Sunday, November 10, 2019

HERC1 mutations

Inspired by a patient

Utine GE, Taşkıran EZ, Koşukcu C, Karaosmanoğlu B, Güleray N, Doğan ÖA, Kiper PÖ, Boduroğlu K, Alikaşifoğlu M. HERC1 mutations in idiopathic intellectual disability. Eur J Med Genet. 2017 May;60(5):279-283.

HERC1 is a member of HERC protein family of ubiquitin ligases and is a negative regulator of the mTOR pathway. It is also a guanine nucleotide exchange factor for ARF and Rab family GTPases. Biallelic mutations in HERC1 were recently shown to cause a human phenotype with overgrowth and intellectual disability as main features. Herein we describe clinical features in another patient with homozygous novel mutation in HERC1. Moderate to severe intellectual disability, hypotonia, macrocephaly, tall stature, and facial features appear as main clinical features of the condition. Kyphoscoliosis and seizures frequently accompany and autistic features might be another feature as recent studies also implicate. HERC1 mutations should be considered in differential diagnosis of severe intellectual disability and behavioural problems, particularly in patients testing negative for fragile X and KANSL1 mutations.

Aggarwal S, Bhowmik AD, Ramprasad VL, Murugan S, Dalal A. A splice site mutation in HERC1 leads to syndromic intellectual disability with macrocephaly and facial dysmorphism: Further delineation of the phenotypic spectrum. Am J Med Genet A. 2016 Jul;170(7):1868-73.

We report on a sib pair of Indian origin presenting with intellectual disability, dysmorphism, and macrocephaly. Exome sequencing revealed a homozygous splice site HERC1 mutation in both probands. Functional analysis revealed use of an alternate splice site resulting in formation of a downstream stop codon and nonsense mediated decay. In the light of recent reports of HERC1 mutations in two families with a similar phenotypic presentation, this report reiterates the pathogenic nature and clinical consequences of HERC1 disruption.

Nguyen LS, Schneider T, Rio M, Moutton S, Siquier-Pernet K, Verny F, Boddaert N, Desguerre I, Munich A, Rosa JL, Cormier-Daire V, Colleaux L. A nonsense variant in HERC1 is associated with intellectual disability, megalencephaly, thick corpus callosum and cerebellar atrophy. Eur J Hum Genet. 2016 Mar;24(3):455-8.

Megalencephaly is a congenital condition characterized by severe overdeveloped brain size. This phenotype is often caused by mutations affecting the RTK/PI3K/mTOR (receptor tyrosine kinase-phosphatidylinositol-3-kinase-AKT) signaling and its downstream pathway of mammalian target of rapamycin (mTOR). Here, using a whole-exome sequencing in a Moroccan consanguineous family, we show that a novel autosomal-recessive neurological condition characterized by megalencephaly, thick corpus callosum and severe intellectual disability is caused by a homozygous nonsense variant in the HERC1 gene. Assessment of the primary skin fibroblast from the proband revealed complete absence of the HERC1 protein. HERC1 is an ubiquitin ligase that interacts with tuberous sclerosis complex 2, an upstream negative regulator of the mTOR pathway. Our data further emphasize the role of the mTOR pathway in the regulation of brain development and the power of next-generation sequencing technique in elucidating the genetic etiology of autosomal-recessive disorders and suggest that HERC1 defect might be a novel cause of autosomal-recessive syndromic megalencephaly.

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