PRDM13 — PR/SET domain 13

The PRDM13 gene provides instructions for making a protein that acts as a master switch during the early development of the eye and brain. This protein, known as a transcription factor, helps determine what specific types of cells will form in the retina, which is the light-sensitive tissue at the back of the eye. It is particularly important for the proper development of the macula, the small central area of the retina responsible for sharp, detailed, and central vision. When there are specific genetic changes associated with the PRDM13 gene, it causes a rare eye condition called North Carolina Macular Dystrophy (NCMD). Interestingly, the mutations that cause NCMD usually do not occur inside the gene itself. Instead, they happen in the "control switches" (regulatory regions) next to the gene, or involve extra copies of the gene. This causes the PRDM13 protein to be produced in the wrong amounts or at the wrong time during eye development before birth, which interferes with the normal formation of the macula. For patients and families, NCMD is an inherited condition that is present from birth. It is passed down in an autosomal dominant pattern, meaning a person only needs one altered copy of the gene to have the condition, and there is a 50% chance of passing it to each child. The good news is that unlike many other inherited retinal diseases, NCMD is generally non-progressive. This means that while a child may be born with abnormal-looking maculas and some degree of central vision loss, their vision typically remains stable throughout their life and does not lead to complete blindness.

Gene description: Encodes a PR-domain containing protein involved in transcriptional regulation and cell differentiation.

Patient and family guide: The PRDM13 gene provides instructions for making a protein that acts as a master switch during the early development of the eye and brain. This protein, known as a transcription factor, helps determine what specific types of cells will form in the retina, which is the light-sensitive tissue at the back of the eye. It is particularly important for the proper development of the macula, the small central area of the retina responsible for sharp, detailed, and central vision. When there are specific genetic changes associated with the PRDM13 gene, it causes a rare eye condition called North Carolina Macular Dystrophy (NCMD). Interestingly, the mutations that cause NCMD usually do not occur inside the gene itself. Instead, they happen in the "control switches" (regulatory regions) next to the gene, or involve extra copies of the gene. This causes the PRDM13 protein to be produced in the wrong amounts or at the wrong time during eye development before birth, which interferes with the normal formation of the macula. For patients and families, NCMD is an inherited condition that is present from birth. It is passed down in an autosomal dominant pattern, meaning a person only needs one altered copy of the gene to have the condition, and there is a 50% chance of passing it to each child. The good news is that unlike many other inherited retinal diseases, NCMD is generally non-progressive. This means that while a child may be born with abnormal-looking maculas and some degree of central vision loss, their vision typically remains stable throughout their life and does not lead to complete blindness.

Gene function: PRDM13 is a transcription factor that plays a role in the development and maintenance of specific neuronal cell types, including those in the retina. It is thought to be involved in regulating gene expression critical for photoreceptor cell fate and survival. While its precise retinal function is still under investigation, its disruption can lead to developmental abnormalities or degeneration of retinal cells, affecting visual function.

Protein structure: The PRDM13 gene encodes the PR domain zinc finger protein 13, a transcription factor that is approximately 707 amino acids in length in humans. The protein is characterized by two main structural features typical of the PRDM family. At the N-terminus, it contains a PR (PRDI-BF1 and RIZ1 homology) domain, which is structurally related to the SET domain found in many histone methyltransferases. This domain is crucial for the protein's epigenetic regulatory functions. Following the PR domain, the C-terminal region of PRDM13 contains an array of four C2H2-type zinc finger domains. These zinc finger motifs are essential for sequence-specific DNA binding, allowing PRDM13 to target specific genes for transcriptional regulation. The protein functions within the nucleus, where it likely interacts with other co-repressors and chromatin-modifying enzymes to form functional repressor complexes that modulate gene expression during cellular differentiation.

Molecular function: PRDM13 functions primarily as a transcriptional repressor and epigenetic regulator during neurogenesis. As a member of the PRDM (PRDI-BF1 and RIZ homology domain containing) family, it is involved in chromatin remodeling and the regulation of gene expression. PRDM13 acts downstream of key proneural transcription factors, such as Ptf1a, to control cell fate specification in the developing nervous system. In the retina, PRDM13 is critical for the specification of specific amacrine cell subtypes, particularly GABAergic and glycinergic inhibitory interneurons. It achieves this by repressing alternative cell fates and regulating the expression of downstream target genes necessary for amacrine cell differentiation. The PR domain of PRDM13 is predicted to possess histone methyltransferase activity, allowing it to modify chromatin structure and stably repress target genes. In the context of North Carolina Macular Dystrophy (NCMD), the disease is not caused by a loss of PRDM13 function, but rather by its dysregulation. Mutations in regulatory elements lead to the inappropriate overexpression or ectopic expression of PRDM13 in the developing macula. This overexpression is thought to disrupt the delicate balance of transcriptional networks required for normal photoreceptor and macular development, ultimately arresting macular development and leading to the characteristic lesions seen in NCMD.

Expression pattern: PRDM13 exhibits a highly specific spatiotemporal expression pattern during embryonic development, primarily restricted to the central nervous system and the developing retina. In the retina, PRDM13 is expressed in the inner neuroblastic layer during development and is specifically localized to a subset of amacrine cells in the mature retina. It is not typically expressed in mature photoreceptors or the retinal pigment epithelium (RPE). Beyond the retina, PRDM13 is expressed in the dorsal region of the developing neural tube, where it plays a role in specifying inhibitory interneurons. It is also expressed in the developing cerebellum, specifically in Purkinje cells, and in the hypothalamus, where it is required for the differentiation of KISS1-expressing neurons in the arcuate nucleus. The precise temporal regulation of PRDM13 is critical, as its transient expression is necessary for proper cell fate determination during neurogenesis.

Mutation spectrum: The mutation spectrum of PRDM13 is highly unusual compared to most inherited retinal disease genes. The vast majority of pathogenic variants associated with North Carolina Macular Dystrophy (NCMD) are not located within the coding sequence of the gene. Instead, they consist of non-coding single nucleotide variants (SNVs) located in a DNase I hypersensitivity site upstream of PRDM13, which acts as a cis-regulatory element or enhancer. In addition to these regulatory SNVs, several large tandem duplications involving the PRDM13 gene and its surrounding regulatory regions have been identified as a major cause of NCMD. These duplications lead to increased gene dosage and overexpression. To date, only a small number of pathogenic variants (fewer than 20) have been described for NCMD, but they are found in families worldwide. Recently, rare recessive coding mutations (such as missense or nonsense mutations) within the PRDM13 gene have been identified, but these cause a severe, fatal neurodevelopmental syndrome rather than isolated retinal disease.

Pathogenic variants: 1. Chr6:g.100040906G>T (GRCh37) - A well-characterized non-coding single nucleotide variant (V1) located in an upstream regulatory element, identified as a founder mutation in the original North Carolina kindred and causing classic NCMD. 2. Chr6:g.100040987G>C (GRCh37) - Another non-coding single nucleotide variant (V2) in the same upstream regulatory region, causing NCMD in multiple families. 3. Chr6:g.100041040C>T (GRCh37) - A non-coding single nucleotide variant (V3) in the upstream enhancer region associated with NCMD. 4. PRDM13 Tandem Duplications - Various large tandem duplications (ranging from ~65 kb to over 1 Mb) encompassing the PRDM13 gene and its regulatory elements, leading to gene overexpression and causing NCMD in diverse populations. 5. p.Arg240* (or similar loss-of-function coding variants) - Rare biallelic nonsense or missense mutations within the PRDM13 coding sequence that cause a fatal perinatal neurodevelopmental disorder with cerebellar hypoplasia, distinct from NCMD.

Clinical significance: Mutations affecting the PRDM13 gene are primarily associated with North Carolina Macular Dystrophy (NCMD, also known as MCDR1), a rare, congenital, autosomal dominant macular malformation. NCMD is typically characterized by non-progressive, bilateral macular lesions that are present at birth or discovered in early childhood. The clinical presentation is highly variable, even within the same family, ranging from mild drusen-like deposits in the central macula (Grade 1) to confluent drusen (Grade 2), and in severe cases, large coloboma-like atrophic macular lesions (Grade 3). Despite the dramatic appearance of the macula in severe cases, visual acuity is often surprisingly well-preserved, typically ranging from 20/20 to 20/200, and rarely deteriorates over the patient's lifetime. Color vision and peripheral vision generally remain intact. The condition is considered a developmental abnormality of the macula rather than a progressive degeneration. In addition to NCMD, recessive coding mutations in PRDM13 have recently been linked to a severe, fatal perinatal neurodevelopmental disorder characterized by brainstem dysfunction and cerebellar hypoplasia, highlighting the gene's critical role in broader central nervous system development.

Inheritance: Autosomal Recessive

Chromosomal location: 6q16.1

Genotype-phenotype correlations: The genotype-phenotype correlation for PRDM13 is distinct and depends heavily on the nature of the mutation. North Carolina Macular Dystrophy (NCMD) is caused by heterozygous, non-coding variants (such as single nucleotide variants in upstream regulatory elements or tandem duplications) that lead to the dysregulation and overexpression of PRDM13 during retinal development. Despite the specific genetic etiology, NCMD exhibits significant phenotypic variability (ranging from mild drusen to severe macular staphyloma) even among individuals carrying the exact same mutation within a family. This suggests that other genetic modifiers or epigenetic factors strongly influence the final clinical phenotype. In contrast, biallelic (homozygous or compound heterozygous) coding mutations within the PRDM13 gene itself result in a completely different and much more severe phenotype: a fatal perinatal neurodevelopmental disorder characterized by cerebellar hypoplasia and brainstem dysfunction. This stark contrast highlights that NCMD is a localized developmental defect caused by ectopic or overexpression in the retina, whereas loss-of-function coding mutations disrupt essential neurodevelopmental processes in the brain.

Research and therapeutic approaches: Currently, there are no approved targeted therapies or gene therapies for PRDM13-associated North Carolina Macular Dystrophy (NCMD). Because NCMD is a developmental malformation that occurs in utero and is generally non-progressive after birth, the window for therapeutic intervention to prevent the macular lesions may be limited to the prenatal period, which presents significant clinical and ethical challenges. Management is primarily supportive, focusing on maximizing existing vision through refractive correction, low vision aids, and educational support for children with significant visual impairment. Research into therapeutic approaches is still in the early preclinical stages. Since NCMD is caused by the overexpression or dysregulation of PRDM13 rather than a loss of function, traditional gene replacement therapy (like Luxturna for RPE65) is not applicable. Instead, potential future strategies would need to focus on downregulating PRDM13 expression. Investigational approaches could include CRISPR/Cas9-based genome editing to correct the regulatory mutations or remove the duplicated gene segments, or the use of antisense oligonucleotides (ASOs) or RNA interference (RNAi) to reduce PRDM13 mRNA levels. However, these approaches are currently only being explored in animal models and cell lines, and no clinical trials for PRDM13 therapies are currently active.

Diagnostic testing: Diagnosis of PRDM13-associated North Carolina Macular Dystrophy (NCMD) relies on a combination of clinical examination, retinal imaging, and molecular genetic testing. Clinical diagnosis is based on the characteristic appearance of congenital, non-progressive macular lesions, often supported by optical coherence tomography (OCT) which can reveal retinal thinning, loss of outer retinal layers, and excavation in the macular region. Molecular confirmation requires targeted genetic testing. Because the pathogenic variants causing NCMD are predominantly non-coding single nucleotide variants (SNVs) in the promoter/enhancer region or tandem duplications involving the PRDM13 gene, standard whole exome sequencing (WES) may miss these mutations. Therefore, whole genome sequencing (WGS) or targeted sequencing of the PRDM13 regulatory regions (specifically the MCDR1 locus on chromosome 6q16) is recommended. Genetic counseling is essential, as NCMD follows an autosomal dominant inheritance pattern with complete penetrance but highly variable expressivity, meaning offspring of an affected individual have a 50% chance of inheriting the condition, though the severity cannot be predicted.

Animal models: Animal models have been crucial in elucidating the role of PRDM13 in retinal development and disease. In mice, Prdm13 is essential for the specification of specific amacrine cell subtypes, particularly those expressing Ebf3, and acts downstream of Ptf1a to promote GABAergic and glycinergic amacrine neurons. Conditional deletion of Prdm13 in the mouse retina results in a significant reduction of these amacrine cell populations. To model North Carolina Macular Dystrophy (NCMD), researchers have developed CRISPR-derived and inducible mouse models that overexpress PRDM13, mimicking the human disease mechanism. These models demonstrate that eye-specific overexpression of PRDM13 leads to photoreceptor degeneration and deregulates genes involved in phototransduction and photoreceptor health. Additionally, zebrafish models have been utilized to study PRDM13-related neurodevelopmental anomalies, confirming its conserved role in central nervous system and retinal development.

Population genetics: North Carolina Macular Dystrophy (NCMD) caused by PRDM13 dysregulation is a rare condition, and specific carrier frequencies in the general population are not well-established but are presumed to be extremely low. The disease was initially described in a large, multi-generational family tracing its roots to North Carolina, USA, where a specific founder mutation (a non-coding SNV) was identified. However, PRDM13-associated NCMD has since been reported in diverse populations worldwide, including families of European, Asian, Middle Eastern, and Hispanic descent. In these different populations, the disease is often caused by distinct non-coding variants or unique tandem duplications, indicating that while the original North Carolina family exhibited a founder effect, mutations affecting PRDM13 regulation arise independently across different ethnicities.

Selected references: 1. Small KW, et al. North Carolina Macular Dystrophy Is Caused by Dysregulation of the Retinal Transcription Factor PRDM13. Ophthalmology, 2016. PMID: 26507665 2. Small KW, et al. North Carolina Macular Dystrophy: Phenotypic Variability and the Role of PRDM13. Invest Ophthalmol Vis Sci, 2019. PMID: 31112581 3. Watanabe S, et al. Prdm13 regulates subtype specification of retinal amacrine cells. J Neurosci, 2015. PMID: 25995483 4. Coolen M, et al. Recessive PRDM13 mutations cause fatal perinatal brainstem dysfunction with cerebellar hypoplasia and disrupt Purkinje cell differentiation. Am J Hum Genet, 2022. PMID: 35395193 5. Chacon-Camacho OF, et al. A novel PRDM13 gene duplication causing congenital North Carolina macular dystrophy phenotype in a Mexican family. Mol Vis, 2024. PMID: 38905659 6. Manes G, et al. A novel duplication of PRMD13 causes North Carolina macular dystrophy: overexpression of PRDM13 orthologue in drosophila eye reproduces the human phenotype. Hum Mol Genet, 2017. PMID: 28973673