Megalencephaly and polymicrogyria with polydactyly syndrome

The genetic basis of hydrocephalus: genes, pathways, mechanisms, and global impact

Hydrocephalus (HC) is a heterogenous disease characterized by alterations in cerebrospinal fluid (CSF) dynamics that may cause increased intracranial pressure. HC is a component of a wide array of genetic syndromes as well as a secondary consequence of brain injury (intraventricular hemorrhage (IVH), infection, etc.) that can present across the age spectrum, highlighting the phenotypic heterogeneity of the disease. Surgical treatments include ventricular shunting and endoscopic third ventriculostomy with or without choroid plexus cauterization, both of which are prone to failure, and no effective pharmacologic treatments for HC have been developed. Thus, there is an urgent need to understand the genetic architecture and molecular pathogenesis of HC. Without this knowledge, the development of preventive, diagnostic, and therapeutic measures is impeded. However, the genetics of HC is extraordinarily complex, based on studies of varying size, scope, and rigor. This review serves to provide a comprehensive overview of genes, pathways, mechanisms, and global impact of genetics contributing to all etiologies of HC in humans.

Genomics of human congenital hydrocephalus

Congenital hydrocephalus (CH), characterized by enlarged brain ventricles, is considered a disease of pathological cerebrospinal fluid (CSF) accumulation and, therefore, treated largely by neurosurgical CSF diversion. The persistence of ventriculomegaly and poor neurodevelopmental outcomes in some post-surgical patients highlights our limited knowledge of disease mechanisms. Recent whole-exome sequencing (WES) studies have shown that rare, damaging de novo and inherited mutations with large effect contribute to ~ 25% of sporadic CH. Interestingly, multiple CH genes are key regulators of neural stem cell growth and differentiation and converge in human transcriptional networks and cell types pertinent to fetal neurogliogenesis. These data implicate genetic disruption of early brain development as the primary pathomechanism in a substantial minority of patients with sporadic CH, shedding new light on human brain development and the pathogenesis of hydrocephalus. These data further suggest WES as a clinical tool with potential to re-classify CH according to a molecular nomenclature of increased precision and utility for genetic counseling, outcome prognostication, and treatment stratification.

Megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome: a case report

This report describes a case of megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome in a 1-year-old boy, born to healthy nonconsanguineous parents. Megalencephaly and bilateral postaxial polydactyly of upper and lower limbs were noted at birth. He had profound developmental delay and moderate hypotonia. Magnetic resonance imaging (MRI) of the brain revealed hydrocephalus, polymicrogyria in both frontal lobes and perisylvian regions, and thin corpus callosum. Array-comparative genomic hybridization was normal. The patient's clinical and radiologic findings fit the classic description of megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome. The possible overlap between megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome and other similar conditions is discussed.

Megalencephaly syndromes and activating mutations in the PI3K-AKT pathway: MPPH and MCAP

The megalencephaly-polymicrogyria-polydactyly-hydrocephalus (MPPH) and megalencephaly-capillary malformation (MCAP) syndromes are highly recognizable and partly overlapping disorders of brain overgrowth (megalencephaly). Both syndromes are characterized by congenital or early postnatal megalencephaly, with a high risk for progressive ventriculomegaly leading to hydrocephalus and cerebellar tonsillar ectopia leading to Chiari malformation, and cortical brain abnormalities, specifically polymicrogyria. MCAP is further characterized by distinct cutaneous capillary malformations, finger or toe syndactyly, postaxial polydactyly, variable connective tissue dysplasia and mild focal or segmental body overgrowth, among other features. MPPH, on the other hand, lacks consistent vascular or somatic manifestations besides postaxial polydactyly in almost half of reported individuals. We identified de novo germline mutations in PIK3R2 and AKT3 in individuals with MPPH, and both postzygotic, mosaic and rare germline mutations in PIK3CA in individuals with MCAP. PIK3R2, AKT3, and PIK3CA are members of the critical phosphatidylinositol-3-kinase (PI3K)-vakt murine thymoma viral oncogene homolog (AKT) pathway that is well implicated in cell growth, proliferation, survival, apoptosis, among other diverse cellular functions. The identified mutations in these three genes have been shown to lead to gain of function and activation of the PI3K-AKT pathway. Germline and postzygotic mutations of PIK3CA and other PI3K-AKT-mTOR pathway genes have also been identified in several other overgrowth syndromes, highlighting the key role of this signaling pathway in normal development and pathophysiology of a large group of congenital anomalies.

Prenatal diagnosis of MPPH syndrome

We report the prenatal sonographic detection of a fetus with megalencephaly, polymicrogyria, postaxial polydactyly and hydrocephaly. Only 14 patients have been reported in the literature so far, all but one were diagnosed postnatally. The polymicrogyria in the frontoparietal lobe was confirmed by prenatal magnetic resonance imaging. Additionally, a hypoplastic thymus as seen in a 22q11 deletion was present. Although polymicrogyria along with pre-axial polydactyly has been described in 22q11 deletion, the diagnosis of Di George syndrome was ruled out. The etiology of megalencephaly, polymicrogyria, postaxial polydactyly and hydrocephaly has not been revealed yet. A dominant as well as recessive inheritance has been suggested.

Megalencephaly-capillary malformation (MCAP) and megalencephaly-polydactyly-polymicrogyria-hydrocephalus (MPPH) syndromes: two closely related disorders of brain overgrowth and abnormal brain and body morphogenesis

The macrocephaly-capillary malformation syndrome (M-CM), which we here propose to rename the megalencephaly-capillary malformation syndrome (MCAP; alternatively the megalencephaly-capillary malformation-polymicrogyria syndrome), and the more recently described megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome (MPPH) are two megalencephaly (MEG) disorders that involve a unique constellation of physical and neuroimaging anomalies. We compare the features in 42 patients evaluated for physical and neuroimaging characteristics of MCAP and MPPH and propose a more global view of these syndromes based on classes of developmental abnormalities that include primary MEG and growth dysregulation, developmental vascular anomalies (primarily capillary malformations), distal limb anomalies (such as syndactyly and polydactyly), cortical brain malformations (most distinctively polymicrogyria, PMG), and variable connective tissue dysplasia. Based on these classes of developmental abnormalities, we propose that MCAP diagnostic criteria include progressive MEG with either vascular anomalies or syndactyly. In parallel, we propose that MPPH diagnostic criteria include progressive MEG and PMG, absence of the vascular anomalies and syndactyly characteristic of MCAP, and absence of brain heterotopia.

MPPH syndrome: two new cases

This report describes 2 additional cases of megalencephaly and perisylvian polymicrogyria with postaxial polydactyly and hydrocephalus syndrome, a recently recognized disorder of infants and young children with macrocrania, developmental delay/mental retardation, and often epilepsy. Medulloblastoma, a previously unreported feature in megalencephaly and perisylvian polymicrogyria with postaxial polydactyly and hydrocephalus syndrome, developed in one child at 3 years of age. Although the disorder is presumed to be genetic, the cause of megalencephaly and perisylvian polymicrogyria with postaxial polydactyly and hydrocephalus syndrome has not yet been determined.

Unbalanced der(5)t(5;20) translocation associated with megalencephaly, perisylvian polymicrogyria, polydactyly and hydrocephalus

The combination of megalencephaly, perisylvian polymicrogyria, polydactyly and hydrocephalus (MPPH) is a rare syndrome of unknown cause. We observed two first cousins affected by an MPPH-like phenotype with a submicroscopic chromosome 5q35 deletion as a result of an unbalanced der(5)t(5;20)(q35.2;q13.3) translocation, including the NSD1 Sotos syndrome locus. We describe the phenotype and the deletion breakpoints of the two MPPH-like patients and compare these with five unrelated MPPH and Sotos patients harboring a 5q35 microdeletion. Mapping of the breakpoints in the two cousins was performed by MLPA, FISH, high density SNP-arrays and Q-PCR for the 5q35 deletion and 20q13 duplication. The 5q35 deletion area of the two cousins almost completely overlaps with earlier described patients with an atypical Sotos microdeletion, except for the DRD1 gene. The five unrelated MPPH patients neither showed submicroscopic chromosomal aberrations nor DRD1 mutations. We reviewed the brain MRI of 10 Sotos patients and did not detect polymicrogyria in any of them. In our two cousins, the MPPH-like phenotype is probably caused by the contribution of genes on both chromosome 5q35 and 20q13. Some patients with MPPH may harbor a submicroscopic chromosomal aberration and therefore high-resolution array analysis should be part of the diagnostic workup.

Syndrome of megalencephaly, polydactyly, and polymicrogyria lacking frank hydrocephalus, with associated MR imaging findings

Megalencephaly, polymicrogyria, polydactyly, and hydrocephalus (MPPH) syndrome has been recently recognized and is very rare. Each case reported so far has demonstrated hydrocephalus to varying degrees. We report an infant with MPPH syndrome, but lacking frank hydrocephalus. The additional finding of an abnormally elongated pituitary infundibulum has not been described in this syndrome and, along with the presence of a regressing cystic cavum septum pellucidum, suggests that chronic underlying hydrocephalus may have been present.

Significant overlap and possible identity of macrocephaly capillary malformation and megalencephaly polymicrogyria-polydactyly hydrocephalus syndromes

We report on three patients with macrocephaly and polymicrogyria, and additional anomalies seen in megalencephaly polymicrogyria-polydactyly hydrocephalus (MPPH) and macrocephaly capillary malformation (MCM) syndromes. Based on their characteristic brain malformations they were originally diagnosed with MPPH. In one patient the phenotype evolved during early infancy, and ultimately resulted in a diagnosis of MCM. A second was prenatally diagnosed with MPPH, but postnatally visualized capillary malformations led to a diagnosis of MCM. In a third, the original MPPH diagnosis was reconsidered after a critical review revealed additional subtle findings suggestive of MCM. Characteristic brain malformations are thought to distinguish between MPPH with perisylvian polymicrogyria, and MCM with megalencephaly with Chiari 1 malformation. However, polymicrogyria was reported in a significant number of patients with MCM. Conversely, upon review of imaging studies of patients with MPPH, we noted progressive crowding of the posterior fossa and acquired tonsillar herniation, a process deemed characteristic for MCM. Thus, neither polymicrogyria nor acquired tonsillar herniation are distinguishing features, and occur in both disorders. In addition to brain abnormalities, shared findings include cognitive impairment, coarse facial features and postaxial polydactyly. Facial nevus flammeus and cutis marmorata are most noticeable in infancy, and ligamentous laxity and redundant soft tissue are somewhat subjective findings. While asymmetric overgrowth is considered typical for MCM, it is not universally present. These variable and subtle findings can be identified in patients with MPPH. We propose that MPPH and MCM may not represent distinct entities and that the term MPPH-CM syndrome be used to describe this spectrum.

Megalencephaly, polymicrogyria, and hydrocephalus (MPPH) syndrome: a new case with syndactyly

Megalencephaly, polymicrogyria, and hydrocephalus (MPPH) syndrome is characterized by megalencephaly, perisylvian polymicrogyria, postaxial polydactyly, and hydrocephalus. Seven cases have been reported. This report presents a new sporadic patient with megalencephaly, polymicrogyria, and hydrocephalus syndrome, a girl born to healthy, nonconsanguineous parents at 38 weeks. Macrocephaly (+4 standard deviation) was present at birth. She had syndactyly instead of the postaxial polydactyly reported in the other patients. Neurologic examination showed severe diffuse hypotonia and profound developmental delay. Magnetic resonance imaging revealed enlarged lateral and third ventricles, with cavum septi pellucidi et vergae, bilateral abnormal white matter intensity, and diffuse polymicrogyria, most prominent in both the frontal and perisylvian regions. A visual evoked potential study showed increased latencies, probably caused by white matter abnormalities. At 16 months, she has never had seizures and shows profound psychomotor retardation. Results of metabolic and genetic studies were normal.