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Primary immunodeficiencies (PIDs) comprise a diverse group of over 400 genetic disorders that result in clinically apparent immune dysfunction. Although PIDs are classically considered as Mendelian disorders with complete penetrance, we now understand that absent or partial clinical disease is often noted in individuals harboring disease-causing genotypes. Despite the frequency of incomplete penetrance in PID, no conceptual framework exists to categorize and explain these occurrences. Here, by reviewing decades of reports on incomplete penetrance in PID we identify four recurrent themes of incomplete penetrance, namely genotype quality, (epi)genetic modification, environmental influence, and mosaicism. For each of these principles, we review what is known, underscore what remains unknown, and propose future experimental approaches to fill the gaps in our understanding. Although the content herein relates specifically to inborn errors of immunity, the concepts are generalizable across genetic diseases.

Abstract Congenital anomalies of the kidney and urinary tract form a spectrum of congenital structural disorders that are generally known under the term CAKUT. The term CAKUT was introduced 20 years ago and has been used extensively in literature since. Prof. Woolf has made a plea for abandoning this term in his “case for the prosecution.” Here, I advocate for the continued use of CAKUT as an umbrella term for these related congenital kidney and urinary tract abnormalities. I explain why the term CAKUT accurately and usefully defines this group of related structural disorders with prenatal origin and why it makes sense to continue grouping these disorders given accumulating evidence for shared etiology of CAKUT phenotypes and the importance of grouping CAKUT phenotypes in genetic counseling.

Inherited cardiomyopathies are genetic diseases that can lead to heart failure and sudden cardiac death. These conditions tend to run in families, following an autosomal dominant pattern where first-degree relatives have a 50% chance of carrying the pathogenic variant. Despite significant advancements and increased accessibility of genetic testing, accurately predicting the phenotypic expression of these conditions remains challenging due to the inherent variability in their clinical manifestations and the incomplete penetrance observed. This poses challenges in providing patient care and effectively communicating the potential risk of future disease to patients and their families. To address these challenges, this review aims to synthesize the available evidence on penetrance, expressivity, and factors influencing disease expression to improve communication and risk assessment for patients with inherited cardiomyopathies and their family members.

KIAA0586 variants have been associated with a wide range of ciliopathies, mainly Joubert syndrome (JS, OMIM #616490) and short-rib thoracic dysplasia syndrome (SRTD, OMIM #616546). However, the hypothesis that this gene is involved with hydrolethalus syndrome (HSL, OMIM #614120) and orofaciodigital syndrome IV (OMIM #258860) has already been raised. Ciliopathies’ clinical features are often overlapped despite differing in phenotype severity. Besides KIAA0586, HYLS1 and KIF7 are also known for being causative of ciliopathies, indicating that all three genes may have similar or converging genomic pathways. Overall, the genotypic and phenotypic spectrum of ciliopathies becomes wider and conflicting while more and more new variants are added to this group of disorders’ molecular pot. In this case report we discuss the first Brazilian individual clinically diagnosed with hydrolethalus syndrome and molecular findings that demonstrate the role of KIAA0586 as a causative gene of a group of genetic disorders. Also, recent reports on individuals with intronic and exonic variants combined leading to ciliopathies support our patient’s molecular diagnosis. At the same time, we discuss variable expressivity and overlapping features in ciliopathies.

Bardet–Biedl Syndrome is a rare non-motile primary ciliopathy with multisystem involvement and autosomal recessive inheritance. The clinical picture is extremely polymorphic. The main clinical features are retinal cone-rod dystrophy, central obesity, postaxial polydactyly, cognitive impairment, hypogonadism and genitourinary abnormalities, and kidney disease. It is caused by various types of mutations, mainly in genes encoding BBSome proteins, chaperonins, and IFT complex. Variable expressivity and pleiotropy are correlated with the existence of multiple genes and variants modifiers. This review is focused on the phenomena of heterogeneity (locus, allelic, mutational, and clinical) in Bardet–Biedl Syndrome, its mechanisms, and importance in early diagnosis and proper management.

The 15q11.2q13 chromosomal region is particularly susceptible to chromosomal rearrangements due to low-copy repeats (LCRs) located inside this area. Specific breakpoints (BP1-BP5) that lead to deletions and duplications of variable size have been identified. Additionally, this specific region contains several imprinted genes, giving rise to complex syndromes (Prader–Willi, Angelman and 15q11-q13 duplication syndromes). 15q11.2-q13 duplication syndrome has been associated with neurodevelopmental disorders (hypotonia, developmental delay, speech delay and seizures) and ASD but is characterized by variable expressivity and reduced penetrance, features that make genetic counseling a complex procedure especially in prenatal cases. In the present study, a total of 14 pre- and postnatal cases were diagnosed as 15q11.2q13 duplication carriers using Affymetrix CytoScan 750 K array-CGH, and our analysis combined these with 120 cases existing in the literature. The inheritance pattern of the cases of this study is unknown, but as a review of the literature revealed, 62.96% of the affected carriers inherited the duplicated area from their mother. The combined results of this analysis (the present study and the literature) show that in the majority of the cases, the phenotype is a compound phenotype, with clinical characteristics that include ASD, intellectual disability, developmental delay and an absence of speech. The aim of this paper is to deliver new possibilities to genetic counseling that can be provided in prenatal and postnatal cases as the phenotype of 15q11.2q13 microduplication carriers cannot be fully predicted; so, clinical diagnoses should be a combination of molecular findings and clinical manifestations that are present.

This review discusses the discovery, epidemiology, pathophysiology, genetic etiology, molecular diagnosis, and medication-based management of fragile X syndrome (FXS). It also highlights the syndrome’s variable expressivity and common comorbid and overlapping conditions. FXS is an X-linked dominant disorder associated with a wide spectrum of clinical features, including but not limited to intellectual disability, autism spectrum disorder, language deficits, macroorchidism, seizures, and anxiety. Its prevalence in the general population is approximately 1 in 5000–7000 men and 1 in 4000–6000 women worldwide. FXS is associated with the  fragile X messenger ribonucleoprotein 1  ( FMR1 ) gene located at locus Xq27.3 and encodes the fragile X messenger ribonucleoprotein (FMRP). Most individuals with FXS have an FMR1 allele with > 200 CGG repeats (full mutation) and hypermethylation of the CpG island proximal to the repeats, which silences the gene’s promoter. Some individuals have mosaicism in the size of the CGG repeats or in hypermethylation of the CpG island, both produce some FMRP and give rise to milder cognitive and behavioral deficits than in non-mosaic individuals with FXS. As in several monogenic disorders, modifier genes influence the penetrance of  FMR1  mutations and FXS’s variable expressivity by regulating the pathophysiological mechanisms related to the syndrome’s behavioral features. Although there is no cure for FXS, prenatal molecular diagnostic testing is recommended to facilitate early diagnosis. Pharmacologic agents can reduce some behavioral features of FXS, and researchers are investigating whether gene editing can be used to demethylate the FMR1 promoter region to improve patient outcomes. Moreover, clustered regularly interspaced palindromic repeats (CRISPR)/Cas9 and developed nuclease defective Cas9 (dCas9) strategies have promised options of genome editing in gain-of-function mutations to rewrite new genetic information into a specified DNA site, are also being studied.

Sudden cardiac death is defined as an unexpected decease of cardiac origin. In individuals under 35 years old, most of these deaths are due to familial arrhythmogenic syndromes of genetic origin, also known as channelopathies. These familial cardiac syndromes commonly follow an autosomal dominant pattern of inheritance. Diagnosis, however, can be difficult, mainly due to incomplete penetrance and variable expressivity, which are hallmarks in these syndromes. The clinical manifestation of these diseases can range from asymptomatic to syncope but sudden death can sometimes be the first symptom of disease. Early identification of at-risk individuals is crucial to prevent a lethal episode. In this review, we will focus on the genetic basis of channelopathies and the effect of genetic and non-genetic modifiers on their phenotypes.

ABSTRACT X‐linked sideroblastic anaemia (XLSA) is a rare hereditary disorder caused by mutations in the ALAS2 gene, essential for haem biosynthesis. We report two male siblings, the first of whom developed severe microcytic hypochromic anaemia requiring regular transfusions, iron chelation and an allogeneic bone marrow transplant, while his brother displayed only mild microcytic hypochromic indices without anaemia. Initial genetic screening did not identify a pathogenic variant. However, duo exome sequencing later revealed an intronic ALAS2 mutation, initially categorised as of uncertain significance and subsequently reclassified as pathogenic. This case underscores the diagnostic challenges posed by intronic mutations and the highly variable expressivity of XLSA, even among siblings. Trial Registration: The authors have confirmed clinical trial registration is not needed for this submission.

Background/Objectives: Cantú syndrome is a rare autosomal dominant genetic disorder caused by gain-of-function variants in the ABCC9 or KCNJ8 genes. Although its phenotypic expression is variable and can go unnoticed postnatally, certain ultrasound findings may raise suspicion during pregnancy. This article presents a case of prenatal diagnosis through exome sequencing, which also enabled retrospective diagnosis in the mother and a previously undiagnosed child, highlighting the clinical and emotional value of diagnostic certainty in fetal medicine. Methods: We conducted a descriptive observational study based on a case identified at the Fetal Medicine Unit of the Regional University Hospital of Málaga. The patient underwent high-resolution ultrasound and trio-based exome sequencing (fetus and both parents). Results: Prenatal exome sequencing revealed a heterozygous pathogenic variant in ABCC9, consistent with Cantú syndrome, identified simultaneously in the fetus and the mother as part of a trio-based analysis, confirming maternal inheritance. The same variant was later detected in the patient’s older daughter, who had been under pediatric evaluation for a suggestive phenotype but had not received a genetic diagnosis until this study. The prenatal diagnosis allowed for obstetric and neonatal planning, genetic counselling, and a reinterpretation of the clinical and emotional meaning of previous pregnancies. Conclusions: Prenatal diagnosis of Cantú syndrome enables anticipation of perinatal complications, planned clinical interventions, and also provides emotional relief and a coherent narrative for families. In scenarios of variable phenotypic expressivity, fetal medicine may represent a gateway to family diagnosis, with significant clinical and psychosocial implications.