Clinical delineation of the PACS1-related syndrome—Report on 19 patients

INTRODUCTION

We have previously described two unrelated boys with a strikingly similar facial appearance, moderate intellectual disability, and cryptorchidism who carry an identical de novo mutation in PACS1 (NM_0180026.2, OMIM 615009: Mental Retardation, Autosomal Dominant 17) [Schuurs-Hoeijmakers et al., 2012]. This finding of an identical de novo c.607C>T mutation in two individuals that shared strikingly similar clinical findings, suggested that mutations in PACS1 define a recognizable syndromic form of intellectual disability. A recent report on a third patient with an identical mutation and a similar facial appearance and comparable clinical presentation provided further confirmation of a specific recognizable PACS1-related syndrome [Gadzicki et al., 2014]. In the present publication we report 16 further cases (9 male and 7 female) ages 2–21 years, with the recurrent de novo c.607C>T mutation in PACS1. All mutations were identified by exome sequencing in a clinical or research setting from all around the world. We describe the phenotype of these unpublished individuals (including 3 from the Deciphering of Developmental Disorders Study [Deciphering Developmental Disorder Study, 2014]) and the 3 recently reported individuals [Schuurs-Hoeijmakers et al., 2012; Gadzicki et al., 2014] and review their clinical characteristics.

GENETICS

All patients have the same de novo c.607C>T mutation in PACS1 that results in an amino acid substitution of Arginine to Tryptophan in the Furin (cargo)-binding region (FBR) of the protein, directly adjacent to a CK2-binding motif. This amino acid is evolutionarily highly conserved in PACS1 and in its close paralog PACS2. All mutations reported here were identified by exome sequencing, and the patients were evaluated in human genetics centers. Previous genetic testing by conventional karyotyping, microarray analysis or targeted gene analysis did not reveal abnormalities, with the exception of a total of five inherited rare CNVs that were detected by microarray analysis in four individuals: two paternally inherited deletions in individual 3 (a 117 kb deletion in 2q13 and a 108 kb deletion in 16p13.2) [Gadzicki et al., 2014], a paternally inherited deletion of 150 kb in 7q11.23 in individual 11, a paternally inherited duplication of 600 kb in 11p11.12, and a 673 kb duplication in 14q32.33 of unknown inheritance in individual 18. All these rare CNVs were considered benign and do not contribute to the phenotype.

BIRTH, CONGENITAL ABNORMALITIES, AND EARLY DEVELOPMENT

Pregnancy was complicated by intrauterine growth retardation in individuals 17 and 18, accompanied by oligohydramnios in individual 17. Nine individuals were born by caesarean (Supplementary Table S1). Early development was characterized by abnormal facial features, frequent neonatal hypotonia, and a variety of congenital abnormalities. (Table I and Supplementary Table SI). Congenital anomalies were present in most individuals (15/19). Heart defects, mostly atrial or ventricular septal defects, were present in eight. Ten individuals had eye abnormalities: one had bilateral iris coloboma, one had bilateral optic nerve coloboma and one individual had an irregular optic disc. High myopia, nystagmus and strabismus were noted as well. Cryptorchidism was present in six of twelve males.

FACIAL APPEARANCE

Facial appearance is strikingly similar in all patients (Fig. 1, Supplementary Fig. S1). Individuals have full and arched eyebrows, hypertelorism with downslanting palpebral fissures (in most), and long eye lashes. Ptosis is seen in many. The ears are prominent, simple and low-set, and the nose has a bulbous tip. The mouth is wide with downturned corners and a thin upper lip with an unusual “wavy” profile and a flat philtrum. There is diastema of the teeth.

GROWTH, DEVELOPMENT AND BEHAVIOR

Failure to thrive in early childhood was noted in five individuals. Growth is generally within the normal range in later childhood, except for individual 1 who had surgery for volvulus in the neonatal period and subsequently developed short bowel syndrome, which contributed to his impaired growth. In general, the patients demonstrate oromotor sensitivity with a strong preference for soft food. Problems with learning to eat solid food are reported by the parents and may persist into childhood, adolescence and adulthood. Slender posture is frequent, and may reflect oral aversion and subsequent poor food intake. The parents of individual 8, now 21 years of age, report that she has always had problems with texture in food. She hardly bites her food with her front teeth and will swallow without chewing. Individual 3 will only eat when distracted by watching a movie. Consultation with a speech therapist and/or dietitian has been of value in some individuals. Four individuals had a G-tube inserted for their feeding problems. Motor development is delayed but eventually, walking is achieved between age 2 and 3 years in most patients, although an unsteady gait persists in some. Individual 1 has an unstable gait without specific neurologic abnormalities, makes intermittent use of a wheelchair. Gradual loss of walking abilities with frequent falling was noted from 10 years on in individual 18. She developed an ataxic, crouching gait, and required a walker to assist with ambulation from age 11 years on. Serial MRIbrain scans showed progressive cerebellar atrophy with childhood onset; MR spectroscopy was unremarkable. Patients may have marked hypotonia (8/19), which tends to improve over time. Hypotonia may contribute to early feeding problems and may further lead to severe pes planus requiring orthopedic shoes for support when initially learning to walk (4/19). Cognitive development is delayed in all individuals. The level of intellectual disability varies from mild to moderate in those who have undergone formal IQ testing. Language is more severely delayed than motor development. Individuals 6, 7, 8, and 18 had no speech at age 4, 3, 20, and 11 years, respectively. While Individual 8 does not use language she is able to read and can communicate by an iPad with a “Touch Chat” app she started using from age 18 years. This application greatly increased her means of communication, and helped in reducing frustration and oppositional behaviors. Other methods that improved communication are child sign language and picture exchange cards. Individual 1 started speaking in sentences at 8 years followed by reading at 11 years. In contrast, we noted that a number of other individuals started speaking in their second year. Parents frequently commented on excellent (visual) memory. The patients' disposition is generally happy and friendly. Individuals with the PACS1 mutation enjoy interacting with other people and appreciate getting personal affection. Difficulties in behavior are reported (12/19): individual 6 and 16 express demanding behavior, with a negative demeanor in drawing attention. Many parents report that individuals are frequently frustrated, leading to temper tantrums with aggressive outbursts towards themselves and others. Individual 6 can be aggressive and has sleeping problems, whilst individual 16 shows self-aggression, anxiety and hyperphagia. Six individuals show behavior within the autism spectrum. They profit from a structured day program. A dislike for loud or sudden noises (hyperacusis) is a common observation of parents. Several parents report that their children prefer to have specific sensational stimuli on their face (e.g., covering the face with a soft scarf).

EPILEPSY AND CEREBRAL IMAGING

Epilepsy is a frequent feature. Twelve individuals developed febrile convulsions or seizures of varying severity and onset. Seizures were mostly well-controlled with anti-epileptic drugs. In some individuals seizures did not return after anti-convulsive treatment was discontinued. Cerebral imaging was performed in 16 individuals and showed abnormalities in 12. Four individuals had cerebellar vermis anomalies, ranging from diminished volume of cerebellar vermis to partial agenesis of the vermis (Supplementary Fig. S2). Ventricular abnormalities were seen in four individuals, accompanied by communicating hydrocephalus in two (Supplementary Fig. S2). Three individuals had subtle white matter abnormalities. Individual 18 had mild cerebellar atrophy with clinically progressive loss of walking abilities (Supplementary Fig. S3). Occasional cerebral abnormalities included a mild delay in myelination, frontal cortical dysplasia, diffuse cerebral parenchymal loss, hypoplasia of the cerebellar hemispheres, partial deficiency of the frontal lobes and mild cerebellar hypoplasia. Craniosynostosis was seen in individual 17 (Supplementary Fig. S2).

OCCASIONAL ABNORMALITIES

A number of other features were observed in two or more individuals. These included gastric reflux (six individuals), and inguinal or umbilical hernia (three individuals). Chest abnormalities were reported: widely spaced nipples (three individuals), scoliosis (two individuals) and pectus excavatum (three individuals). Single palmar crease was present in four individuals. Kidney abnormalities were found in three: a right duplex kidney and posterior urethral diverticulum in individual 5, an extra renal pelvis in individual 7 and a duplex right kidney in individual 8. Nine individuals have constipation that required treatment.

DISCUSSION

The present study, performed on 19 individuals, shows that the recurrent de novo c.607C>T mutation in PACS1 results in a clinically recognizable intellectual disability (ID) syndrome. The hallmarks of this PACS1 related ID syndrome are a recognizable facial appearance (Fig. 1) and intellectual disability with marked speech delay. Seizures are frequent and respond well to treatment. There is a wide range of congenital anomalies, notably cardiac, cerebral, eye, and kidney abnormalities. Cryptorchidism is frequent. The clinical presentation of this larger series of individuals strongly resembles the clinical presentation of the first two cases confirming that the mutation causes a specific syndrome. Syndromes with overlapping facial characteristics are Baraitser Winter syndrome (OMIM: #243310), Cornelia de Lange syndrome (OMIM: #122470), Mowat-Wilson syndrome (OMIM: #235730) and Kabuki syndrome (OMIM: #147920). Although diagnostic sequencing of PACS1 has been available at Radboudumc Nijmegen we are not aware of any clinically diagnosed cases. All individuals were diagnosed by exome sequencing. We expect this to change as clinicians become more familiar with the phenotype.

The individuals described in this study were collected via two routes: (i) clinicians contacting the corresponding author of our previous study [Schuurs-Hoeijmakers et al., 2012], and (ii) the parents who found each other via social media (there is an active PACS1-parent Facebook community—PACS1 closed group). This second route was a valuable source of information about feeding and behavioral issues and it nicely demonstrated the drive that parents have to meet and share life experiences and concerns, and consult each other about daily problems. Parents regularly provided each other with practical information that clinicians could not provide. We have become convinced that clinicians and researchers should recognize the potential for bringing together individuals with the same rare genetic condition through social media.

We are not aware of de novo mutations at other genomic positions in PACS1 apart from the c.607C>T mutation. Together, the recurrence of this particular mutation and the absence of mutations at other positions in the gene, strengthens our previous proposal of a dominant-negative or a gain-of-function effect at the protein level [Schuurs-Hoeijmakers et al., 2012]. The mutation is located in the furin cargo binding domain, directly adjacent to a CK2-binding motif that is essential for PACS1 autoregulation and might affect the auto regulatory property of the protein as well as interaction with cargo, or adaptor binding. This may lead to sequestration and mislocalization of the protein partners of PACS1. Further definite proof of this hypothesis has to come from additional functional studies. Our previous studies in zebrafish embryos documented a role for the PACS1 protein in cranial neural crest migration. This may explain the obvious facial similarity in our patients [Schuurs-Hoeijmakers et al., 2012].

Several individuals in this report were recruited via large exome sequencing projects. The mutation was identified in 0.2–0.3% of the studied cohorts (four mutations in 2000 studied individuals with intellectual disability at Baylor college of Medicine, and 4 mutations in the 1133 studied individuals in the Deciphering Developmental Disorders Study [Deciphering Developmental Disorder Study, ]). The rapid discovery of almost 20 individuals with the same de novo mutation is interesting in light of recent data that suggest that some disease causing mutations confer a positive selection during spermatogenesis. The paradigmatic example for such a scenario is the c.1138G>A mutation in FGFR3 (NM_000142​.4) in achondroplasia (OMIM:#100800) [Goriely and Wilkie, 2012; Shinde et al., 2013]. The mutation rate of the c.1138G>A in achondroplasia is much higher than the genome average. There is evidence that there is positive selection of this mutation in sperm cells [Goriely and Wilkie, 2012; Shinde et al., 2013], which is due to a gain-of-function effect. Whether the c. 607C>T mutation in PACS1 has a similar selective advantage in sperm cells will require further studies.

In summary, the PACS1-related syndrome constitutes a clinically recognizable intellectual disability syndrome with multiple congenital anomalies. Based on the clinical findings in the 19 individuals that we report here, we offer the following suggestions for clinical work-up and management. Early evaluation should include cardiac, neurologic, ophthalmologic and renal evaluation. The high incidence of feeding difficulties requires special attention. Furthermore, the language delay requires proactive speech therapy. Early introduction of alternative means of communication can improve and facilitate communication, prevent frustration and advance development. Paying attention to hyperacusis and avoiding exposure to high volume can prevent discomfort. Given the recognizable facial gestalt, we hope that the present study will facilitate clinical recognition of further cases.