Recurrent arginine substitutions in the ACTG2 gene are the primary driver of disease burden and severity in visceral myopathy.

Visceral myopathy with abnormal intestinal and bladder peristalsis includes a clinical spectrum with megacystis-microcolon intestinal hypoperistalsis syndrome and chronic intestinal pseudo-obstruction. The vast majority of cases are caused by dominant variants in ACTG2; however, the overall genetic architecture of visceral myopathy has not been well-characterized. We ascertained 53 families, with visceral myopathy based on megacystis, functional bladder/gastrointestinal obstruction, or microcolon. A combination of targeted ACTG2 sequencing and exome sequencing was used. We report a molecular diagnostic rate of 64% (34/53), of which 97% (33/34) is attributed to ACTG2. Strikingly, missense mutations in five conserved arginine residues involving CpG dinucleotides accounted for 49% (26/53) of disease in the cohort. As a group, the ACTG2-negative cases had a more favorable clinical outcome and more restricted disease. Within the ACTG2-positive group, poor outcomes (characterized by total parenteral nutrition dependence, death, or transplantation) were invariably due to one of the arginine missense alleles. Analysis of specific residues suggests a severity spectrum of p.Arg178>p.Arg257>p.Arg40 along with other less-frequently reported sites p.Arg63 and p.Arg211. These results provide genotype-phenotype correlation for ACTG2-related disease and demonstrate the importance of arginine missense changes in visceral myopathy.

Expanding the clinical and molecular characteristics of PIGT-CDG, a disorder of glycosylphosphatidylinositol anchors.

PIGT-CDG, an autosomal recessive syndromic intellectual disability disorder of glycosylphosphatidylinositol (GPI) anchors, was recently described in two independent kindreds [Multiple Congenital Anomalies-Hypotonia-Seizures Syndrome 3 (OMIM, #615398)]. PIGT encodes phosphatidylinositol-glycan biosynthesis class T, a subunit of the heteropentameric transamidase complex that facilitates the transfer of GPI to proteins. GPI facilitates attachment (anchoring) of proteins to cell membranes. We describe, at ages 7 and 6 years, two children of non-consanguineous parents; they had hypotonia, severe global developmental delay, and intractable seizures along with endocrine, ophthalmologic, skeletal, hearing, and cardiac anomalies. Exome sequencing revealed that both siblings had compound heterozygous variants in PIGT (NM_015937.5), i.e., c.918dupC, a novel duplication leading to a frameshift, and c.1342C > T encoding a previously described missense variant. Flow cytometry studies showed decreased surface expression of GPI-anchored proteins on granulocytes, consistent with findings in previous cases. These siblings further delineate the clinical spectrum of PIGT-CDG, reemphasize the neuro-ophthalmologic presentation, clarify the endocrine features, and add hypermobility, low CSF albumin quotient, and hearing loss to the phenotypic spectrum. Our results emphasize that GPI anchor-related congenital disorders of glycosylation (CDGs) should be considered in subjects with early onset severe seizure disorders and dysmorphic facial features, even in the presence of a normal carbohydrate-deficient transferrin pattern and N-glycan profiling. Currently available screening for CDGs will not reliably detect this family of disorders, and our case reaffirms that the use of flow cytometry and genetic testing is essential for diagnosis in this group of disorders.

McArdle Disease Presenting With Muscle Pain in a Teenage Girl: The Role of Whole-Exome Sequencing in Neurogenetic Disorders.

We present the case of a young woman with worsening attacks of muscle pain and rhabdomyolysis beginning at age 14. Initial metabolic testing and electromyography revealed findings of a nonspecific myopathy. Diagnostic options were discussed among the members of a neurogenetics clinic team. Whole-exome sequencing was selected as a first tier test. This testing revealed a known disease causing mutation in the PYGM gene consistent with McArdle disease. We discuss the decision to use whole-exome sequencing in diagnostics and the rationale for making this our choice as a first-level test modality.

Persistent congenital hyperinsulinism in two patients with Beckwith-Wiedemann syndrome due to mosaic uniparental disomy 11p.

Congenital hyperinsulinism (CHI) is a rare metabolic disease characterized by inappropriate insulin secretion in the presence of hypoglycemia. We describe the clinical presentation and management of congenital hyperinsulinism and persistent hypoglycemia in two infants. Both patients had an initial clinical diagnosis of Beckwith-Wiedemann syndrome (BWS) but normal methylation analysis for LIT1 and H19 status. Both patients were eventually found to have mosaic uniparental disomy 11p diagnosed by single nucleotide polymorphism (SNP) array in DNA isolated from lymphoblasts and fibroblasts, respectively. We report that patients with mosaic BWS are at increased risk for both transient and refractory hypoglycemia that may need aggressive management with diazoxide, octreotide, high glucose infusion rates, and a frequent feeding regime. Our patient experience supports the case for pursuing further testing in patients with features of BWS with normal methylation studies, karyotype, and SNP arrays on blood. The next logical step is SNP array on skin biopsy to rule out mosaicism.

Recurrent reciprocal 1q21.1 deletions and duplications associated with microcephaly or macrocephaly and developmental and behavioral abnormalities.

Chromosome region 1q21.1 contains extensive and complex low-copy repeats, and copy number variants (CNVs) in this region have recently been reported in association with congenital heart defects, developmental delay, schizophrenia and related psychoses. We describe 21 probands with the 1q21.1 microdeletion and 15 probands with the 1q21.1 microduplication. These CNVs were inherited in most of the cases in which parental studies were available. Consistent and statistically significant features of microcephaly and macrocephaly were found in individuals with microdeletion and microduplication, respectively. Notably, a paralog of the HYDIN gene located on 16q22.2 and implicated in autosomal recessive hydrocephalus was inserted into the 1q21.1 region during the evolution of Homo sapiens; we found this locus to be deleted or duplicated in the individuals we studied, making it a probable candidate for the head size abnormalities observed. We propose that recurrent reciprocal microdeletions and microduplications within 1q21.1 represent previously unknown genomic disorders characterized by abnormal head size along with a spectrum of developmental delay, neuropsychiatric abnormalities, dysmorphic features and congenital anomalies. These phenotypes are subject to incomplete penetrance and variable expressivity.