ARL2BP

The name ARL2BP stands for "ARF-like GTPase 2 binding protein," and the gene is also known by the nickname BART. The protein it makes attaches tightly to the active form of a small signaling protein called ARL2 and acts as its main working partner, or effector. ARL2BP works at the base of cilia, the thin projections that many cells use to sense and organize their environment. In the retina, it sits at the base of each photoreceptor (definition: A light-sensing cell in the retina. Rods (used for night and peripheral vision) and cones (used for color and detail) are the two types.)'s cilium, where it is needed to build and maintain the outer segment, the part of the cell that actually captures light. Because it is essential to healthy cilia throughout the body, ARL2BP disease is considered a ciliopathy (definition: A disease caused by a problem with cilia — tiny hair-like structures on cells, including the light-sensing cells in the retina, that the cell needs to work properly.), a condition rooted in faulty cilia.

The retinitis pigmentosa linked to ARL2BP typically causes progressive loss of night vision and of peripheral, or side, vision. Retinal symptoms generally begin during the third decade of life, meaning a person's twenties. In one detailed, long-term case, light sensitivity (photophobia) was the first symptom at age 20, followed about ten years later by night blindness and loss of both central sharpness and peripheral field. Progression can be slow, and the person in that same report still retained useful residual vision at age 63. Eye testing such as electroretinography, which measures the retina's electrical response to light, and detailed retinal imaging has shown features of a mixed rod-cone dystrophy. Because these observations come from a small number of individuals, the age of onset, symptoms, and pace of change can vary from person to person.

ARL2BP was established as a cause of autosomal recessive (definition: An inheritance pattern where a person needs an altered copy of a gene from both parents to be affected. Parents with only one copy are usually unaffected carriers.) retinitis pigmentosa in 2013, using gene mapping and sequencing in affected families. Later reports of additional families with different splice-site changes confirmed the association. It is a rare cause of retinal degeneration; in one large French study, ARL2BP variants accounted for about 0.1% of recessive rod-cone dystrophies. Variants have also been found in consanguineous families in Pakistani and other populations, often in the setting of retinitis pigmentosa with situs inversus. Reported features beyond the eye include situs inversus, male infertility with abnormal sperm, kidney findings such as renal agenesis and cysts, and in some cases loss of smell (anosmia). Laboratory work in cells and studies in knockout mice show that loss of ARL2BP shortens cilia and disrupts the structure of the photoreceptor (definition: A light-sensing cell in the retina. Rods (used for night and peripheral vision) and cones (used for color and detail) are the two types.) outer segment, which helps explain how the vision loss develops.

Because only a small number of families have been described, the full range of symptoms and the typical rate of vision change are not yet fully defined. The precise steps by which loss of ARL2BP leads to photoreceptor (definition: A light-sensing cell in the retina. Rods (used for night and peripheral vision) and cones (used for color and detail) are the two types.) death remain under study. Not everyone with ARL2BP variants shows the same combination of features; one man with complete absence of the protein had infertility and cilia problems but no retinal symptoms at age 23, and long-term eye follow-up was recommended. Whether the amount of remaining normal gene product influences how severe the disease becomes is still an open question.

Research on ARL2BP so far has focused on understanding how the protein works and how its loss damages photoreceptors, using cells and animal models rather than testing therapies in people. These studies have shown that ARL2BP is needed for proper cilia and outer-segment structure, but they are laboratory and animal findings, not treatments that are available now. No therapy directed at this gene has been approved, and the mechanistic work is best understood as groundwork that may inform future research.

• Discovery of ARL2BP as a cause of recessive retinitis pigmentosa

  Human genetic study combined with laboratory and mouse experiments.

  What was found:

  Mutations in ARL2BP caused recessive RP (RP66), one family also had situs inversus, and the protein was found at the base of the photoreceptor cilium, where its loss shortened cilia.

  Why it matters:

  This established ARL2BP as a genuine cause of inherited retinal disease and pointed to faulty cilia as the underlying problem.

  Limitation: The disease mechanism was inferred from a small number of families and from model systems rather than proven in patients.

• Knockout mouse shows why photoreceptors fail

  Animal (mouse) laboratory study.

  What was found:

  The mice showed early, progressive decline in vision, with disorganized outer segments, shortened cilia, and defective ciliary microtubule structure.

  Why it matters:

  It clarifies the biological role of ARL2BP in maintaining photoreceptors, which may guide future therapy research.

  Limitation: Mouse results do not always translate directly to humans and do not represent an available treatment.

• Additional families confirm ARL2BP splice-site variants in RP

  Observational human genetic study with RNA analysis.

  What was found:

  Two more unrelated individuals had homozygous splice-site variants that disrupted ARL2BP, confirming it as a rare cause of recessive RP.

  Why it matters:

  Independent confirmation strengthens the link between ARL2BP and retinitis pigmentosa.

  Limitation: Only two affected people were found, and the authors noted the disease mechanism and any genotype-phenotype correlation still need to be defined.

• How often ARL2BP variants cause rod-cone dystrophy

  Observational human cohort study.

  What was found:

  ARL2BP variants explained only about 0.1% of cases, indicating they are a very rare cause.

  Why it matters:

  This helps put the gene's frequency in context for patients and clinicians.

  Limitation: Frequency estimates can differ across populations and cohorts, so this single figure may not apply everywhere.

• Long-term follow-up of syndromic ARL2BP disease

  Single-patient case report with long-term natural history.

  What was found:

  He had rod-cone dystrophy alongside situs inversus, abnormal sperm, unilateral kidney absence and cysts, with slow progression and useful residual vision at 63.

  Why it matters:

  It documents the range of body-wide features and the sometimes slow visual course seen with ARL2BP variants.

  Limitation: Findings from one person cannot predict how the condition will behave in others.

ARL2BP-related disease is inherited in an autosomal recessive (definition: An inheritance pattern where a person needs an altered copy of a gene from both parents to be affected. Parents with only one copy are usually unaffected carriers.) pattern, meaning a person is affected only when both gene copies are altered, and parents are typically unaffected carriers. Siblings of an affected person have a chance of being carriers or affected, which a genetic counselor can explain in detail. Because this is a ciliopathy (definition: A disease caused by a problem with cilia — tiny hair-like structures on cells, including the light-sensing cells in the retina, that the cell needs to work properly.), families may want to be aware of possible non-eye features that have been reported, including situs inversus, kidney abnormalities, reduced male fertility, and loss of smell, so these can be raised with appropriate specialists. Since retinal symptoms often begin in adulthood, some relatives who carry two variants may not yet have obvious eye changes, and ophthalmology follow-up can be discussed with their doctors.

• Do my specific ARL2BP variants explain my symptoms, and are both copies of the gene affected?
• Given the ciliopathy link, should I have my kidneys, and if relevant fertility, evaluated or monitored?
• What pattern of vision change might I expect, and how often should my retina be checked?
• Which eye tests, such as electroretinography or imaging, would help track my condition over time?
• What does autosomal recessive inheritance mean for my siblings, children, or other relatives?
• Are there any registries or research studies for ARL2BP-related disease that I could learn more about?

Confirming the specific ARL2BP variants with a genetics professional can help clarify the diagnosis and what it may mean for you. Because this gene is tied to a ciliopathy (definition: A disease caused by a problem with cilia — tiny hair-like structures on cells, including the light-sensing cells in the retina, that the cell needs to work properly.), it may be worth discussing with your care team whether checks of other organ systems, such as the kidneys, or a conversation about fertility are appropriate. A guided tour of RP Hope's resources can help you find support at whatever stage you are in. Reading first-person accounts from others navigating an RP diagnosis may also help.