Bothnia Retinal Dystrophy is a rare atypical type of retinitis pigmentosa. Patients usually present in early childhood with night blindness, which progresses to retinitis punctata albescens and macular degeneration by the early teenage years. Affected individuals suffer from progressive loss of visual acuity and are legally blind by early adulthood.
Electrophysiological studies have helped identify the progressive course of this disorder. Patients early in the disease course have a normal fundus appearance, but begin to show a loss of rod function within the first decade. Over time, affected individuals lose cone responses as well. Dark adaptometry tests have found that rod responses recover after prolonged dark adaptation of 20-24 hours but cone responses do not recover.
The condition has a particularly high prevalence in the Vasterbotten County of northern Sweden and is hence also known as Vasterbotten Dystrophy. It can be diagnosed based on ophthalmological studies and molecular diagnosis of the RLBP1 gene. There is currently no cure for the condition. However, patients may benefit from tinted lenses and low-vision aids as well as therapy to help adapt to the progressive vision loss. Prenatal testing and genetic counselling may be useful for family members of affected individuals.
Bothnia Retinal Dystrophy follows an autosomal recessive pattern of inheritance and is caused by homozygous mutations in the RLBP1 gene. This gene encodes a soluble retinoid-carrier protein found only in the retina and pineal gland. The RLBP1 protein is believed to play a role in the visual cycle, which is the cycling of retinoids between photoreceptors and adjacent pigment epithelium cells, and hence it is essential for the functioning of rod and cone photoreceptors. A homozygous substitution (c.9096C>T) in exon 7 of the gene, resulting in a p.R234W missense mutation, has been implicated in this disorder.
Monies et al. (2017) analyzed the findings of 1000 diagnostic panels and exomes carried out at a next generation sequencing lab in Saudi Arabia. One patient, a 16-year-old male, suffered from seizures, retinitis pigmentosa and obesity. His parents were consanguineous and he had a family history of this phenotype. Whole exome sequencing helped identify a dual molecular diagnosis in this patient. A homozygous mutation (c.773T>G, p.L258W) was found in exon 8 of the patient’s RLBP1 gene, associated with Bothnia retinal dystrophy. The patient also had a heterozygous variant (c.1471C>T, p.R491X) in exon 7 of the SYT9 gene and this was suggested to be associated with epilepsy. The authors noted that dual molecular diagnoses were rare and only occurred in 1.5% of the cohort.
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