Autosomal Recessive ACTG2-Related Visceral Myopathy in Brothers

**Background:** Pediatric intestinal pseudo-obstruction (PIPO) is a chronic disorder characterized by the inability of the gastrointestinal tract to propel its contents in the absence of mechanical obstruction. ACTG2-related visceral myopathy, first described in 2012, is the most common molecular cause of familial or sporadic PIPO and was initially reported as an autosomal dominant condition. However, recent reports of homozygous truncating variants and deletions have suggested autosomal recessive inheritance for some mild variants. This report describes a unique familial quartet illustrating carriers of mild ACTG2 variants with mild gastrointestinal symptoms and their children with severe PIPO.

**Methods:** Two brothers with PIPO, referred at ages 18 and 8 years, underwent clinical evaluation including antroduodenal manometry. Genome sequencing (GS) was performed on DNA from whole blood of the mother and both brothers using the Illumina TruSeq PCR-free kit on the HiSeq X system with paired-end 150 bp reads. Data were aligned to GRCh37/hg19 and analyzed using Strelka for SNVs and Canvas for CNVs. Immunohistochemistry was performed on paraffin-embedded intestinal tissue from the brothers and neonatal controls using antibodies against ACTG2 (1:100; Novus Biologicals), neurofilament (1:600; Monosan), and smooth muscle actin α2 (ACTA2; ready to use; Dako).

**Key Results:** Genome sequencing identified a maternally inherited NM_001615.3:c.28G>A (p.Val10Met) missense variant and a 6.8 kb 2p13.1 loss [NG_034140.1:g.906_7729del(GRCh37)] in both brothers. The deletion includes the entire noncoding exon 1 and partial intron 1 of ACTG2. The p.Val10Met variant was not found in gnomAD and was interpreted as likely pathogenic per ACMG guidelines. The deletion was interpreted as a variant of unknown significance. The mother, maternal aunts, and maternal grandmother had severe childhood-onset constipation requiring large amounts of laxatives. The father reportedly had chronic diarrhea and nephrolithiasis and died at age 50 from lung cancer. Immunohistochemistry showed a slight visual decrease in ACTG2 expression in the brothers' intestinal tissue compared to controls, with no difference in neurofilament or ACTA2 expression. Both brothers developed acute volvulus requiring right hemicolectomy and small bowel resection with ileostomy, becoming TPN-dependent. Antroduodenal manometry revealed mixed myopathic and neuropathic motility patterns with low amplitude contractions and lack of motor migrating complexes. Both brothers showed initial benefit from pyridostigmine (up to 80 mg TID) with less abdominal distension, increased oral intake, and higher ostomy output.

**Clinical Implications:** This case solidifies the principle that some ACTG2 variants lead to mild gastrointestinal phenotypes (e.g., chronic constipation) in the monoallelic state and severe PIPO in the biallelic state. The deletion of noncoding ACTG2 exon 1 represents an under-recognized cause of mild gastrointestinal symptoms that would not be detectable by exome sequencing, explaining some instances of intrafamilial variability with apparent autosomal dominant inheritance. The authors recommend genome sequencing as the genetic work-up for primary or idiopathic PIPO due to genetic heterogeneity and the ability of GS to detect noncoding exon deletions missed by exome sequencing and standard SNP microarrays. This case adds to the growing evidence that loss-of-function is the mechanism for mild ACTG2 variants and that autosomal recessive inheritance with variable expressivity and incomplete penetrance occurs in ACTG2-related disease.