No abstract

Pitt-Hopkins syndrome is an emerging neurodevelopmental disorder caused by haploinsufficiency of the TCF4 gene on chromosome 18q21. It is characterized by severe intellectual disability, seizures, microcephaly, constipation and a distinctive facial gestalt. Although the overlapping phenotype of microcephaly, epilepsy, absent speech and constipation represents a challenge for the differential diagnosis with Angelman syndrome, Rett syndrome and Mowat-Wilson syndrome, distinctive of Pitt-Hopkins syndrome are breathing abnormalities, that can occur as either hyperventilation episodes or apnea crises, and a typical facial dysmorphism, including bitemporal narrowing, squared forehead, deep-set eyes, peculiar nose conformation, with broad nasal bridge, down-turned nasal tip and flaring nostrils, typical shape of the mouth, with a tented and M shaped upper lip, and widely spaced teeth. The occurrence of these signs in variable association of uncoordinated movements, microcephaly of postnatal onset, eye abnormalities, constipation, epilepsy and subtle brain abnormalities is highly predictive of a TCF4 mutation, making it possible to plan a genetic test of choice among severe encephalopathies. Angelman syndrome represents the nosological condition closest to Pitt-Hopkins syndrome.

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The Noonan syndrome and the cardio-faciocutaneous (CFC) syndrome have been described as phenotypically and genetically distinct entities. However, the resemblance between them led some authors to question the validity of this separation. We review available clinical evidence to support the opposite view, namely, that the Noonan and CFC syndromes are indeed distinct and separate conditions, both falling within the broad and causally heterogeneous spectrum of the Noonan/congenital lymphedema phenotype.

We report on 3 new cases of C trigonocephaly syndrome. In addition to the findings characteristic of this condition, one of the patients also had a large omphalocele. This patient was referred from a suburban hospital with a diagnosis of Down syndrome, stressing the fact that C syndrome is still under-recognized and underdiagnosed. Another patient was diagnosed at birth and immediately submitted to craniosynostectomy. A second operation was performed 7 months later resulting in normal brain growth and close to normal psychomotor development at 3 years, in contrast to the third patient, who was not treated surgically and was severely retarded at 4 years.

We report on 4 new cases of mildly retarded patients with marfanoid habitus and a characteristic constellation of minor anomalies. These patients, although sporadic, are likely to be affected by the same X-linked type of mental retardation described by Lujan et al. (American Journal of Medical Genetics 17:311-322, 1984) and more recently by Fryns and Buttiens (American Journal of Medical Genetics 28:267-274, 1987). The similar psychiatric history in 2 of our patients suggests that psychotic behaviour could be an additional manifestation, previously unrecognized in this condition. Late diagnosis of this relatively new syndrome in all our patients confirms the difficulty of the nosologic definition of mentally retarded individuals on clinical grounds alone. On the other hand, the Lujan-Fryns syndrome appears to be more common than one would have thought.

A previously unreported X-linked MCA/MR syndrome is described in 4 members of a large family. Phenotypic manifestations include mental retardation, microcephaly, failure to thrive, severe congenital hypotonia, characteristic face, hypogenitalism, pachygyria. This appears to be an X-linked dominant trait with decreased penetrance and expressivity in carrier females.

Variants in RAC3, encoding a small GTPase RAC3 which is critical for the regulation of actin cytoskeleton and intracellular signal transduction, are associated with a rare neurodevelopmental disorder with structural brain anomalies and facial dysmorphism. We investigated a cohort of 10 unrelated participants presenting with global psychomotor delay, hypotonia, behavioural disturbances, stereotyped movements, dysmorphic features, seizures and musculoskeletal abnormalities. MRI of brain revealed a complex pattern of variable brain malformations, including callosal abnormalities, white matter thinning, grey matter heterotopia, polymicrogyria/dysgyria, brainstem anomalies and cerebellar dysplasia. These patients harboured eight distinct de novo RAC3 variants, including six novel variants (NM_005052.3): c.34G > C p.G12R, c.179G > A p.G60D, c.186_188delGGA p.E62del, c.187G > A p.D63N, c.191A > G p.Y64C and c.348G > C p.K116N. We then examined the pathophysiological significance of these novel and previously reported pathogenic variants p.P29L, p.P34R, p.A59G, p.Q61L and p.E62K. In vitro analyses revealed that all tested RAC3 variants were biochemically and biologically active to variable extent, and exhibited a spectrum of different affinities to downstream effectors including p21-activated kinase 1. We then focused on the four variants p.Q61L, p.E62del, p.D63N and p.Y64C in the Switch II region, which is essential for the biochemical activity of small GTPases and also a variation hot spot common to other Rho family genes, RAC1 and CDC42. Acute expression of the four variants in embryonic mouse brain using in utero electroporation caused defects in cortical neuron morphology and migration ending up with cluster formation during corticogenesis. Notably, defective migration by p.E62del, p.D63N and p.Y64C were rescued by a dominant negative version of p21-activated kinase 1. Our results indicate that RAC3 variants result in morphological and functional defects in cortical neurons during brain development through variant-specific mechanisms, eventually leading to heterogeneous neurodevelopmental phenotypes.