Clinical characteristics.

Muenke syndrome is defined by the presence of the specific FGFR3 pathogenic variant – c.749C>G – that results in the protein change p.Pro250Arg. Muenke syndrome is characterized by considerable phenotypic variability: features may include coronal synostosis (more often bilateral than unilateral); synostosis of other sutures, all sutures (pan synostosis), or no sutures; or macrocephaly. Bilateral coronal synostosis typically results in brachycephaly (reduced anteroposterior dimension of the skull), although turribrachycephaly (a "tower-shaped" skull) or a cloverleaf skull can be observed. Unilateral coronal synostosis results in anterior plagiocephaly (asymmetry of the skull and face). Other craniofacial findings typically include: temporal bossing; widely spaced eyes, ptosis or proptosis (usually mild); midface retrusion (usually mild); and highly arched palate or cleft lip and palate. Strabismus is common. Other findings can include: hearing loss (in 33%-100% of affected individuals); developmental delay (~33%); epilepsy; intracranial anomalies; intellectual disability; carpal bone and/or tarsal bone fusions; brachydactyly, broad toes, broad thumbs, and/or clinodactyly; and radiographic findings of thimble-like (short and broad) middle phalanges and/or cone-shaped epiphyses. Phenotypic variability is considerable even within the same family. Of note, some individuals who have the p.Pro250Arg pathogenic variant may have no signs of Muenke syndrome on physical or radiographic examination.

Diagnosis/testing.

The diagnosis of Muenke syndrome is established by the identification of the FGFR3 pathogenic variant c.749C>G (p.Pro250Arg).

Management.

Treatment of manifestations: Children with Muenke syndrome and craniosynostosis are best managed by a pediatric craniofacial clinic that typically includes a craniofacial surgeon and neurosurgeon, clinical geneticist, ophthalmologist, otolaryngologist, pediatrician, radiologist, psychologist, dentist, audiologist, speech therapist, and social worker. Depending on severity, the first craniosynostosis repair (fronto-orbital advancement and cranial vault remodeling) is typically performed between ages three and six months. An alternative approach is endoscopic strip craniectomy, which is a less invasive procedure and is typically performed prior to age three months.

Postoperative increased intracranial pressure and/or the need for secondary or tertiary extracranial contouring may occur. The need for secondary revision procedures is inversely related to the age of the affected individual at the time of initial repair. The location of the fused/synostotic suture, type of fixation, and the use of bone grafting do not have a significant effect on the need for revision.

Standard treatments for sensorineural hearing loss; early speech therapy and intervention programs for those with developmental delay, intellectual impairment, behavioral problems, and/or hearing loss; surgical correction for strabismus; lubrication for exposure keratopathy.

Prevention of secondary complications: Early surgical reconstruction for craniosynostosis may reduce the risk for complications including sequelae related to increased intracranial pressure (e.g., behavioral changes).

Surveillance: Affected individuals benefit from integrated multidisciplinary care and protocol-driven management from birth to maturity that includes: annual multidisciplinary reviews and periodic review by a social work team; regular developmental assessments; periodic repeat audiograms to screen for acquired or progressive hearing loss; periodic assessment for strabismus.

Genetic counseling.

Muenke syndrome is inherited in an autosomal dominant manner with incomplete penetrance and variable expressivity. If the defining FGFR3 pathogenic variant cannot be detected in either parent of a proband, germline mosaicism in a parent or a de novo pathogenic variant in the proband are possible. Each child of an individual with Muenke syndrome has a 50% chance of inheriting the pathogenic variant. Prenatal testing for a pregnancy at increased risk is possible if the pathogenic variant has been identified in the family. Prenatal ultrasound examination may be used as an adjunct to prenatal genetic testing.

Suggestive Findings

Muenke syndrome should be suspected in individuals with the following clinical and radiographic findings.

Clinical features

• Facial asymmetry
• Brachycephaly (reduced anteroposterior dimension of the skull), turribrachycephaly (a "tower-shaped" skull), or cloverleaf skull
• Sutural ridging over both (or less commonly one) of the coronal sutures accompanied by:
  • Ipsilateral
    • Flattening of the forehead
    • Elevation of the superior orbital rim
    • Elevation of the eyebrow
    • Anterior placement of the ear
    • Deviation of the nasal root
  • Contralateral
    • Frontal bossing of the forehead
    • Depression of the eyebrow
• Temporal bossing
• Macrocephaly without craniosynostosis
• Craniosynostosis with sensorineural hearing loss

Radiographic findings

• Head CT with three-dimensional reconstruction demonstrating:
  • Unilateral coronal craniosynostosis
  • Bilateral coronal craniosynostosis
  • Synostosis of other sutures (lambdoid, metopic, sagittal, squamosal)
• Extracranial radiographic features can include:
  • Fusion of the carpal bones (commonly the capitate and hamate or trapezoid and trapezium bones) [Muenke et al 1997, Trusen et al 2003, Kruszka et al 2016]
  • Fusion of the tarsal bones (commonly the calcaneus and cuboid bones) [Muenke et al 1997, Trusen et al 2003, Agochukwu et al 2013, Kruszka et al 2016]
  • Thimble-like (short and broad) middle phalanges of the hands and feet [Muenke et al 1997, Graham et al 1998, Kruszka et al 2016]
  • Epiphyseal coning [Muenke et al 1997, Graham et al 1998, Kruszka et al 2016]

Establishing the Diagnosis

The diagnosis of Muenke syndrome is established in a proband by the identification of a heterozygous c.749C>G (p.Pro250Arg) pathogenic variant in FGFR3 by molecular genetic testing (see Table 1).

Molecular genetic testing approaches can include single-gene testing or use of a multigene panel:

• Single-gene testing. Sequence analysis of FGFR3 can performed first. Note: Targeted analysis for the c.749C>G pathogenic variant in FGFR3 is rarely performed because the clinical features of Muenke syndrome overlap with those of other craniosynostosis conditions caused by different heterozygous pathogenic variants in FGFR3 and other craniosynostosis-related genes (see Differential Diagnosis).
• A multigene panel that includes FGFR3 and other genes of interest (see Differential Diagnosis) may also be considered. Note: (1) The genes included in the panel and the diagnostic sensitivity of the testing used for each gene vary by laboratory and are likely to change over time. (2) Some multigene panels may include genes not associated with the condition discussed in this GeneReview; thus, clinicians need to determine which multigene panel is most likely to identify the genetic cause of the condition while limiting identification of variants of uncertain significance and pathogenic variants in genes that do not explain the underlying phenotype. (3) In some laboratories, panel options may include a custom laboratory-designed panel and/or custom phenotype-focused exome analysis that includes genes specified by the clinician. (4) Methods used in a panel may include sequence analysis, deletion/duplication analysis, and/or other non-sequencing-based tests.
  For an introduction to multigene panels click here. More detailed information for clinicians ordering genetic tests can be found here.

Clinical Description

Craniosynostosis and craniofacial features. Coronal synostosis may be bilateral (~2/3 of affected individuals) or unilateral (~1/3 of affected individuals) [Renier et al 2000, Keller et al 2007, Kruszka et al 2016]. Occasionally, other sutures may be involved, including the metopic suture (leading to trigonocephaly), the sagittal suture, the squamosal suture, or, rarely, all sutures (pan synostosis) [Golla et al 1997, van der Meulen et al 2006, Doumit et al 2014, Kruszka et al 2016].

Craniosynostosis is not always present in individuals with Muenke syndrome:

• In 72 individuals from 24 families, nine persons (12.5%) known to be heterozygous for the FGFR3 p.Pro250Arg pathogenic variant had no evidence of craniosynostosis [Renier et al 2000].
• In a family of seven, five had coronal synostosis and two were phenotypically normal [Moko & Blandin de Chalain 2001]. All seven individuals were heterozygous for the FGFR3 p.Pro250Arg pathogenic variant.
• In a large international cohort of individuals with Muenke syndrome, 16 (15.5%) of 103 did not have craniosynostosis [Kruszka et al 2016].

In these cases, extracranial findings (i.e., hearing loss [often mild to moderate], radiographic findings of carpal and tarsal fusions, short and broad middle phalanges, and cone-shaped epiphysis), when present, helped support the diagnosis of Muenke syndrome.

Craniofacial features that may result from craniosynostosis are summarized in Suggestive Findings. Rarer craniofacial features include ptosis, malar flattening, a short nose with anteverted nares, an overhanging nasal tip, deviation of the nasal septum, a short nose with a depressed nasal bridge, high-arched palate and cleft lip and/or palate, dental malocclusion, mild retrognathia, hypoplastic auricles, and low-set ears.

Hearing loss. Initial studies revealed that at least one third of individuals with Muenke syndrome have mild to moderate sensorineural hearing loss [Muenke et al 1997, Kress et al 2006, de Jong et al 2010], with some even suggesting that all individuals with Muenke syndrome are likely to have some degree of hearing loss, usually mild [Doherty et al 2007, Honnebier et al 2008, Mansour et al 2009, de Jong et al 2011b]. There is evidence that sensorineural hearing loss (usually mild and mid-to-low frequency) is specific to Muenke syndrome compared to other FGFR-related craniosynostosis syndromes [Agochukwu et al 2014a], sometimes occurring in individuals with Muenke syndrome who do not have craniosynostosis [Hollway et al 1998].

• In a large-cohort international study, more than 70% of individuals with Muenke syndrome had hearing loss, with a majority (70.8%) having bilateral sensorineural hearing loss and the remainder having conductive (22%) and mixed forms (8.6%).
• Overall, a majority of hearing loss observed in craniosynostosis syndromes is conductive, except in Muenke syndrome, where sensorineural hearing loss is more likely [de Jong et al 2010, Agochukwu et al 2014b].
• Some children with Muenke syndrome and craniosynostosis develop hearing loss despite passing their newborn hearing screens [Doherty & Muenke, personal observation]. Additionally, some affected individuals may have hearing loss that progresses or becomes more severe as they age.
• Some individuals with Muenke syndrome have recurrent episodes of otitis media treated with myringotomy tube placement [Didolkar et al 2009, Kruszka et al 2016], which may explain the occurrence of conductive hearing loss in some affected individuals.

Developmental delay and behavioral functioning. Developmental delay and/or intellectual disability, usually mild, has been reported in approximately one third of individuals [Muenke et al 1997, Kress et al 2006]. Compared to normative populations, individuals with Muenke syndrome have also been reported to be at increased risk for developing some behavioral and emotional problems [Maliepaard et al 2014, Yarnell et al 2015].

In a large international study of Muenke syndrome, 40.8% were reported to have intellectual disability and 66.3% had developmental delay, with speech delay the most common type (61.1%) [Kruszka et al 2016]. Approximately 24% had a diagnosis of ADHD.

• In a study of intellectual outcomes following protocol management in four persons with Muenke syndrome followed from birth to skeletal maturity compared to persons with Crouzon syndrome and Pfeiffer syndrome, Flapper et al [2009] found that individuals with Muenke syndrome and Pfeiffer syndrome were more likely to be intellectually impaired than were individuals with Crouzon syndrome. One of the four with Muenke syndrome had moderate intellectual disability (IQ <70) and a history of behavioral problems; two had borderline intellectual disability (IQ 70-80) and required special education; and one was of average intelligence (IQ 90-110), completed high school without difficulty, and is currently training to be a pilot.
• In a study by de Jong et al [2012] of individuals with syndromic craniosynostosis, it was found that all of the studied syndromes had a high prevalence of speech delay. In this study, cognitive delay was mainly reported in Apert, Crouzon, and Muenke syndromes. A study of syndromic craniosynostosis by Bannink et al [2011] found behavioral problems to be more common in boys with Apert and Muenke syndromes, with a prevalence of 67% and 50%, respectively.
• One study found a slightly lower IQ in individuals with craniosynostosis with Muenke syndrome compared to individuals with craniosynostosis who do not have the defining pathogenic variant [Arnaud et al 2002].
• After evaluating 13 children with Muenke syndrome, a study by Maliepaard et al [2014] found that children with Muenke syndrome had more social, attention, and inattention problems compared to a normative population and children with other craniosynostosis syndromes.
• In a cohort on 44 affected persons, Yarnell et al [2015] found that individuals with Muenke syndrome were at increased risk for developing adaptive and executive functioning problems compared to their unaffected (mutation negative) sibs and the normative population. Interestingly, the change in behavior in the affected cohort was not dependent on the presence or absence of craniosynostosis or hearing loss, raising the question of an intrinsic brain effect of the pathogenic FGFR3 p.Pro250Arg variant that is distinct from the change in skull shape.

Neurologic abnormalities. Differences in patterns of the expression, formation, and structure of the central nervous system may be partly responsible for the developmental delay and intellectual disability observed in Muenke syndrome.

• The following intracranial anomalies have been reported:
  • Hippocampus and bilateral medial temporal dysgenesis in one person [Grosso et al 2003], who was described as developmentally normal
  • Bilateral lateral ventricular dilatation and a small cerebellum in one person [Yu et al 2010]
  • Porencephalic cyst of the occipital horn of left ventricle and absence of the corpus callosum in one person [Escobar et al 2009]
• Epilepsy was reported in 13 individuals with Muenke syndrome by Agochukwu et al [2012]. More recently, 20 (20.2%) of 99 individuals with Muenke syndrome had a history of seizures [Kruszka et al 2016].
• One individual had a cranial nerve VI deficit leading to paralytic strabismus [Lowry et al 2001].

Ocular anomalies. Strabismus is the most common ocular finding in Muenke syndrome [Yu et al 2010], with one large study reporting an incidence of 31/69 (45%) [Kruszka et al 2016] and another study finding strabismus in 14 (39%) of 26 individuals [de Jong et al 2010].

• Compared to the average pediatric population, children with Muenke syndrome have a higher incidence of strabismus (66%), anisometropia, epicanthal fold changes, widely spaced eyes, downward lateral canthal dystopia, and amblyopia [Jadico et al 2006].
• Ptosis of the upper eyelids has been described in 13 affected individuals [de Jong et al 2011a, Kruszka et al 2016].
• Nystagmus has been described in four affected individuals [Jadico et al 2006, Singh et al 2014, Kruszka et al 2016].

Limb findings. Most individuals with Muenke syndrome have normal-appearing hands and feet with normal range of motion of all joints; therefore, many of the limb findings in Muenke syndrome are identified during the diagnostic evaluation when radiographs reveal findings such as short, broad middle phalanges of the fingers, absent or hypoplastic middle phalanges of the toes, carpal and/or tarsal fusion, and cone-shaped epiphyses [Hughes et al 2001, Kruszka et al 2016]. Broad toes and great thumbs have also been described in individuals with Muenke syndrome.

Cutaneous syndactyly has been described in 13 affected individuals [Golla et al 1997, Passos-Bueno et al 1999, Chun et al 2002, Trusen et al 2003, Shah et al 2006, Baynam & Goldblatt 2010, de Jong et al 2011a].

Obstructive sleep apnea (OSA), a common finding in craniosynostosis syndromes in general, is less prevalent in those with Muenke syndrome [Bannink et al 2011, Dentino et al 2015].

Minor clinical signs / asymptomatic heterozygotes

• Some individuals heterozygous for the FGFR3 p.Pro250Arg pathogenic variant have no clinical or radiographic features of Muenke syndrome [Robin et al 1998, Moko & Blandin de Chalain 2001, Kruszka et al 2016].
• Some individuals with Muenke syndrome have minor clinical signs such as macrocephaly [Gripp et al 1998] and subtle facial findings without craniosynostosis [Gripp et al 1998, Robin et al 1998, Moko & Blandin de Chalain 2001, Sabatino et al 2004, Didolkar et al 2009]; some appear clinically unaffected until their radiographs are examined [Muenke et al 1997]. These individuals may not come to medical attention until the birth of a more severely affected child.

Penetrance

Penetrance is reduced. In a familial study of seven affected individuals, six of eight individuals had coronal synostosis and two of eight were phenotypically normal [Moko & Blandin de Chalain 2001], yielding a penetrance of approximately 75% for that family.

There was no sex-based difference in penetrance and severity of craniosynostosis in a large international cohort of individuals with Muenke syndrome [Kruszka et al 2016]. In this study involving 106 people, 50% of females had bilateral coronal craniosynostosis compared to 44.4% of males (p=0.84), and the penetrance of craniosynostosis trait was similarly prevalent in males (86.7%) and females (82.8%). Previous smaller studies have found penetrance to be higher in females than in males [Moloney et al 1997, Lajeunie et al 1999, Doherty et al 2007, Honnebier et al 2008, Solomon & Muenke 2010].

Nomenclature

Several terms in the literature may be confusing to the reader:

• The term "nonsyndromic" means that a condition does not fit the pattern of a recognizable genetic syndrome [Mulliken 2002]. The term "nonsyndromic craniosynostosis" is variably used either:
  • As a synonym for single-suture craniosynostosis because most instances of single-suture craniosynostosis are of unknown etiology. However, the two terms are not identical in meaning.
    OR
  • To describe bilateral coronal suture synostosis that is not identifiable as a classic syndrome (e.g., Pfeiffer syndrome, Crouzon syndrome). When appropriate, the authors suggest the alternate terms "nonspecific craniosynostosis" or "unclassified brachycephaly."
• The phrase "Muenke nonsyndromic coronal craniosynostosis" is occasionally used to mean Muenke syndrome. The authors discourage the use of this phrase because it inaccurately implies a "non-genetic" cause of Muenke syndrome.
• An individual with "isolated craniosynostosis" has no extracranial manifestations [Cohen & MacLean 2000]. However, some authors use the term "isolated craniosynostosis" to mean that only one type of suture (e.g., coronal, sagittal) is fused. The correct term for uni- or bilateral premature fusion of one suture is "simple craniosynostosis." "Complex craniosynostosis" is correctly used to describe the involvement of two or more sutures.
• In the field of genetics, the term "sporadic" has been used to describe a variety of disparate phenomena. The term "sporadic craniosynostosis" is used by some authors to mean a case without a family history. However, "sporadic craniosynostosis" may incorrectly imply a low recurrence risk. The correct term for a single occurrence of Muenke syndrome in a family is "simplex."
• The term "Adelaide-type craniosynostosis" is no longer used to describe Muenke syndrome.

Prevalence

The birth prevalence of Muenke syndrome is approximately one in 30,000.

In a prospective study of 214 individuals with craniosynostosis born between 1993 and 2005, Morriss-Kay & Wilkie [2005] reported that of the 60 who had a specific molecular diagnosis, 28.5% had the p.Pro250Arg pathogenic variant; thus, 8% of the 214 had Muenke syndrome.

Muenke syndrome is estimated to account for 25%-30% of all genetic causes of craniosynostosis [Morriss-Kay & Wilkie 2005, Wilkie et al 2010].

The c.749C>G pathogenic variant (p.Pro250Arg) in FGFR3 is estimated to occur at a rate of 7.6-8x10^-6 per haploid genome, one of the highest known rates for a human transversion [Moloney et al 1997, Rannan-Eliya et al 2004].

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with Muenke syndrome, the following evaluations are recommended:

• Assessment of suture involvement by skull radiographs or preferably 3D skull CT
• Assessment for hydrocephalus with brain CT or MRI
• Assessment for exposure keratopathy
• Hearing assessment
• Developmental assessment in children
• Ophthalmologic assessment including screening for strabismus and vision. Additionally, this assessment should include fundoscopy to assess for papilledema, a finding that is present when intracranial pressure (ICP) is increased.
• Consultation with a clinical geneticist and/or genetic counselor

Treatment of Manifestations

Children with Muenke syndrome and craniosynostosis should be referred to a craniofacial clinic with pediatric experience. These individuals benefit most from a multidisciplinary approach to care. A craniofacial clinic associated with a major pediatric medical center usually includes: a surgical team (craniofacial surgeon and neurosurgeon), clinical geneticist, ophthalmologist, otolaryngologist, pediatrician, radiologist, psychologist, dentist, audiologist, speech therapist, and social worker. Other disciplines are involved as needed.

Craniosynostosis. Depending on the severity, the first craniosynostosis repair is typically performed between age three and six months. This procedure is usually transcranial (i.e., the skull is opened down to the dura so that the bones can be physically repositioned during a procedure such as a midface advancement).

Newer approaches being performed include endoscopic strip craniectomy and posterior distraction (PD):

• Endoscopic strip craniectomy is typically performed before the affected child reaches age three months and has an overall long-term improved symmetry compared to traditional cranial vault remodeling and fronto-orbital advancement.
• Posterior distraction (PD) is used to manage individuals with severe brachycephaly or turribrachycephaly. The procedure has associated risks and more studies to establish long-term outcomes are needed [Wiberg et al 2012, Thomas et al 2014].

Following craniosynostosis repair, the need for a second procedure is increased in those with Muenke syndrome compared to those with craniosynostosis without the defining pathogenic variant. The reasons for a second procedure vary by individual and can include:

• Severe initial clinical presentation requiring a staged repair
• Cranial vault abnormalities including temporal bulging and recurrent supraorbital retrusion requiring extracranial contouring (i.e., use of a cement such as calcium phosphate to contour the surface of the skull)
• Postoperative increased ICP
• Recurrent deformity requiring a second transcranial repair:
  • The need for a surgical revision for aesthetic reasons (typically temporal bulging) has been reported in multiple series [Renier et al 2000, Cassileth et al 2001, Arnaud et al 2002, Thomas et al 2005, Honnebier et al 2008].
  • According to Thomas et al [2005], individuals with craniosynostosis and the defining pathogenic variant for Muenke syndrome were more likely to require early intervention with a posterior release operation (at age ~6 months) to prevent excess frontal bulging than were those without the defining pathogenic variant.
  • Seven (24.1%) of 29 individuals with the p.Pro250Arg pathogenic variant underwent a second surgery (6/7 had increased ICP) as compared to two (4.3%) of 47 without the pathogenic variant. This difference in reoperation rate was statistically significant (p=0.048) [Thomas et al 2005].
  • In the report of Honnebier et al [2008], 16 individuals with Muenke syndrome required a second procedure: seven required a second transcranial procedure; 15 were expected to undergo extracranial contouring. Note that none had increased ICP.
  • However, a study by Ridgway et al [2011] challenges the above findings, reporting a frequency of frontal revision in individuals with Muenke syndrome who had fronto-orbital advancements that was lower than previously reported. This study found that the need for secondary revision procedures was inversely related to the age of the affected individual at the time of the initial repair. The location of the fused/synostotic suture, type of fixation, and the use of bone grafting do not have a significant effect on the need for revision.

In Muenke syndrome a discrepancy between severity of the craniofacial findings (e.g., severe midface retrusion, widely spaced eyes) and neurologic findings (e.g., increased ICP, hydrocephalus, structural brain anomalies, severe developmental delay, or severe intellectual disability) has been noted [Lajeunie et al 1999, Arnaud et al 2002, Honnebier et al 2008]: severe early clinical findings such as recurrent deformity and the need for a second major procedure did not correlate with postoperative risk for increased ICP.

Hearing loss. Hearing loss is often sensorineural. Standard treatments for hearing loss apply, including special accommodations for school-aged children, hearing aids, and (potentially) cochlear implants (see Hereditary Hearing Loss and Deafness Overview) [Agochukwu et al 2014b].

Developmental delay. Individuals with Muenke syndrome are at increased risk for behavioral problems, intellectual disability, and developmental delay [Maliepaard et al 2014, Yarnell et al 2015]; thus, referral for speech therapy and early intervention is indicated. Referral to a developmental and/or behavioral specialist for assessment and treatment is recommended.

Ocular abnormalities

• Strabismus surgery/correction is indicated to prevent amblyopia.
  Because surgical correction of craniosynostosis is a priority, delay in strabismus surgery in the first two years of life is common; however, earlier correction of strabismus should be considered to achieve binocularity.
• In those with proptosis, lubrication for exposure keratopathy is indicated.

Prevention of Secondary Complications

Early surgical reconstruction for craniosynostosis may reduce the risk for complications including sequelae related to increased intracranial pressure (e.g., behavioral changes).

Surveillance

The following are appropriate:

• Regular developmental assessments of affected children
• Periodic repeat audiograms
• Periodic assessment for strabismus
• As part of protocol-driven care and management: annual multidisciplinary reviews and periodic review by a social work team. Protocol-driven approaches to surveillance currently in use include those of Flapper et al [2009] and de Jong et al [2010].

Evaluation of Relatives at Risk

It is appropriate to evaluate relatives at risk in order to identify as early as possible those who would benefit from institution of treatment and preventive measures (particularly in individuals affected with craniosynostosis, hearing loss, developmental delay, and/or cognitive disability).

Evaluation includes targeted molecular genetic testing for the FGFR3 c.749C>G pathogenic variant.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Mansour et al [2009] determined that in the mouse model of Muenke syndrome all mice had low-frequency sensorineural hearing loss. The characteristic sensorineural hearing loss is probably due to abnormal development of the auditory sensory epithelium of the inner ear including excess pillar cells, too few Deiters' cells, and extra outer hair cells in the organ of Corti. A further study revealed that the rescue of cochlear function and hearing loss phenotype of these mice is possible with a reduction in FGF-10, which normally activates FGFR-2b or FGFR-1b [Mansour et al 2013]. Aberrant signaling through the FGF signaling pathway that includes FGFR3 may be the cause of the abnormal development of auditory sensory cells in Muenke syndrome [Agochukwu et al 2014a].

Animal models indicate that FGFR3 is expressed at its highest levels in the developing central nervous system.

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions.

Mode of Inheritance

Muenke syndrome is inherited in an autosomal dominant manner.

Risk to Family Members

Parents of a proband

• Most individuals diagnosed with Muenke syndrome have an affected parent.
• A proband with Muenke syndrome may have the disorder as the result of de novo pathogenic variant. The proportion of cases caused by a de novo pathogenic variant is unknown.
• As in achondroplasia, de novo pathogenic variants causing Muenke syndrome appear to be exclusively of paternal origin [Rannan-Eliya et al 2004] and to be associated with advanced paternal age.
• Recommendations for the evaluation of parents of a proband with an apparent de novo pathogenic variant include: physical examination; radiographs of the skull, hands, and feet; and testing for the FGFR3 p.Pro250Arg pathogenic variant. Evaluation of the parents may determine that one is heterozygous for the p.Pro250Arg pathogenic variant but has a mild phenotype.
• If the p.Pro250Arg pathogenic variant cannot be detected in the leukocyte DNA of either parent, possible explanations include de novo occurrence of pathogenic variant in the proband or germline mosaicism in a parent. Though theoretically possible, no instances of germline mosaicism have been reported.
• Note: Although most individuals diagnosed with Muenke syndrome have an affected parent, the family history may appear to be negative because of subtle/absent clinical findings (reduced penetrance) or failure to recognize the disorder in family members.

Sibs of a proband. The risk to the sibs of the proband depends on the genetic status of the proband's parents:

• If a parent of the proband is affected or has the defining FGFR3 p.Pro250Arg pathogenic variant, the risk to the sibs of inheriting this pathogenic variant is 50%. Because penetrance is reduced and the phenotype is variable within a family, some individuals who inherit the FGFR3 p.Pro250Arg pathogenic variant have no (or extremely mild) signs of Muenke syndrome.
• When the parents are clinically unaffected and do not have the FGFR3 p.Pro250Arg pathogenic variant, the risk to the sibs is low.
• If the defining pathogenic variant found in the proband cannot be detected in the leukocyte DNA of either parent, the risk to sibs is presumed to be slightly greater than that of the general population (though still <1%) because of the theoretic possibility of parental germline mosaicism [Rannan-Eliya et al 2004].

Offspring of a proband. Each child of an individual with Muenke syndrome has a 50% chance of inheriting the pathogenic variant.

Other family members. The risk to other family members depends on the genetic status of the proband's parents: if a parent is affected and/or has the FGFR3 p.Pro250Arg pathogenic variant, his or her family members are at risk.

Related Genetic Counseling Issues

Consideration of molecular genetic testing of young, at-risk family members is appropriate for guiding medical management (see Management, Evaluation of Relatives at Risk). Prior to testing sibs, parents, and extended family members, discussion should be held with a genetic counselor regarding the risks, benefits, and limitations of testing.

Generally, in individuals of school age and older who have no developmental issues, developmental delay, hearing loss, craniosynostosis, or other features of Muenke syndrome, the likelihood of Muenke syndrome is quite low, though the FGFR3 p.Pro250Arg pathogenic variant has been identified in seemingly unaffected individuals [Kruszka et al 2016]. Children who inherit the FGFR3 p.Pro250Arg pathogenic variant may be more or less severely affected than their parents. Uni- and bilateral coronal synostosis as well as absence of synostosis may be seen in individuals in the same family.

Considerations in families with an apparent de novo pathogenic variant. When neither parent of a proband with an autosomal dominant condition has the pathogenic variant, the variant is likely de novo. However, germline mosaicism or non-medical explanations, including alternate paternity or maternity (e.g., with assisted reproduction) or undisclosed adoption, could also be explored.

Family planning

• The optimal time for determination of genetic risk and discussion of the availability of prenatal/preimplantation genetic testing is before pregnancy.
• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected or at risk.

Prenatal Testing and Preimplantation Genetic Testing

Molecular genetic testing. Once the FGFR3 p.Pro250Arg pathogenic variant has been identified in an affected family member, prenatal testing for a pregnancy at increased risk and preimplantation genetic testing for Muenke syndrome are possible.

Ultrasound examination. Craniosynostosis should be suspected when the cephalic index, cranial shape, or fetal face shape is abnormal [Tonni et al 2011]. Although difficult, prenatal diagnosis may be possible by ultrasound examination of the calvarial sutures. When present, additional craniofacial features of Muenke syndrome (i.e., midface hypoplasia, ocular hypertelorism) may also be apparent [Shaw et al 2011].

In a family known to have the pathogenic variant, if craniosynostosis or other craniofacial features (i.e., midface hypoplasia, ocular hypertelorism) are seen on prenatal ultrasound examination, the index of suspicion for Muenke syndrome should be high.

On prenatal ultrasound examination of twins with Muenke syndrome, Escobar et al [2009] found normal anatomy in one twin and congenital anomalies in the other twin. Molecular diagnosis of Muenke syndrome was made after birth in both twins.

Differences in perspective may exist among medical professionals and within families regarding the use of prenatal testing. While most centers would consider use of prenatal testing to be a personal decision, discussion of these issues may be helpful.

Literature Cited

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Author Notes

We have ongoing studies at the National Institutes of Health (NIH) that focus on various aspects of Muenke syndrome, and we hope to improve our understanding of hearing, cognitive function, and development in people with Muenke syndrome.

We are currently conducting research on the relationship between development, cognitive function, and hearing in individuals with Muenke syndrome. The goal of this study is to better understand the development of the central nervous system as well as to understand the causes of developmental delay and intellectual disabilities that can occur in some individuals with Muenke syndrome. This study will also help us to learn much about the long-term outcomes and functioning of adults with Muenke syndrome. In addition, we also hope to be able to outline factors that may contribute to and predict mental prognosis in individuals with Muenke syndrome. Please note: You do not need to have developmental delay, intellectual disabilities, or hearing loss in order to participate in our study.

Acknowledgments

We are indebted to the affected individuals we work with and their families. This work was supported by the Division of Intramural Research, National Human Genome Research Institute, National Institutes of Health.

Revision History

• 10 November 2016 (ma) Comprehensive update posted live
• 19 June 2014 (me) Comprehensive update posted live
• 7 December 2010 (me) Comprehensive update posted live
• 10 May 2006 (me) Review posted live
• 30 January 2006 (mm) Original submission