, minor brain abnormalities, flat feet, nearsightedness, crossed eyes, cryptorchidism in males, a short stature, and distinctive facial features [1] [11]. Some of these common facial and brain anomalies can be seen in Figure 3.

Figure 3: MRI scans displaying common abnormalities seen in PTHS including dysplasia of the corpus callosum, bulging caudate, global atrophy, and scattered T2 hyperintensities [9].

Patients typically start experiencing symptoms in childhood. Children with PTHS may experience delays in reaching certain developmental milestones such as sitting, rolling over, walking, and talking [11]. Children with PTHS may also be easily agitated or overexcited, showing happy hand-flapping movements [1] [12]. These children may also experience any or all of the symptoms mentioned above.

There is conflicting information regarding whether PTHS can be classified as an autism spectrum disorder (ASD), however, it is clear that many affected by PTHS experience symptoms of ASD [1] [11] [12].

Incidence: The exact prevalence of PTHS is unknown, however, about 1,286 people with PTHS have been reported worldwide and it is estimated that 1 in 34,000 to 1 in 41,000 people could be affected [1] [13] [14].

Brief history:

• 1978: PTHS was first described by David Pitt and Ian Hopkins in the Australian Pediatric Journal [15].

• 2007: Haploinsufficiency of TCF4 was first shown to be connected to PTHS [16].

• 2009: Pitt Hopkins Syndrome Foundation was founded in the Netherlands [44].

• 2013: The Pitt Hopkins Research Foundation became its own non-profit organization [17].

• 2014: The Pitt Hopkins Syndrome Italian Association was formed [24].

• 2016: Pitt Hopkins UK was founded on May 17th [23].

• 2019: The Pitt Hopkins Research Foundation announces a clinical trial for microbiota transfer therapy [18].

• 2022: Clinical trials began to study the safety, tolerability, pharmacokinetics, and efficacy of NNZ-2591 in children and adolescents with PTHS [19].

The State of the Disease Today: Today, patients can live for many years by managing the symptoms of PTHS which can include intellectual disabilities, epilepsy, and breathing abnormalities. There are no targeted treatments currently available, but there are currently ongoing clinical trials to study the safety and efficacy of two different treatment options. The first is a fecal transplant to improve gastrointestinal issues and behavioral problems caused by PTHS and the second is a synthetic peptide drug developed by Neuren Pharmaceuticals which has been shown to restore cognitive function and other behavioral issues caused by PTHS in mice [18] [20].

What is it like to be a patient with this disease?

Who are the patients? People with PTHS typically start experiencing symptoms as a newborn or infant [21]. These symptoms are life-long and often require specialized care for issues relating to breathing, digestion, and other behavioral and medical issues [22]. PTHS does not directly affect life expectancy, however, particular symptoms may be life-threatening [12].

What do current treatment options look like? Current treatment options for PTHS are limited to managing symptoms. Treatments differ between patients depending on the severity of symptoms but may include occupational therapy, speech therapy, dietary changes and laxatives (to treat constipation), and surgical correction of eyes [12][10]. PTHS symptoms such as epilepsy have also been treated with valporate as well as other antiepileptics and anticonvulsants [9] [12].

Are there advocacy groups? Yes. In 2008, the Pitt-Hopkins Syndrome Support Group was formed to provide support for patients and families of PTHS as well as information on PTHS [23]. Later, the Pitt Hopkins Research Foundation was established to advocate for individuals with PTHS and drive research towards finding a cure [17]. Around the globe, there are organizations such as Pitt Hopkins UK, and Pitt Hopkins Syndrome Italian Association as well as several other rare disease networks working to help patients and families of patients with PTHS [23] [24].

Figure 4: TCF4 is located on chromosome 18q21.2 [25] [26].

Are there genetic tests? Yes, there are genetic tests available to patients. Individuals can check their eligibility for free genetic testing for PTHS through Probably Genetic [27]. Genetic testing for PTHS involves sequence analysis of the TCF4 gene, which can detect approximately 70% of pathogenic variants. If nothing is found, then a deletion/duplication analysis is performed which can detect whole or partial gene deletions, which comprise of 30% of pathogenic variants in TCF4 [28]. Finally, if a genetic variant is still not detected, patients could have karyotyping done to detect any chromosomal deletions or rearrangements [28]. A diagram of where TCF4 is located can be seen above in Figure 4.

How do scientists and clinicians study this disease?

Are there good/any model systems scientists can use to develop drugs? The most common model system to study PTHS are mouse models with disrupted TCF4 function.

Figure 5: Tcf4 truncation mouse models display increased hyperactivity compared to healthy mice of the same litter [29].

Tcf4 truncation mouse models have been used to study PTHS and have shown to mimic similar breathing issues and increased hyperactivity associated with PTHS (shown in Figure 5), but do not show seizure-like activity [29]. Mouse models have also been generated to mimic common pathogenic point mutations and deletions in TCF4, which impair the function of TCF4 and lead to microcephaly, and deficient hippocampus-dependent learning and memory [30]. Conditional mouse models of PTHS have also been developed to study the disorder [31].

Have natural history studies been done? A study published in 2016 collected information on 101 individuals with a molecular confirmed diagnosis of PTHS through an internet questionnaire and gathered data on behavioral aspects, developmental milestones, health problems and appearance of individuals [32]. Additionally, the Pitt Hopkins Research Foundation has partnered with Invitae’s Rare Patient Network to collect patient health history data to help move clinical trials forward. So far, the data collected has been used to submit Investigational New Drug (IND) findings to the FDA. PTHS patients and families are encouraged to sign up on the Ciitizen website [33] [34].

Certain physicians or centers that are experts? There are many clinics across the world that are experts in PTHS, including the Pitt-Hopkins Clinic at Massachusetts General Hospital, Children’s Health University of Texas (UT) Southwestern Dallas Texas, University of California San Francisco, Safra Children’s Hospital, Sheba Medical Center in Israel and Children’s Hospital Colorado [35] [36] [37]. Dr. Kimberly Goodspeed at UT Southwestern has expertise in rare genetic developmental disorders including PTHS and has published case studies and reviews on PTHS [38]. Dr. Jessica Duis founded the Chromosome 15 and Related Disorders Clinic that treat rare genetic diseases such as PTHS [37].

What are the major challenges in studying and curing this disease? A major challenge is the ability to clinically diagnose PTHS as there are limited natural history studies, limiting the available clinical diagnostic scoring tools. This could be due to the lack of patients in addition to the variability between the molecular and clinical diagnosis within the patient population [9].

The Cure Corner: What is needed for a cure?

What does an ideal therapeutic look like? An ideal therapeutic would restore normal levels of TCF4 protein, in theory, this would correct symptoms of PTHS.

Are there companies already developing drugs? Neuren Pharmaceuticals has developed a drug called NNZ-2591 that is currently in Phase II for Prader-Willi Syndrome and recruiting for an open-label study in PTHS [19] [39]. It is an analogue of cyclic glycine proline which is important for neuronal development. When treating mice with a Tcf4 mutation, NNZ-2591 restored deficits such as learning and memory, motor performance and hypoactivity compared to a wild-type mouse [20]. Mahzi Therapeutics, in collaboration with the Muotri lab at UCSD, are developing an AAV9 based gene therapy drug to improve clinical phenotypes of PTHS patients by replacing TCF4 deficiency [40] [41].

What are current therapies and treatments lacking? There is no cure or targeted therapeutic for PTHS. Current treatment options address only symptoms caused by PTHS, which vary from patient to patient. There are currently no drugs in clinical trials that target TCF4, however, Neuren Pharmaceuticals has a drug in clinical trials that has shown to normalize deficits in a Pitt Hopkins Tcf4 knockout mouse model [20].

Could an RNA therapeutic fit the need? In theory, modulation of TCF4 expression by RNA therapeutics may correct PTHS phenotypes. For example, TCF4 can be upregulated through antagonizing microRNAs that have been shown to be negative regulators of TCF4 [31] [42]. La Jolla Labs is partnered with Pitt-Hopkins Research Foundation to investigate the feasibility of using antisense oligonucleotides (ASOs) to restore protein levels of TCF4 and reverse PTHS phenotypes [43].

Conclusion

Although a cure has not yet been established for Pitt-Hopkins syndrome, increased research in how RNA therapeutics can be used to modify TCF4 expression gives us hope that one day there may be a cure for this life-altering disorder. La Jolla Labs continues to work with established partners to one day reach our goal of discovering a cure for all patients diagnosed with PTHS.

A special thanks to Melissa Keenan, Pitt Hopkins Research Foundation, and Pitt Hopkins UK for their insight and review of this article prior to publication.

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