The Cell Division Cycle 42 (CDC42) gene encodes a Cdc42 molecule which is a member of the family of Rho GTPases belonging to the Ras superfamily of small GTPases. The Rho-family GTPases are characterized by a multiplicity of physiological functions influencing crucial developmental signals in cell cycle regulation. These biological processes include the establishing and controlling of the cell actin cytoskeleton, vesicle trafficking, cell polarity, proliferation, motility and migration, transcription activation, reactive oxygen species production, and malignant transformation (Ueyama, 2019; Lauri et al., 2021). The Cdc42 protein plays a fundamental role in cell biology and is implicated in a wide array of physiologically pivotal, tissue-specific activities. Cdc42-dependent cytoskeletal reorganization and cell polarity are essential for cardiac organogenesis, tubulogenesis of the pancreas, kidney, lung, and salivary and mammary glands, and differentiation of keratinocytes in the skin and hair follicles (Melendez et al., 2011; Liu et al., 2013). In the central nervous system (CNS), Cdc42 is a positive regulator of its development through neurite initiation, axon growth, branching, myelination and specification, neuronal migration as well as neuronal polarity (Melendez et al., 2011; Liu et al., 2013). Cdc42 coordinates actin turnover and maintenance of actin structures stability in inner ear hair cells and the polarization of the sensory organ of the cochlea which determines the hearing function. The processes of cell polarization and actin microtubule dynamics also play a role in eye morphogenesis and photoreceptor differentiation (Ueyama et al., 2014).

Cdc42 participates in a wide spectrum of immune functions related to hematopoiesis and immune homeostasis as well as effector mechanisms of the innate and adaptive immune responses. In the bone marrow, Cdc42 regulates the multilineage development of blood progenitors and their egress from the bone marrow to the periphery. While Cdc42 is involved in the transcriptional program, it plays an important role in the differentiation of stem and progenitor cells, thus influencing the tight balance between myelopoiesis and erythropoiesis (Mulloy et al., 2010; Nayak et al., 2013), and thereby contributing to the immune system regulation and activation. In neutrophil granulocytes, the Cdc42 molecule, through the cytoskeleton rearrangement, controls migration, activation, degranulation, and reactive oxygen species formation thus influencing pathogen killing efficiency (Tackenberg et al., 2020). The migratory function of dendritic cells upon antigen stimulation and homing to the draining lymph nodes is also Cdc42 activity-depending, reflecting the Cdc42 substantial contribution to the initiation of the adaptive response of T CD4⁺ helper and CD8⁺ cytotoxic/suppressor lymphocytes (Luckashenak et al., 2013). The Cdc42 GTPase is an essential coordinator of B lymph cell development, adhesion, motility, antigen and mitogen-driven B cell receptor activation, cognate interaction with T lymph cells, and differentiation into antibody-producing plasma cells (Song et al., 2014; Burbage et al., 2015; Gerasimcik et al., 2015). It has also been shown that Cdc42 plays an important role in T cell thymopoiesis, proliferation, and survival (Smits et al., 2010).

Referring to the multiplicity of key regulatory functions of the Cdc42 GTPase in human biology, governing morphogenesis and functional homeostasis, the growing spectrum of clinical syndromes and pathogenic variants in CDC42 underpinning the diverse phenotypes have been reported. The patients with a germlin heterozygous missense c.191A>G (p.Tyr64Cys) variant demonstrated a range of congenital phenotypic features termed as Takenouchi-Kosaki syndrome (TKS). The syndrome is characterized by facial dysmorphism, developmental delay, sensorineural hearing loss accompanied by macrothrombocytopenia which compose a universal clinical phenotype observable in all eight hitherto described TKS patients (Takenouchi et al., 2015; Takenouchi et al., 2016; Martinelli et al., 2018; Motokawa et al., 2018; Uehara et al., 2019; Bucciol et al., 2020; Ishikawa et al., 2021; Santoro et al., 2021). Short stature (Takenouchi et al., 2015; Takenouchi et al., 2016; Martinelli et al., 2018; Bucciol et al., 2020; Ishikawa et al., 2021; Santoro et al., 2021), failure to thrive (Takenouchi et al., 2015; Takenouchi et al., 2015; Motokawa et al., 2018; Bucciol et al., 2020; Ishikawa et al., 2021; Santoro et al., 2021) with camptodactyly (Takenouchi et al., 2015; Takenouchi et al., 2016; Martinelli et al., 2018; Motokawa et al., 2018; Bucciol et al., 2020; Ishikawa et al., 2021; Santoro et al., 2021), structural abnormalities of the brain (Takenouchi et al., 2015; Takenouchi et al., 2016; Martinelli et al., 2018; Motokawa et al., 2018; Uehara et al., 2019; Bucciol et al., 2020; Santoro et al., 2021), and antibody deficiency (Martinelli et al., 2018; Motokawa et al., 2018; Bucciol et al., 2020; Ishikawa et al., 2021) represent not constant but frequent findings belonging to the symptomatology in TKS. Herein, we report an another patient presenting with distinctive symptoms of TKS (OMIM #616737) due to the heterozygousp.Tyr64Cys variant in CDC42 and the clinical phenotype with congenital malformations, immunodeficiency, and gastrointestinal disorders.

Referring to the heterogeneity of clinical manifestations and the complexity of mutual phenotype-genotype interrelations in hitherto reported alterations in CDC42, it must be highlighted that establishing the definitive diagnosis of a CDC42- related syndrome, including TKS, is challenging for clinicians. The phenotypic features of TKS related to the p.Tyr64Cys variant in CDC42 comprise psychomotor developmental delay, intellectual disability, dysmorphism, sensorineural hearing loss, cardiac and genitourinary malformations, and macrothrombocytopenia (Takenouchi et al., 2015; Takenouchi et al., 2016; Martinelli et al., 2018; Motokawa et al., 2018; Uehara et al., 2019; Bucciol et al., 2020; Ishikawa et al., 2021; Santoro et al., 2021), consistent also with the herein reported patient. The summary of phenotypic features of patients with TKS due to the c.191A>G; p.Tyr64Cys variant in CDC42 are summarized in Table 2. Noteworthy, in a single patient with the same p.Tyr64Cys variant, localized in the switch II domain of the CDC42 gene, affecting Cdc42 binding to multiple effectors and regulators, and causing TKS (Martinelli et al., 2018), immune dysregulation with autoinflammation was observable. It was characterized by interleukin (IL)-6, IL-18, IL-18 binding protein (BPa), and CXCL9 chemokine hypercytokinemia accompanied by myelofibrosis (Bucciol et al., 2020). This phenomenon reflects the clinical intersubject variability within the p.Tyr64Cys variant group and possible overlap with C-terminal variants in the CDC42 gene and highlights that the p.Tyr64Cys variant does support the hypothesis that the gene is associated with distinct diseases. The clinical phenotypes related to C-terminal variants are characterized by severe neonatal-onset dyshematopoiesis and immune dysregulation including autoinflammation, rash, and HLH (NOCARH syndrome) (Gernez et al., 2019; Lam et al., 2019; He et al., 2020; Coppola et al., 2022) with a chronic excess of the inflammatory cytokine, IL-18 as a hallmark of these disorders (Miyazawa and Wada, 2022; Shimizu et al., 2022). A similar phenotype distinguished by the hyperinflammatory state, full-blown HLH, and hitherto not reported, development of non-Hodgkin lymphoma has also been described by our group in the patient with a novel p.Cys81Tyr variant (Szczawińska-Popłonyk et al., 2020).

The ever-increasing use of the next-generation sequencing (NGS) approach enabling uncovering molecular genetic diagnosis underpinning the clinical symptomatology has led to better recognition of variable CDC42 gene alterations and the spectrum of clinical immunophenotypes. A variable degree of immunodeficiency has been primarily linked to the variants affecting the switch II domain in the CDC42 gene (Takenouchi et al., 2015; Takenouchi et al., 2016; Martinelli et al., 2018; Motokawa et al., 2018; Uehara et al., 2019; Bucciol et al., 2020; Asiri et al., 2021; Ishikawa et al., 2021; Kashani et al., 2021; Santoro et al., 2021). In the reported patients, the antibody immune response ranged from normal referenced serum immunoglobulin levels to panhypogammaglobulinemia affecting the production of all immunoglobulin isotypes with low specific antibody response to diphtheria, tetanus, and mumps vaccine antigens. Antibody deficiency is a consequence of disrupted Cdc42 activity affecting actin cytoskeleton mobility, cellular trafficking, regulation of transcription, and proliferation which occur in multiple cell lineages in CDC42 mutants, including the B and T cell compartments. Furthermore, in the actin cytoskeleton remodeling and cell polarity regulation, the Cdc42 GTPase activating interaction is required with the GTPase-binding domain of the Cdc42 effector, the Wiskott-Aldrich protein (WASP) that is essential for B and T cell differentiation (Tangye et al., 2019). The presently described patient has normal for age IgG, and IgM, decreased IgA levels, normal distribution of IgG subclasses, low antigen-specific anti-tetanus and anti-diphtheria antibodies, accompanied by lymphopenia and aberrant B and T lymph cell, primarily T helper cell development that might be underpinning the proneness to infections and immune dysregulation. Referring to the clinical reports, in 2022, disorders of the actin cytoskeleton affecting immunity have been included by the International Union of Immunological Societies Expert Committee in the updated classification of human inborn errors of immunity (Tangye et al., 2022).

In this report, we add feeding disorders as well as gastric and abdominal walls infection to the clinical symptomatology of the p.Tyr64Cys variant-related syndrome. Noteworthy, the aberrant immune response in our patient may be an important factor contributing to the chronic inflammation and malfunctioning of the gastrointestinal tract leading to a vicious circle of immunodeficiency with immune dysregulation and malnutrition with poor weight gaining, which, in turn, may exacerbate dysfunction of the immune system. This hypothesis may be supported by the master regulatory role of Cdc42 GTPase in maintaining the integration and immune homeostasis of the intestinal mucosa, and as a key player in the actin cytoskeleton dynamics, it controls mechanisms of inflammation, autoimmunity, and cancerogenesis. Intestinal cell proliferation, epithelial integrity, and cellular turnover as well as enterocyte extrusion and shedding are tightly controlled processes regulated by Cdc42-dependent actin cytoskeletal contractility and reorganization (Pradham et al., 2021; Ngo et al., 2022).

The patient’s perspective is largely dependent on the detection of the causative variant and establishing a definitive genetic diagnosis which primarily means an explanation for the multifaceted disease. Both for the family and leading physicians, it paves the way for future diagnostic and therapeutic interventions, shedding light on the expanded phenotype with immunodeficiency and gastrointestinal disorders. Prophylaxis against infections with azithromycin is legitimate as the first-line protective measure against respiratory tract infections. Administration of inactive vaccines needs to be recommended as an attempt to trigger the mounting of the immune memory yet the antigen-specific response in the patient may not be adequate (Szczawińska-Popłonyk et al., 2015). Regular monitoring is required by a multidisciplinary team including specialists in neurology, hematology, otorhinolaryngology, gastroenterology, cardiology, and nephrology, under the pediatric immunologist’s supervision. Further clinical observation is needed as other phenotypic features may appear and novel treatment modalities may be proposed according to the course of the disease. It is worth noting that the Rho GTPase Cdc42 lies downstream of the master regulator Ras, crucial for cell malignant transformation. The crosstalk between the Cdc42 molecule and the network of effectors in the immune system is increasingly recognized, and new therapeutic targets based on new regulator partners are expected, and their translation into the treatment strategies for cancer creating a future therapeutic perspective (El Masri and Delon, 2021).

Importantly, the everyday struggle with the child’s intellectual and motor disabilities, failure to thrive and feeding disorders, recurrent infections, increased risk of tumorigenesis, and frequent medical consultations is a disease burden for the patient’s relatives. To cope with the multisystemic syndrome of TKS, medical and social support is needed for the family to enhance undertaking positive health-promoting stimulating activities. Therefore, the de novo nature of the pathogenic CDC42 variant has an important informative role for the family.

The ever-increasing progress in molecular genetic studies contributes to better definition and delineation of the Takenouchi-Kosaki syndrome. It is a multifaceted disease with complex phenotypic features and a broad dysmorphology, neurological, hematological, hormonal, respiratory as well as immunological symptomatology. In this report, we add gastrointestinal and feeding disorders to the spectrum of syndromic manifestations. Uncovering the causative pathogenic variant in the CDC42 gene is of utmost importance for understanding the complex mutual genetic and phenotypic relationships and disease pathophysiology as well as for the informativeness of the patient’s family and the leading physician, contributing to diagnostic and therapeutic management and anticipating the prognosis.