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What is Blue Cone Monochromacy?

Blue Cone Monochromacy (BCM) is a rare genetic retinal disorder estimated to affect 1 in 100,000 people. It is an inherited retinal degeneration (IRD) caused by mutations in the OPN1LW/OPN1MW gene cluster, encoding long (L)- and middle (M)-wavelength sensitive (i.e. red-green) opsins [38,39,41] of the cone photoreceptor cells in the retina. The visual disabilities are serious and include reduced visual acuity, abnormal color vision, myopia, nystagmus and photophobia [11,20,21,34,52].

Blue Cone Monochromacy affects the retina, on the eye’s back

It is a recessive X-linked disease therefore it almost exclusively affects males (XY) while female carriers (XX) only rarely show some of the symptoms in a mild form.

Blue Cone Monochromacy is usually considered a stationary disease, with symptoms first manifesting in early infancy, although there is evidence of disease progression with macular changes associated with macular degeneration in many patients [5,6,34,52]. The first symptom observed is nystagmus in 2-3 months old newborns.

Video of a 4-months baby with Blue Cone Monochromacy showing Nystagmus.​

Symptoms

There are three types of cones in the human retina that are responsible for daylight vision, visual acuity and color vision: they are sensitive to Long (red), Middle (green) and Short (blue) wavelength light[41] . When people have Blue Cone Monochromacy, both the red and green cones do not function properly, while the blue cones work normally [5,38,39]. Signs and symptoms may include low visual acuity (clarity or sharpness), impaired color vision, photophobia (light sensitivity), myopia (nearsightedness), and nystagmus (fast, uncontrollable movements of the eye).

A variety of symptoms characterize Blue Cone Monochromacy:

  • Low visual acuity ranging between 20/60 and 20/200.
  • Poor color discrimination (impaired ability or inability to distinguish between colours)
  • Intolerance to light (characterized by difficulty seeing in bright light, especially during daylight) and associated photophobia (sensitivity to light).
  • Myopia, patients often have myopia
  • Nystagmus (characterized by involuntary, rhythmic eye movements) which is present since the age of 2-months and may slowly decrease with age.

For the majority of subjects with Blue Cone Monochromacy, symptoms are usually stationary. However, clinical studies show evidence of disease progression with macular changes [6,34,52].

A child with low vision, forced to read very close to the book

Synonyms:

  • Blue cone monochromatism;
  • S-cone monochromacy;
  • Atypical X-linked achromatopsia;
  • X-linked incomplete achromatopsia.

Causes: genes and mutations

There are three genes involved in Blue Cone Monochromacy, and are located at position Xq28, at the end of the q arm of the X chromosome. The three genes are in tandem and are:

  1. LCR (Locus Control Region)
  2. OPN1LW (L-cone opsin gene)
  3. OPN1MW (M-cone opsin gene)

These genes encode proteins that are needed in the process of converting light into electrical signals that the brain uses for visual processing. The proteins are called L- and M-cone opsins, which play a crucial role in this process.

The Locus Control Region (LCR) serves as a promoter for the expression of two of the two genes thereafter, OPN1LW and OPN1MW, encoding opsin proteins responsible for red and green light capture in the human retina. LCR ensures exclusive expression of one opsin gene in each cone [49,54,55].

There are many genetic mutations that can affect this group of genes leading to BCM  [9,17,25,26,28,38,39]: a deletion of the LCR, intragenic deletion of exons within the genes OPN1LW and OPN1MW and a 2 steps mechanism with an homologous recombination and a punctual inactivation.

Picture taken from the he aggregated data of the BCM Patient Registry – BCMRegistry-Poster-2022-Final – [1] The BCM Patient Registry is at  www.BCMRegistry.org

How is it transmitted?

Blue Cone Monochromacy is transmitted genetically through genes that are passed from parents to children.

In particular, Blue Cone Monochromacy is a recessive X-linked disease which means the disease is expressed in males (XY) who are hemizygous for the mutation and in the rare females (XX) who are homozygous for the genetic mutation, i.e. in females who have the genetic mutation on both X chromosomes. It is therefore not impossible for a female (XX) to have BCM, but it is an extremely rare condition.

Diagnosis

In a male child, from 2 months upwards, an aversion to light and nystagmus may lead to the suspicion of a case of Blue Cone Monochromacy, but it does not provide sufficient indications to establish the form of the condition. To identify a case of Blue Cone Monochromacy, it is necessary to reconstruct the family history, with the condition linked to the transmission of the X chromosome, if there are other cases in the family. In adult individuals, visual acuity and color vision can be tested and a clinical diagnosis can be made. However, the most important step is the genetic confirmation through a DNA test.

The most suitable diagnostic tools are:

  • A DNA test.
  • A color test such as Farnsworth D-15 or Farnsworth Munsell 100 Hue test.
  • The reconstruction of the family history or family Pedigree of the disease.
  • The electroretinogram (ERG), which can demonstrate the loss of the functions of L/M cones with the consequent maintenance of the function of S-cones and rods[4].

It’s important consider differential diagnosis distinguishing Blue Cone Monochromacy from others diseases that present with similar clinical features, for example, achromatopsia [7]. It is important to reach the correct diagnosis because the evolution of the disease, the possible therapies and necessary aids change depending on the specific condition. The crucial step to confirm the diagnosis of Blue Cone Monochromacy is to test the DNA.

 

Gene therapy and Treatments

As of today, there is no known cure for Blue Cone Monochromacy; however, the efficacy and safety of various prospective treatments are currently being evaluated, gene therapy being the most promising one.

The goal of gene therapy studies[15,16,23,51,59,61,62] is to virally supplement retinal cells expressing mutant genes associated with the Blue Cone Monochromacy phenotype with healthy forms of the gene; thus, allowing the repair and proper functioning of retinal photoreceptor cells in response to the instructions associated with the inserted healthy gene.

Additionally, corrective visual aids and personalized vision therapy provided by Low Vision Specialists may help patients correct glare and optimize their visual acuity.

Epidemiology

Blue Cone Monochromacy is a cause of inherited low vision estimated to affect approximately 1 in 100,000 people [27]. The disease affects male recipients of the X-linked mutation, while females usually remain unaffected carriers of the BCM trait.

History

Early discoveries

Blue Cone Monochromacy has been known for many years, with the first detailed description dating back to Huddart in 1777 [24], who recognized individuals affected by Blue Cone Monochromacy had difficulties in distinguishing colors, but could differentiate between white, black, and various light or bright colors.

Subsequent studies on Blue Cone Monochromacy were conducted by Sloan in 1954 [48] and Blackwell and Blackwell in 1961 [8], who described patients capable of distinguishing between blue and yellow signals that seemed to have functional rods and S-cones cells. Furthermore, Spivey in 1965 [50] indicated that affected individuals were able to see small blue objects on a large yellow field and vice versa.

The disease has also been studied also by Alpern et al. (1960) [2,3] and by Fleischman (1981) but the most important results have been obtained in 1989 and 1993 by Nathans et al. [38,39] and in 1991 by Reyniers et al. [46] who identified the genes causing Blue Cone Monochromacy.

Recent studies

Only in the last few years and thanks to the support of the BCM Families Foundation, Blue Cone Monochromacy has been studied with the aim to find a treatment.

Particularly since 2010 the BCM Families Foundation have been financing clinical studies at the University of Pennsylvania with the aim to understand if gene therapy can be a treatment for Blue Cone Monochromacy. Positive results, never obtained before, show the presence of sufficient cone cells in the retina to warrant gene therapy [11]. These cone photoreceptor cells can be treated with gene therapy.  Another study compared patients belonging to the two major Blue Cone Monochromacy causative mutations, deletions and C203R missense mutations, and identified age windows of opportunity for intervention through gene therapy [52]. This set of studies represents a natural history study of Blue Cone Monochromacy. Further studies have led to the identification of outcome measures for a clinical trial and the identification of inclusion and exclusion criteria [12,30,33,47,51].

With the financial support and/or the collaboration of the BCM Families Foundation, several studies on gene therapy vectors have been done. The group of Dr. W.W. Hauswirth at the University of Florida before and the group of Dr. Wen Tao Deng at the West Virginia University then, have been working on animal models of Blue Cone Monochromacy to identify the best AAV vector for gene therapy [15,16,18,23,59,61,62]. Adverum Biotechnologies. Inc, has evaluated a novel intravitreal gene therapy vector for the treatment of Blue Cone Monochromacy [23].  It used ADVM-062, a vector optimized for cone-specific expression of human L-opsin. Unlike existing therapies that involve subretinal vector injection, ADVM-062 can be administered using a single intravitreal (IVT) injection, which poses less risk to the central retinal structure of BCM patients.

Picture taken from ref. [23]

Several recent scientific studies have been done on the Blue Cone Monochromacy. [9,17,25,26,28, 38,39,46,58] to identify all causative mutations and to deeply understand main causative mutations. Next Generation Sequencing (NGS) of DNA technologies have been introduced in labs and allow sequencing the entire exome or the entire genome. These technologies represent the future of DNA testing giving the possibility of sequencing an entire exome or genome within days at reasonable costs and enabling the detection of otherwise missed genetic disease. However, the widely utilized short-read next-generation sequencing (NGS) is inappropriate for the analysis of the OPN1LW/OPN1MW gene cluster of Blue Cone Monochromacy and a new tool has been recently developed [22].

Videos

  1. Video from the Low Vision Centers of Indiana, that shows symptoms of Blue Cone Monochromacy and the use of magenta filtered contact lenses:
  1. Video of a 4-months baby with Blue Cone Monochromacy showing Nystagmus

External resources

References:

  1. “Patient Registry – Blue Cone Monochromacy”.
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