Retinitis pigmentosa and molar tooth sign caused by novel AHI1 compound heterozygote pathogenic variants

Background

Joubert syndrome (JS) is a group of rare congenital disorders characterized by cerebellar vermis dysplasia, developmental delay, and retina dysfunctions. Herein, we reported a Chinese patient carrying a new variant in the AHI1 gene with mild JS, and the 3D structure of the affected Jouberin protein was also predicted.

Case presentation

The patient was a 31-year-old male, who presented difficulty at finding toys at the age of 2 years, night blindness from age of 5 years, intention tremor and walking imbalance from 29 years of age. Tubular visual field and retina pigmentation were observed on ophthalmology examinations, as well as molar tooth sign on brain magnetic resonance imaging (MRI). Whole exome sequence revealed two compound heterozygous variants at c.2105C>T (p.T702M) and c.1330A>T (p.I444F) in AHI1 gene. The latter one was a novel mutation. The 3D protein structure was predicted using I-TASSER and PyMOL, showing structural changes from functional β-sheet and α-helix to non-functional D-loop, respectively.

Conclusions

Mild JS due to novel variants at T702M and I444F in the AHI1 gene was reported. The 3D-structural changes in Jouberin protein might underlie the pathogenesis of JS.

Identification of a novel truncating variant in AHI1 gene and a brief review on mutations spectrum

Joubert syndrome (JS) is a rare inherited neurodevelopmental condition characterized by hypotonia, ataxia, developmental delay, abnormal eye movements, neonatal respiratory disturbance and unique midbrain-hindbrain malformation, known as the molar tooth sign. JS is a genetically heterogeneous disorder with nearly 35 ciliary genes are implicated in its pathogenesis. AHI1 gene is one of the most frequently mutated gene in JS patients which is accounted for 8-11% of cases, particularly in Arab population. AHI1 encodes a cilium-localized protein with a significant role in mediating vesicle trafficking, ciliogenesis and cell polarity. Here, we report a novel pathogenic variant in AHI1 gene and review previously published mutations in AHI1 gene briefly. Whole exome sequencing was employed to determine the causative mutation in an Iranian Arab family with JS from southwestern Iran. Segregation analysis of the candidate variant in the family members was performed using PCR-Sanger sequencing. This approach found a novel homozygous nonsense variant c.832C > T (p.Gln278Ter) in AHI1. Segregation analysis was consistent with individual's phenotype and an autosomal recessive pattern in the family. The variant residing in a relatively highly conserved region and fulfilled the criteria required to be classified as a pathogenic variant based on American College of Medical Genetics and Genomics guidelines. This study confirms the diagnosis of JS in this family and highlights the efficiency of next-generation sequencing-based technique to identify the genetic causes of hereditary disorders with locus heterogeneity.

Novel mutations of PKHD1 and AHI1 identified in two families with cystic renal disease

Objective: To report newly identified mutations in two families in China with cystic renal disease. Case presentations: Two fetuses were found by prenatal ultrasound to have symmetrically enlarged kidneys with increased echogenicity and cystic changes. We isolated fetal and parental genomic DNAs from umbilical cord blood and circulating leukocytes, performed next generation sequencing for mutations, followed by Sanger sequencing for confirmation. We discovered two new heterozygous mutations in PKHD1: c.2507_2515delTGAAGGAGG (p.Val836_Glu838del) in exon 24 among the fetus and father, as well as c.6840G>A (p.Trp2280*) in exon 42 among the fetus and mother. A mutation of c.2507_2515delTGAAGGAGG caused deletion of three amino acids. Two heterozygous mutations in AHI1, c.1304G>A (p.Arg435Gln), and c.3257A>G (p.Glu1086Gly) were identified in the second fetus, while the former was also found in the mother. The mutated locus in AHI1 is highly conserved among humans, dogs, mice, and monkeys. Conclusions: We report two newly identified mutations in PKHD1 and AHI1. An accurate genetic diagnosis is crucial for genetic counseling of parents with offspring carrying cystic renal disease.