Clinical characteristics.

MPPH (megalencephaly-postaxial polydactyly-polymicrogyria-hydrocephalus) syndrome is a developmental brain disorder characterized by megalencephaly (brain overgrowth) with the cortical malformation bilateral perisylvian polymicrogyria (BPP). At birth the occipital frontal circumference (OFC) ranges from normal to 6 standard deviations (SD) above the mean for age, sex, and gestational age; in older individuals the range is from 3 to 10 SD above the mean. A variable degree of ventriculomegaly is seen in almost all children with MPPH syndrome; nearly 50% of individuals have frank hydrocephalus. Neurologic problems associated with BPP include oromotor dysfunction (100%), epilepsy (50%), and mild-to-severe intellectual disability (100%). Postaxial hexadactyly occurs in 50% of individuals with MPPH syndrome.

Diagnosis/testing.

The clinical diagnosis of MPPH syndrome can be established in individuals with the two core features: megalencephaly and polymicrogyria (PMG). The molecular diagnosis of MPPH syndrome is established in a proband with some of the suggestive clinical and imaging features by identification of a heterozygous pathogenic variant in one of three genes: AKT3, CCND2, or PIK3R2. While most individuals with MPPH syndrome have a germline pathogenic variant in one of these genes, some have a somatic mosaic pathogenic variant (most commonly reported in PIK3R2 or AKT3).

Management.

Treatment of manifestations: Hydrocephalus warrants early neurosurgical intervention. Treatment per neurooncologist for those with medulloblastoma. Oromotor difficulties, epilepsy, developmental delays, intellectual disability, polydactyly, vision issues, cardiac anomalies, thyroid abnormalities, and renal anomalies are treated as per usual clinical care standards. Social worker support and care coordination for families of affected individuals.

Surveillance: Follow up with a pediatric neurologist regularly to monitor and treat epilepsy. Brain MRI to detect hydrocephalus and/or cerebellar tonsillar ectopia is provisionally recommended every six months from birth to age two years, and yearly from age two to six years. In older individuals, the frequency should be determined based on prior brain imaging findings as well as clinical findings. Brain imaging (with particular attention to the posterior fossa) may be considered every six months to assess for medulloblastoma. Assess growth and feeding at each visit. Routine follow up with a developmental pediatrician given the high risk of developmental delays and/or intellectual disability. Ophthalmology examination annually or as needed; endocrine follow up as recommended by endocrinologist. Assess need for social work support and care coordination at each visit.

Genetic counseling.

MPPH syndrome is an autosomal dominant disorder typically caused by a de novo pathogenic variant. Almost all individuals with MPPH syndrome have the disorder as the result of a de novo germline AKT3, CCND2, or PIK3R2 pathogenic variant; somatic mosaic pathogenic variants in PIK3R2 and AKT3 have been reported in a few affected individuals. Vertical transmission of a PIK3R2 pathogenic variant from an affected heterozygous parent to several affected children has been reported in one family to date. Presumed parental germline mosaicism has been suggested in three families. Each child of an individual with a germline AKT3, CCND2, or PIK3R2 pathogenic variant has a 50% chance of inheriting the pathogenic variant. The risk for transmission to offspring of an individual with somatic mosaicism for an MPPH-related pathogenic variant is expected to be less than 50%. Once the MPPH syndrome-related pathogenic variant has been identified in an affected family member, prenatal testing for a pregnancy at increased risk and preimplantation genetic testing for MPPH syndrome are possible.

Suggestive Findings

MPPH syndrome should be suspected in individuals with the following clinical and imaging findings [Mirzaa et al 2004, Mirzaa et al 2012]. Note: Findings shown in bold are core features.

Clinical findings

• Macrocephaly or megalencephaly (occipital frontal circumference ≥2 SD above the mean); onset either prenatally or postnatally
• Postaxial polydactyly of one or more extremities
• Hypotonia
• Early-onset epilepsy
• Intellectual disability
• Oromotor dysfunction (including speech/swallowing difficulties, excessive drooling, expressive speech delays)

Imaging findings

• Cortical brain malformations including polymicrogyria and specifically, bilateral perisylvian polymicrogyria
• Progressive ventriculomegaly leading to hydrocephalus
• Cerebellar tonsillar ectopia or Chiari malformations
• Thick corpus callosum (or mega corpus callosum)

Establishing the Diagnosis

The clinical diagnosis of MPPH syndrome is established in a proband with the two core features: megalencephaly and polymicrogyria.

The molecular diagnosis of MPPH syndrome is established in a proband with some of the suggestive clinical and imaging findings by identification of a heterozygous pathogenic variant in one of three genes: AKT3, CCND2, or PIK3R2 (see Table 1). While most individuals with MPPH syndrome have a germline (i.e., constitutional) pathogenic variant in one of these three genes, some individuals have been reported with a somatic mosaic pathogenic variant in one of these genes (most commonly PIK3R2 and AKT3).

Note: Failure to detect either a germline or somatic mosaic pathogenic variant in one of these three genes does not exclude a clinical diagnosis of MPPH syndrome in a proband with the two core clinical and imaging features.

Molecular genetic testing approaches used to identify germline and somatic pathogenic variants can include use of a multigene panel or comprehensive genomic testing (exome sequencing, genome sequencing), testing for somatic mosaicism, and chromosomal microarray analysis (CMA):

• A multigene panel that includes AKT3, CCND2, PIK3R2, and other genes of interest (see Differential Diagnosis) should be considered to detect germline and somatic variants in the MPPH-related genes. 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. (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. (5) Somatic mosaicism for variants in the three MPPH-related genes may not be detected by all commercially available multigene panels due primarily to the inability to test tissues other than blood (e.g., skin or buccal cells) and/or technical limitations in detecting low-level mosaicism; thus, clinicians considering use of a multigene panel need to select a panel specifically optimized to detect mosaicism for the three MPPH-related genes.
  For an introduction to multigene panels click here. More detailed information for clinicians ordering genetic tests can be found here.
• Comprehensive genomic testing does not require the clinician to determine which gene is likely involved. Exome sequencing is most commonly used; genome sequencing is also possible.
  For an introduction to comprehensive genomic testing click here. More detailed information for clinicians ordering genomic testing can be found here.

If no germline pathogenic variant is found in any of the three genes, sequence analysis for AKT3, PIK3R2, or CCND2 with methods to detect somatic mosaicism and/or testing for a large duplication of 1q43-q44 that includes AKT3 may be warranted:

• Testing for somatic mosaicism. Sequence analysis of DNA derived from saliva or skin (whether visibly affected or not) may detect a pathogenic variant not detected in DNA isolated from blood.
  Note: Sensitivity to detect low-level mosaicism of a somatic pathogenic variant is greatest using massively parallel sequencing (i.e., next-generation sequencing) in tissues other than blood, and in particular will be of high yield when analyzing affected tissues.
• CMA analysis for duplications or triplications of 1q43-q44 that includes AKT3. Because not all gene-targeted deletion/duplication methods are designed to size large copy number variants, CMA is the most appropriate for detection of this duplication or triplication.

Clinical Description

MPPH syndrome is a developmental brain disorder characterized by megalencephaly (brain overgrowth) with the cortical malformation bilateral perisylvian polymicrogyria. To date, fewer than than 100 individuals with features of MPPH syndrome have been reported with either a clinical diagnosis (presence of the 2 core clinical and imaging findings: megalencephaly and polymicrogyria) , and/or a molecularly confirmed diagnosis [Mirzaa et al 2004, Colombani et al 2006, Garavelli et al 2007, Tohyama et al 2007, Pisano et al 2008, Tore et al 2009, Verkerk et al 2010, Osterling et al 2011, Mirzaa et al 2012, Rivière et al 2012, Kariminejad et al 2013, Zamora & Roberts 2013, Mirzaa et al 2014, Nakamura et al 2014, Tapper et al 2014, Demir et al 2015, Mirzaa et al 2015, Nellist et al 2015, Terrone et al 2016, Shi et al 2020].

Phenotype Correlations by Gene

AKT3. Features including connective tissue laxity and cutaneous capillary malformations can overlap with megalencephaly-capillary malformation (MCAP) syndrome (see Differential Diagnosis) [Mirzaa et al 2012, Rivière et al 2012, Nakamura et al 2014, Nellist et al 2015].

CCND2

• PMG appears to be more severe and widespread, typically extending to the frontal and/or occipital lobes. These extensive cortical malformations correlate with increased severity of epilepsy and intellectual disability [Mirzaa et al 2014].
• Postaxial polydactyly is more commonly observed in individuals with CCND2-related MPPH syndrome than in those with a pathogenic variant in either PIK3R2 or AKT3 [Mirzaa et al 2014].

Genotype-Phenotype Correlations

In general, no differences in phenotype have been observed between individuals with a molecularly confirmed diagnosis and those with only a clinical diagnosis. The exceptions are several individuals with a molecularly confirmed diagnosis of MPPH syndrome who had BPP but lacked the core clinical feature of megalencephaly [Mirzaa et al 2015].

Penetrance

Penetrance is predicted to be 100% in individuals with a germline variant in AKT3, CCND2, or PIK3R2.

Prevalence

MPPH syndrome has been reported to date in fewer than 100 individuals from various ethnic backgrounds. Therefore, data regarding prevalence are limited.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with megalencephaly-postaxial polydactyly-polymicrogyria-hydrocephalus (MPPH) syndrome, the evaluations summarized in Table 5 (if not performed as part of the evaluation that led to diagnosis) are recommended.

Treatment of Manifestations

Surveillance

Given the limited number of individuals reported with MPPH syndrome, formal surveillance guidelines do not exist.

Evaluation of Relatives at Risk

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

Therapies Under Investigation

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. Note: There may not be clinical trials for this disorder.

Mode of Inheritance

MPPH syndrome is an autosomal dominant disorder typically caused by a de novo pathogenic variant.

Risk to Family Members

Parents of a proband

• Almost all individuals with MPPH syndrome have the disorder as the result of a de novo germline AKT3, CCND2, or PIK3R2 pathogenic variant. Somatic mosaic pathogenic variants in PIK3R2 and AKT3 have been reported in a few individuals with MPPH syndrome [Mirzaa et al 2015, Alcantara et al 2017].
• Vertical transmission of a PIK3R2 pathogenic variant from an affected heterozygous parent to several affected children has been reported in one family to date [Mirzaa et al 2015].
• Parental germline mosaicism was suggested in three families by recurrence of MPPH syndrome in sibs and failure to detect the pathogenic variant in DNA isolated from parental blood samples [Rivière et al 2012, Mirzaa et al 2015, Szalai et al 2020].
• Recommendations for the evaluation of parents of a proband include molecular genetic testing for the pathogenic variant identified in the proband and a baseline neurologic assessment including measurement of head size.
• If the pathogenic variant identified in the proband is not identified in either parent and parental identity testing has confirmed biological maternity and paternity, the proband most likely has a de novo pathogenic variant.
• Another possible explanation is parental germline (or somatic and germline) mosaicism [Rivière et al 2012, Mirzaa et al 2015, Szalai et al 2020]. A parent with somatic and germline mosaicism for an MPPH syndrome-related pathogenic variant may be mildly/minimally affected.
• Note: Testing of parental leukocyte DNA may not detect all instances of somatic mosaicism and will not detect a pathogenic variant that is present in the germ cells only.

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 and/or is known to have the pathogenic variant identified in the proband, the risk to the sibs is 50%. While there may be phenotypic variability within the same family, all sibs who inherit a pathogenic variant will have features of MPPH syndrome.
• If the AKT3, CCND2, or PIK3R2 pathogenic variant found in the proband cannot be detected in the leukocyte DNA of either parent, the risk to sibs is slightly greater than that of the general population (but as-yet unknown) because of the possibility of parental germline mosaicism; further data are needed to establish the recurrence risk for sibs [Rivière et al 2012, Mirzaa et al 2015, Szalai et al 2020].

Offspring of a proband

• Each child of an individual with a germline AKT3, CCND2, or PIK3R2 pathogenic variant has a 50% chance of inheriting the pathogenic variant.
• The risk for transmission to offspring of an individual with somatic mosaicism for an MPPH-related pathogenic variant (i.e., the pathogenic variant is thought to have occurred post-fertilization in one cell of the multicellular embryo) is expected to be less than 50%.

Other family members. The risk to other family members depends on the status of the proband's parents: if a parent has a germline AKT3, CCND2, or PIK3R2 pathogenic variant, the parent's family members may be at risk.

Related Genetic Counseling Issues

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 parents of affected individuals.

DNA banking. Because it is likely that testing methodology and our understanding of genes, pathogenic mechanisms, and diseases will improve in the future, consideration should be given to banking DNA from probands in whom a molecular diagnosis has not been confirmed (i.e., the causative pathogenic mechanism is unknown).

Prenatal Testing and Preimplantation Genetic Testing

Once the MPPH syndrome-related pathogenic variant has been identified in an affected family member, prenatal testing for a pregnancy at increased risk and preimplantation genetic testing for MPPH syndrome are possible.

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.

Molecular Pathogenesis

AKT3, CCND2, and PIK3R2 encode key proteins within the phosphatidylinositol-4,5-bisphosphate-3-kinase (PI3K)-protein kinase B (AKT)-mammalian target of rapamycin (mTOR) pathway, a major signaling pathway involved with key cellular functions including protein synthesis, metabolism, cell cycle, survival, growth, and proliferation [Engelman et al 2006, Vanhaesebroeck et al 2012].

Pathogenic gain-of-function variants in these and other genes within the pathway (including PIK3CA and MTOR) are associated with a spectrum of diffuse and segmental developmental brain disorders including megalencephaly, hemimegalencephaly, polymicrogyria, and focal cortical dysplasia [Mirzaa & Poduri 2014].

AKT3 is a serine/threonine kinase and the principal target of phosphatidylinositol 3,4,5-trisphosphate (PIP3). PIK3R2 encodes the beta regulatory subunit of the PI3K enzymatic complex, a kinase complex that phosphorylates phosphatidylinositol 4,5-bisphosphate, to generate PIP3. Binding of PIP3 to the AKT complex leads to phosphorylation of multiple downstream PI3K-AKT-MTOR targets, including MTOR (mammalian target of rapamycin) itself.

CCND2, a protein that mediates the G1-S transition of the cell cycle, is among the downstream targets of the PI3K-AKT-MTOR pathway [Engelman et al 2006, Mirzaa et al 2014].

Mechanism of disease causation. Gain of function

Cancer and Benign Tumors

AKT3 is a key modulator of several sporadic tumors (including melanoma, glioma, and ovarian cancer) that occur in the absence of any other findings of MPPH syndrome. Somatic pathogenic gain-of-function missense variants across all functional domains of AKT3 are seen in a variety of tumors in the Catalogue of Somatic Mutations in Cancer (COSMIC). These somatic variants in AKT3 are not present in the germline; thus, predisposition to these tumors is not heritable.

CCND2. Sporadic tumors (including ovarian and testicular tumors) occurring in the absence of any other findings of MPPH syndrome have shown high-level expression of CCND2. CCND2 is also overexpressed in astrocytomas and glioblastomas [Parry & Engh 2012, Koyama-Nasu et al 2013]. Importantly, three individuals with an MPPH or CCND2 pathogenic variant have developed medulloblastoma [Osterling et al 2011; Hadzipasic et al 2021; Author, unpublished data] suggesting a causal link.

PIK3R2. Somatic pathogenic gain-of-function variants in PIK3R2 occur in sporadic tumors, particularly endometrial cancer in the absence of any other findings of MPPH syndrome. These somatic PIK3R2 variants are not present in the germline; thus, predisposition to these tumors is not heritable.

Author Notes

Dr Ghayda Mirzaa is a clinical and molecular geneticist at the Seattle Children's Research Institute and the University of Washington School of Medicine. Her research is focused on understanding the developmental basis and genetic causes of developmental brain disorders, including brain growth abnormalities, cortical malformations, and epilepsy. Dr Mirzaa's research team studies the natural history of MPPH syndrome.

Acknowledgments

We thank our patients and their families and health care providers for their collaboration and contribution to our knowledge of MPPH syndrome.

Revision History

• 28 July 2022 (sw) Comprehensive update posted live
• 17 November 2016 (bp) Review posted live
• 31 March 2016 (gm) Original submission

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