Introduction

Aarskog syndrome, also known as Aarskog-Scott syndrome or faciogenital dysplasia, is a rare genetic disorder that primarily affects males and is characterised by distinct facial features, genital abnormalities, and skeletal anomalies. The syndrome was first described by Norwegian paediatrician Dagfinn Aarskog in 1970, and later independently characterised by American geneticist Charles I. Scott, Jr., in 1971.^1,2 Although the syndrome's prevalence is unknown, it is considered a rare disorder, with less than 200 cases reported in the literature.³ This article explores the genetic basis and inheritance patterns of Aarskog syndrome, detailing its clinical features, genetic mutations, inheritance mechanisms, diagnostic approaches, management strategies, and potential avenues for future research.

Clinical manifestations

Facial Features

Individuals with Aarskog syndrome typically present with distinctive facial dysmorphism. These facial characteristics include a round face with a broad forehead and a widow's peak hairline.4,5 There is often hypertelorism, which refers to widely spaced eyes, and down-slanting palpebral fissures, giving the eyes a characteristic appearance6. Additional features include ptosis, drooping eyelids, and a short, upturned nose⁵. The philtrum, which is the groove between the nose and upper lip, may be prominent, adding to the distinct facial structure of those affected.⁴ Moreover, individuals may have a wide mouth with a pointed chin and a crease below the lower lip. It is important to note that these facial features may change with age, and the prominence of these characteristics can vary among affected individuals.⁶

Genital abnormalities

Genital abnormalities are a hallmark of Aarskog syndrome, particularly in males, who may present with several notable features.⁷ One common manifestation is cryptorchidism, where the testes fail to descend into the scrotum. Additionally, males with Aarskog syndrome may have a shawl scrotum, a condition where the scrotum surrounds the penis, creating a characteristic appearance.⁸ Another key feature is micropenis, which is an abnormally small penis. In females, genital abnormalities associated with Aarskog syndrome are typically milder and may include labial hypoplasia or other minor anomalies.⁹

Skeletal anomalies

Skeletal abnormalities are common in Aarskog syndrome and can affect various parts of the body.^2,4 Individuals may present with brachydactyly, which refers to short fingers and toes, and clinodactyly, which involves the curvature of the fifth finger.¹ Short stature is another characteristic feature, with growth delays often apparent in childhood.³ Additionally, individuals with Aarskog syndrome may exhibit joint hyperextensibility, particularly in the hands and fingers, as well as chest abnormalities such as pectus excavatum, a sunken chest, or pectus carinatum, a protruding chest.⁵ Spinal abnormalities, such as scoliosis or kyphosis, may also be observed, although the extent and severity of these skeletal anomalies can vary widely among affected individuals.⁶

Other clinical features

In addition to the primary features mentioned above, individuals with Aarskog syndrome may exhibit a range of other clinical manifestations. Dental issues are common and can include delayed eruption of teeth, malocclusion, and overcrowding.^1,7 While most individuals have intelligence within the normal range, mild to moderate cognitive and developmental delays can occur, and behavioural challenges such as attention deficit hyperactivity disorder (ADHD) and social difficulties are not uncommon.⁸ Ocular issues, including strabismus (crossed eyes) and refractive errors, can also be part of the syndrome's presentation. These additional features contribute to the clinical variability and complexity of Aarskog syndrome.⁹

Genetic basis of aarskog syndrome

FGD1 gene and its function

The primary genetic cause of Aarskog syndrome is mutations in the FGD1 gene, located on the short arm of the X chromosome (Xp11.21).⁸ The FGD1 gene encodes a guanine nucleotide exchange factor (GEF) that specifically activates the Rho GTPase Cdc42. Rho GTPases are a family of signalling molecules that regulate various cellular processes, including cytoskeletal dynamics, which are crucial for cell shape, migration, and division.⁶ These dynamics are vital for the proper functioning of cells, affecting everything from cellular movement to intracellular transport. Additionally, Rho GTPases play a role in vesicular trafficking, impacting intracellular transport and communication, and in cell proliferation and differentiation, essential for tissue development and growth.⁹ The FGD1 protein, by activating Cdc42, plays a vital role in skeletal formation, morphogenesis, and the development of facial and urogenital structures.⁵

Types of FGD1 mutations

Mutations in the FGD1 gene can disrupt the normal function of the encoded protein, leading to the clinical manifestations of Aarskog syndrome. These mutations can be categorised into several types, each affecting the gene's function in different ways. Missense mutations involve a single nucleotide change that results in the substitution of one amino acid for another, potentially altering protein function.² Nonsense mutations introduce a premature stop codon, leading to a truncated, non-functional protein. Splice site mutations affect RNA splicing, potentially resulting in abnormal protein products. Lastly, small deletions or insertions can cause a frameshift, disrupting the normal reading frame of the gene.³ The diversity of these mutations contributes to the phenotypic variability observed among affected individuals.⁶

Molecular mechanisms

The molecular mechanisms underlying Aarskog syndrome involve the disruption of Rho GTPase signalling pathways, particularly those involving Cdc42. One proposed mechanism involves cytoskeletal abnormalities, where the disruption of FGD1 function leads to impaired actin cytoskeleton organisation, affecting cell shape and movement.⁷ This impairment can contribute to skeletal and facial anomalies, characteristic features of the syndrome. Another mechanism is defective vesicular trafficking; abnormal vesicular transport may affect the delivery of essential molecules during development, leading to urogenital and skeletal defects.⁴ Additionally, altered cellular proliferation and differentiation, caused by impaired signalling pathways, can disrupt normal growth and developmental processes, contributing to the growth delays and developmental abnormalities observed in Aarskog syndrome.⁸ Research continues to investigate the precise molecular mechanisms by which FGD1 mutations lead to the diverse clinical features of the disorder.⁵

Inheritance patterns

X-linked recessive inheritance

Aarskog syndrome follows an X-linked recessive inheritance pattern, which explains why the condition is more commonly expressed in males.² Males have one X and one Y chromosome, so a single altered copy of the FGD1 gene on the X chromosome is sufficient to cause the disorder.⁴ In contrast, females have two X chromosomes and would need mutations in both copies of the FGD1 gene to express the syndrome, which is exceedingly rare.³ Thus, females typically serve as carriers, passing the gene to their offspring without expressing the disorder themselves.⁷

Carrier females

Females who carry one mutated copy of the FGD1 gene are usually asymptomatic or may exhibit mild features of the syndrome.⁶ These carrier females have a 50% chance of passing the mutated gene to their offspring. Sons who inherit the mutated gene will be affected by the syndrome, while daughters who inherit the gene will be carriers like their mothers.⁹ This pattern of inheritance highlights the gender-specific manifestation of the disorder and the critical role of genetic counselling in families affected by Aarskog syndrome.⁹

Rare occurrences in females

Although Aarskog syndrome predominantly affects males, rare cases have been reported in females, with several possible explanations for this occurrence.¹ One possibility is skewed X-inactivation, a process where one of the two X chromosomes in females is randomly inactivated in each cell.⁸ If the normal X chromosome is preferentially inactivated, the mutated gene may be expressed, leading to the syndrome. Another possibility is compound heterozygosity, where females with mutations in both copies of the FGD1 gene express the syndrome if they inherit a different mutation from each parent.⁷ Lastly, females with Turner syndrome (45, X), who have only one X chromosome, may express Aarskog syndrome if they inherit a mutated FGD1 gene.⁶ These rare occurrences highlight the complexity of genetic inheritance and expression in Aarskog syndrome.⁹

Diagnosis and genetic testing

Clinical evaluation

The diagnosis of Aarskog syndrome is primarily based on clinical evaluation, considering the distinctive facial, skeletal, and genital features.¹ Given the variability in presentation, a thorough examination and detailed family history are crucial in identifying potential X-linked inheritance.⁵ Key steps in the diagnostic process include a comprehensive physical examination, focusing on evaluating facial features, skeletal abnormalities, and genital anomalies.³ Developmental assessment is also important to understand growth patterns, cognitive development, and behavioural challenges that may accompany the syndrome.² Additionally, investigating the family history can help identify potential carriers and affected individuals, providing valuable information for genetic counselling and risk assessment.⁷

Genetic testing

Genetic testing plays a crucial role in confirming the diagnosis of Aarskog syndrome by identifying mutations in the FGD1 gene.⁸ Several genetic testing methods are commonly used to detect these mutations.² Sanger sequencing is a standard method for detecting point mutations and small insertions or deletions in the FGD1 gene, offering precise identification of genetic changes.⁷ Another technique, multiplex ligation-dependent probe amplification (MLPA), is used to detect larger deletions or duplications within the gene. Additionally, next-generation sequencing (NGS) provides a comprehensive approach to identifying mutations, offering the ability to sequence the entire FGD1 gene and detect a wide range of genetic variations.⁸ Genetic testing is invaluable for confirming diagnoses, detecting carriers in females, and facilitating prenatal diagnosis in families with a known history of Aarskog syndrome.⁶

Management and prognosis

Medical management

There is currently no cure for Aarskog syndrome, so treatment focuses on managing symptoms and improving quality of life. A multidisciplinary approach is often required, involving various healthcare professionals to address the diverse needs of affected individuals. Orthopaedic interventions may be necessary to correct skeletal deformities and improve mobility, while surgical procedures can address genital abnormalities such as cryptorchidism.⁶ Dental care is essential for managing dental issues and ensuring proper oral health. Additionally, developmental support, including educational interventions and therapies, is crucial for assisting individuals with learning and behavioural challenges.³ Ongoing medical care and supportive therapies play a vital role in addressing specific challenges and enhancing the overall well-being of those with Aarskog syndrome.

Long-term outlook

The prognosis for individuals with Aarskog syndrome varies depending on the severity of symptoms and the effectiveness of management strategies. While most individuals lead relatively normal lives, they may face ongoing challenges related to growth, development, and health. Regular medical follow-up and early intervention can significantly improve outcomes, addressing potential complications and supporting the individual’s development.¹ With appropriate care and support, individuals with Aarskog syndrome can achieve a good quality of life, although ongoing medical attention may be required to manage specific symptoms and prevent complications.²

Research and future directions

Ongoing research into the molecular mechanisms of Aarskog syndrome is crucial for developing targeted therapies and improving understanding of the disorder. Advances in genetic technologies, such as next-generation sequencing and gene editing, are facilitating the identification of novel mutations and their effects on cellular pathways.⁸ A better understanding of the pathways affected by FGD1 mutations may lead to the development of novel treatments that address the root cause of the disorder, rather than just managing symptoms.⁹ Additionally, research into the variability of phenotypic expression and the impact of genetic modifiers may provide insights into the mechanisms underlying the disorder’s clinical variability.⁸

Potential therapeutic approaches

Several potential therapeutic approaches are being explored in the context of Aarskog syndrome. Gene therapy, which involves correcting or replacing the defective gene, holds promise for addressing the underlying genetic cause of the disorder.⁹ Additionally, small molecule drugs that target specific pathways disrupted by FGD1 mutations are being investigated for their potential to ameliorate symptoms and improve outcomes.⁸ Furthermore, advances in stem cell therapy and regenerative medicine may offer opportunities for repairing or regenerating damaged tissues, providing novel treatment options for individuals with Aarskog syndrome.⁹

Conclusion

Aarskog syndrome is a rare genetic disorder characterised by a distinct set of clinical features resulting from mutations in the FGD1 gene. The X-linked recessive inheritance pattern explains the predominance of affected males, while females typically serve as carriers. Although there is currently no cure, management strategies aim to alleviate symptoms and support affected individuals. Continued research into the genetic and molecular underpinnings of Aarskog syndrome holds promise for future therapeutic developments, with potential advances in gene therapy and targeted treatments offering hope for improved outcomes. Understanding the complexity of Aarskog syndrome is essential for developing effective interventions and providing comprehensive care to affected individuals and their families.