SPEN haploinsufficiency causes a neurodevelopmental disorder overlapping proximal 1p36 deletion syndrome with an episignature of X chromosomes in females.

Deletion 1p36 (del1p36) syndrome is the most common human disorder resulting from a terminal autosomal deletion. This condition is molecularly and clinically heterogeneous. Deletions involving two non-overlapping regions, known as the distal (telomeric) and proximal (centromeric) critical regions, are sufficient to cause the majority of the recurrent clinical features, although with different facial features and dysmorphisms. SPEN encodes a transcriptional repressor commonly deleted in proximal del1p36 syndrome and is located centromeric to the proximal 1p36 critical region. Here, we used clinical data from 34 individuals with truncating variants in SPEN to define a neurodevelopmental disorder presenting with features that overlap considerably with those of proximal del1p36 syndrome. The clinical profile of this disease includes developmental delay/intellectual disability, autism spectrum disorder, anxiety, aggressive behavior, attention deficit disorder, hypotonia, brain and spine anomalies, congenital heart defects, high/narrow palate, facial dysmorphisms, and obesity/increased BMI, especially in females. SPEN also emerges as a relevant gene for del1p36 syndrome by co-expression analyses. Finally, we show that haploinsufficiency of SPEN is associated with a distinctive DNA methylation episignature of the X chromosome in affected females, providing further evidence of a specific contribution of the protein to the epigenetic control of this chromosome, and a paradigm of an X chromosome-specific episignature that classifies syndromic traits. We conclude that SPEN is required for multiple developmental processes and SPEN haploinsufficiency is a major contributor to a disorder associated with deletions centromeric to the previously established 1p36 critical regions.

Recurrent Xp22.31-Yq11 Unbalanced Translocations: Molecular Diagnosis and Clinical Implications in Three Families.

Unbalanced translocation between chromosomes X and Y is a recurring chromosomal rearrangement. The presence of a derivative chromosome X (derX), where a Yq11-qter segment is attached to the short arm of chromosome X, replacing a terminal Xpter-p22.31, poses challenges for interpretation of findings by prenatal cell-free DNA (cfDNA) screening, establishing genotype-phenotype correlation in male and female individuals, and for genetic counseling. In this report, we provide clinical outcomes, inheritance, and clinical implications of derX in three families referred to diagnostic testing due to discrepant results for sex chromosomes reported by cfDNA, abnormal prenatal ultrasound findings, recurrent pregnancy losses, or affected family members with derX transmitted through multiple generations. Reports of discrepant sex and risk for sex chromosome aneuploidy such as 45,X, 47,XXY and 47,XYY are common false positive outcomes of a prenatal cfDNA screening if either a mother or a fetus has unbalanced Xp-Yq translocation. In addition, mothers who carry der(X) facing a recurrent risk of ambiguity in prenatal testing. Pregnancy loss and neonatal death/stillbirth of male offspring are common in affected families, but this risk does not directly correlate with the size of deleted Xp region. This study emphasizes the importance of CMA and familial testing for accurate diagnosis and genetic counseling.

The clinical and genetic spectrum of autosomal-recessive TOR1A-related disorders.

In the field of rare diseases, progress in molecular diagnostics led to the recognition that variants linked to autosomal-dominant neurodegenerative diseases of later onset can, in the context of biallelic inheritance, cause devastating neurodevelopmental disorders and infantile or childhood-onset neurodegeneration. TOR1A-associated arthrogryposis multiplex congenita 5 (AMC5) is a rare neurodevelopmental disorder arising from biallelic variants in TOR1A, a gene that in the heterozygous state is associated with torsion dystonia-1 (DYT1 or DYT-TOR1A), an early-onset dystonia with reduced penetrance. While 15 individuals with AMC5-TOR1A have been reported (less than 10 in detail), a systematic investigation of the full disease-associated spectrum has not been conducted. Here, we assess the clinical, radiological and molecular characteristics of 57 individuals from 40 families with biallelic variants in TOR1A. Median age at last follow-up was 3 years (0-24 years). Most individuals presented with severe congenital flexion contractures (95%) and variable developmental delay (79%). Motor symptoms were reported in 79% and included lower limb spasticity and pyramidal signs, as well as gait disturbances. Facial dysmorphism was an integral part of the phenotype, with key features being a broad/full nasal tip, narrowing of the forehead and full cheeks. Analysis of disease-associated manifestations delineated a phenotypic spectrum ranging from normal cognition and mild gait disturbance to congenital arthrogryposis, global developmental delay, intellectual disability, absent speech and inability to walk. In a subset, the presentation was consistent with foetal akinesia deformation sequence with severe intrauterine abnormalities. Survival was 71%, with higher mortality in males. Death occurred at a median age of 1.2 months (1 week-9 years), due to respiratory failure, cardiac arrest or sepsis. Analysis of brain MRI studies identified non-specific neuroimaging features, including a hypoplastic corpus callosum (72%), foci of signal abnormality in the subcortical and periventricular white matter (55%), diffuse white matter volume loss (45%), mega cisterna magna (36%) and arachnoid cysts (27%). The molecular spectrum included 22 distinct variants, defining a mutational hotspot in the C-terminal domain of the Torsin-1A protein. Genotype-phenotype analysis revealed an association of missense variants in the 3-helix bundle domain to an attenuated phenotype, while missense variants near the Walker A/B motif as well as biallelic truncating variants were linked to early death. In summary, this systematic cross-sectional analysis of a large cohort of individuals with biallelic TOR1A variants across a wide age-range delineates the clinical and genetic spectrum of TOR1A-related autosomal-recessive disease and highlights potential predictors for disease severity and survival.

Neurosurgical Management of Rubinstein-Taybi Syndrome: An Institutional Experience.

INTRODUCTION: Rubinstein-Taybi syndrome (RTS) is a rare genetic condition with a distinctive set of physical features. This case series reports a single institutional experience of RTS cases, highlighting the role of neurosurgery in the comprehensive management of RTS patients. METHODS: A retrospective review of patients with genetically confirmed RTS presenting between 2010 and 2023 at Children's Hospital of Pittsburgh was performed. Patient demographics, genetic profile, clinical symptoms, radiographic characteristics, and neurosurgical management were recorded for all patients. RESULTS: Twenty-one patients (13 females, 8 males) aged 0 to 22 years presented for formal genetic counseling and diagnosis. Twenty patients (95%) had CREBBP pathogenic variants (RTS type 1), and 1 patient (5%) had EP300 pathogenic variants (RTS type 2). Ten patients (48%) had a low-lying conus medullaris, and 3 patients (30%) underwent subsequent spinal cord detethering. Four patients (19%) had a Chiari malformation, and three (75%) underwent Chiari decompression surgeries. One patient (5%) had Chiari-associated syringomyelia. CONCLUSIONS: RTS patients have an increased rate of tethered cord syndrome requiring detethering. The incidence of symptomatic Chiari I malformation requiring decompression has not been previously reported. The RTS series presented here demonstrates a high incidence of symptomatic Chiari I malformation in addition to tethered cord syndrome.

A comparative analysis of RAS variants in patients with disorders of somatic mosaicism.

PURPOSE: RAS genes (HRAS, KRAS, and NRAS) are commonly found to be mutated in cancers, and activating RAS variants are also found in disorders of somatic mosaicism (DoSM). A survey of the mutational spectrum of RAS variants in DoSM has not been performed. METHODS: A total of 938 individuals with suspected DoSM underwent high-sensitivity clinical next-generation sequencing-based testing. We investigated the mutational spectrum and genotype-phenotype associations of mosaic RAS variants. RESULTS: In this article, we present a series of individuals with DoSM with RAS variants. Classic hotspots, including Gly12, Gly13, and Gln61 constituted the majority of RAS variants observed in DoSM. Furthermore, we present 12 individuals with HRAS and KRAS in-frame duplication/insertion (dup/ins) variants in the switch II domain. Among the 18.3% individuals with RAS in-frame dup/ins variants, clinical findings were mainly associated with vascular malformations. Hotspots were associated with a broad phenotypic spectrum, including vascular tumors, vascular malformations, nevoid proliferations, segmental overgrowth, digital anomalies, and combinations of these. The median age at testing was higher and the variant allelic fraction was lower in individuals with in-frame dup/ins variants than those in individuals with mosaic RAS hotspots. CONCLUSION: Our work provides insight into the allelic and clinical heterogeneity of mosaic RAS variants in nonmalignant conditions.

Expanding the Spectrum of BAF-Related Disorders: De Novo Variants in SMARCC2 Cause a Syndrome with Intellectual Disability and Developmental Delay.

SMARCC2 (BAF170) is one of the invariable core subunits of the ATP-dependent chromatin remodeling BAF (BRG1-associated factor) complex and plays a crucial role in embryogenesis and corticogenesis. Pathogenic variants in genes encoding other components of the BAF complex have been associated with intellectual disability syndromes. Despite its significant biological role, variants in SMARCC2 have not been directly associated with human disease previously. Using whole-exome sequencing and a web-based gene-matching program, we identified 15 individuals with variable degrees of neurodevelopmental delay and growth retardation harboring one of 13 heterozygous variants in SMARCC2, most of them novel and proven de novo. The clinical presentation overlaps with intellectual disability syndromes associated with other BAF subunits, such as Coffin-Siris and Nicolaides-Baraitser syndromes and includes prominent speech impairment, hypotonia, feeding difficulties, behavioral abnormalities, and dysmorphic features such as hypertrichosis, thick eyebrows, thin upper lip vermilion, and upturned nose. Nine out of the fifteen individuals harbor variants in the highly conserved SMARCC2 DNA-interacting domains (SANT and SWIRM) and present with a more severe phenotype. Two of these individuals present cardiac abnormalities. Transcriptomic analysis of fibroblasts from affected individuals highlights a group of differentially expressed genes with possible roles in regulation of neuronal development and function, namely H19, SCRG1, RELN, and CACNB4. Our findings suggest a novel SMARCC2-related syndrome that overlaps with neurodevelopmental disorders associated with variants in BAF-complex subunits.

Retrospective analysis of a clinical exome sequencing cohort reveals the mutational spectrum and identifies candidate disease-associated loci for BAFopathies.

PURPOSE: BRG1/BRM-associated factor (BAF) complex is a chromatin remodeling complex that plays a critical role in gene regulation. Defects in the genes encoding BAF subunits lead to BAFopathies, a group of neurodevelopmental disorders with extensive locus and phenotypic heterogeneity. METHODS: We retrospectively analyzed data from 16,243 patients referred for clinical exome sequencing (ES) with a focus on the BAF complex. We applied a genotype-first approach, combining predicted genic constraints to propose candidate BAFopathy genes. RESULTS: We identified 127 patients carrying pathogenic variants, likely pathogenic variants, or de novo variants of unknown clinical significance in 11 known BAFopathy genes. Those include 34 patients molecularly diagnosed using ES reanalysis with new gene-disease evidence (n = 21) or variant reclassifications in known BAFopathy genes (n = 13). We also identified de novo or predicted loss-of-function variants in 4 candidate BAFopathy genes, including ACTL6A, BICRA (implicated in Coffin-Siris syndrome during this study), PBRM1, and SMARCC1. CONCLUSION: We report the mutational spectrum of BAFopathies in an ES cohort. A genotype-driven and pathway-based reanalysis of ES data identified new evidence for candidate genes involved in BAFopathies. Further mechanistic and phenotypic characterization of additional patients are warranted to confirm their roles in human disease and to delineate their associated phenotypic spectrums.

A homozygous CAP2 pathogenic variant in a neonate presenting with rapidly progressive cardiomyopathy and nemaline rods.

Nemaline Myopathy (NM) is a disorder of skeletal muscles caused by mutations in sarcomere proteins and characterized by accumulation of microscopic rod or thread-like structures (nemaline bodies) in skeletal muscles. Patients diagnosed with both NM and infantile cardiomyopathy are very rare. A male infant presented, within the first few hours of life, with severe dilated cardiomyopathy, biventricular dysfunction and left ventricular noncompaction. A muscle biopsy on the 8th day of life from the right sternocleidomastoid muscle identified nemaline rods. Whole exome sequencing identified a c.1288 delT (homozygous pathogenic variant) in the CAP2 gene (NM_006366), yielding a CAP2 protein (NP_006357.1) with a p.C430fs. Both parents were heterozygous for the same variant but have no history of heart or muscle disease. Analysis of patient derived fibroblasts and cardiomyocytes derived from induced pluripotent stem cells confirmed the p.C430fs mutation (pathogenic variant), which appears to cause loss of both CAP2 protein and mRNA. The CAP2 gene encodes cyclase associated protein 2, an actin monomer binding and filament depolymerizing protein and CAP2 knockout mice develop severe dilated cardiomyopathy and muscle weakness. The patient underwent a heart transplant at 1 year of age. Heart tissue explanted at that time also showed nemaline rods and additionally disintegration of the myofibrillar structure. Other extra cardiac concerns include mild hypotonia, atrophic and widened scarring. This is the first description of a patient presenting with nemaline myopathy associated with a pathogenic variant of CAP2.

CSNK2B: A broad spectrum of neurodevelopmental disability and epilepsy severity.

CSNK2B has recently been implicated as a disease gene for neurodevelopmental disability (NDD) and epilepsy. Information about developmental outcomes has been limited by the young age and short follow-up for many of the previously reported cases, and further delineation of the spectrum of associated phenotypes is needed. We present 25 new patients with variants in CSNK2B and refine the associated NDD and epilepsy phenotypes. CSNK2B variants were identified by research or clinical exome sequencing, and investigators from different centers were connected via GeneMatcher. Most individuals had developmental delay and generalized epilepsy with onset in the first 2 years. However, we found a broad spectrum of phenotypic severity, ranging from early normal development with pharmacoresponsive seizures to profound intellectual disability with intractable epilepsy and recurrent refractory status epilepticus. These findings suggest that CSNK2B should be considered in the diagnostic evaluation of patients with a broad range of NDD with treatable or intractable seizures.

Phenotypic expansion in KIF1A-related dominant disorders: A description of novel variants and review of published cases.

KIF1A is a molecular motor for membrane-bound cargo important to the development and survival of sensory neurons. KIF1A dysfunction has been associated with several Mendelian disorders with a spectrum of overlapping phenotypes, ranging from spastic paraplegia to intellectual disability. We present a novel pathogenic in-frame deletion in the KIF1A molecular motor domain inherited by two affected siblings from an unaffected mother with apparent germline mosaicism. We identified eight additional cases with heterozygous, pathogenic KIF1A variants ascertained from a local data lake. Our data provide evidence for the expansion of KIF1A-associated phenotypes to include hip subluxation and dystonia as well as phenotypes observed in only a single case: gelastic cataplexy, coxa valga, and double collecting system. We review the literature and suggest that KIF1A dysfunction is better understood as a single neuromuscular disorder with variable involvement of other organ systems than a set of discrete disorders converging at a single locus.

Correction: DYRK1A-related intellectual disability: a syndrome associated with congenital anomalies of the kidney and urinary tract.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Copy number alterations involving 59 ACMG-recommended secondary findings genes.

In clinical exome/genome sequencing, the American College of Medical Genetics and Genomics (ACMG) recommends reporting of secondary findings unrelated to a patient's phenotype when pathogenic single-nucleotide variants (SNVs) are observed in one of 59 genes associated with a life-threatening, medically actionable condition. Little is known about the incidence and sensitivity of chromosomal microarray analysis (CMA) for detection of pathogenic copy number variants (CNVs) comprising medically-actionable genes. Clinical CMA has been performed on 8865 individuals referred for molecular cytogenetic testing. We retrospectively reviewed the CMA results to identify patients with CNVs comprising genes included in the 59-ACMG list of secondary findings. We evaluated the clinical significance of these CNVs in respect to pathogenicity, phenotypic manifestations, and heritability. We identified 23 patients (0.26%) with relevant CNV either deletions comprising the entire gene or intragenic alterations involving one or more secondary findings genes. A number of patients and/or their family members with pathogenic CNVs manifest or expected to develop an anticipated clinical phenotype and would benefit from preventive management similar to the patients with pathogenic SNVs. To improve patients' care standardization should apply to reporting of both sequencing and CNVs obtained via clinical genome-wide analysis, including chromosomal microarray and exome/genome sequencing.

Four children with postnatally diagnosed mosaic trisomy 12: Clinical features, literature review, and current diagnostic capabilities of genetic testing.

Children or adults with mosaic trisomy 12 diagnosed postnatally are extremely rare. Only a small number of patients with this mosaicism have been reported in the literature. The clinical manifestation of mosaic trisomy 12 is variable, ranging from mild developmental delay to severe congenital anomaly and neonatal death. The trisomy 12 cells are not usually able to be detected by phytohemagglutinin stimulated peripheral blood chromosome analysis. The variability of phenotypes and the limited number of patients with this anomaly pose a challenge to predict the clinical outcomes. In this study, we present the phenotypes and laboratory findings in four patients and review the 11 previously reported patients with mosaic trisomy 12 diagnosed postnatally, as well as 11 patients with mosaic trisomy 12 diagnosed prenatally. The findings of this study provide useful information for laboratory diagnosis and clinical management of these patients.

Phenotypic expansion of Bosch-Boonstra-Schaaf optic atrophy syndrome and further evidence for genotype-phenotype correlations.

Bosch-Boonstra-Schaaf Optic Atrophy Syndrome (BBSOAS) is an autosomal dominant neurodevelopmental disorder caused by loss-of-function variants in NR2F1 and characterized by visual impairment, developmental delay, and intellectual disability. Here we report 18 new cases, provide additional clinical information for 9 previously reported individuals, and review an additional 27 published cases to present a total of 54 patients. Among these are 22 individuals with point mutations or in-frame deletions in the DNA-binding domain (DBD), and 32 individuals with other types of variants including whole-gene deletions, nonsense and frameshift variants, and point mutations outside the DBD. We corroborate previously described clinical characteristics including developmental delay, intellectual disability, autism spectrum disorder diagnoses/features thereof, cognitive/behavioral anomalies, hypotonia, feeding difficulties, abnormal brain MRI findings, and seizures. We also confirm a vision phenotype that includes optic nerve hypoplasia, optic atrophy, and cortical visual impairment. Additionally, we expand the vision phenotype to include alacrima and manifest latent nystagmus (fusional maldevelopment), and we broaden the behavioral phenotypic spectrum to include a love of music, an unusually good long-term memory, sleep difficulties, a high pain tolerance, and touch sensitivity. Furthermore, we provide additional evidence for genotype-phenotype correlations, specifically supporting a more severe phenotype associated with DBD variants.

DYRK1A-related intellectual disability: a syndrome associated with congenital anomalies of the kidney and urinary tract.

PURPOSE: Haploinsufficiency of DYRK1A causes a recognizable clinical syndrome. The goal of this paper is to investigate congenital anomalies of the kidney and urinary tract (CAKUT) and genital defects (GD) in patients with DYRK1A variants. METHODS: A large database of clinical exome sequencing (ES) was queried for de novo DYRK1A variants and CAKUT/GD phenotypes were characterized. Xenopus laevis (frog) was chosen as a model organism to assess Dyrk1a's role in renal development. RESULTS: Phenotypic details and variants of 19 patients were compiled after an initial observation that one patient with a de novo pathogenic variant in DYRK1A had GD. CAKUT/GD data were available from 15 patients, 11 of whom presented with CAKUT/GD. Studies in Xenopus embryos demonstrated that knockdown of Dyrk1a, which is expressed in forming nephrons, disrupts the development of segments of embryonic nephrons, which ultimately give rise to the entire genitourinary (GU) tract. These defects could be rescued by coinjecting wild-type human DYRK1A RNA, but not with DYRK1AR205* or DYRK1AL245R RNA. CONCLUSION: Evidence supports routine GU screening of all individuals with de novo DYRK1A pathogenic variants to ensure optimized clinical management. Collectively, the reported clinical data and loss-of-function studies in Xenopus substantiate a novel role for DYRK1A in GU development.

Clinical exome sequencing reveals locus heterogeneity and phenotypic variability of cohesinopathies.

PURPOSE: Defects in the cohesin pathway are associated with cohesinopathies, notably Cornelia de Lange syndrome (CdLS). We aimed to delineate pathogenic variants in known and candidate cohesinopathy genes from a clinical exome perspective. METHODS: We retrospectively studied patients referred for clinical exome sequencing (CES, N = 10,698). Patients with causative variants in novel or recently described cohesinopathy genes were enrolled for phenotypic characterization. RESULTS: Pathogenic or likely pathogenic single-nucleotide and insertion/deletion variants (SNVs/indels) were identified in established disease genes including NIPBL (N = 5), SMC1A (N = 14), SMC3 (N = 4), RAD21 (N = 2), and HDAC8 (N = 8). The phenotypes in this genetically defined cohort skew towards the mild end of CdLS spectrum as compared with phenotype-driven cohorts. Candidate or recently reported cohesinopathy genes were supported by de novo SNVs/indels in STAG1 (N = 3), STAG2 (N = 5), PDS5A (N = 1), and WAPL (N = 1), and one inherited SNV in PDS5A. We also identified copy-number deletions affecting STAG1 (two de novo, one of unknown inheritance) and STAG2 (one of unknown inheritance). Patients with STAG1 and STAG2 variants presented with overlapping features yet without characteristic facial features of CdLS. CONCLUSION: CES effectively identified disease-causing alleles at the mild end of the cohensinopathy spectrum and enabled characterization of candidate disease genes.

De novo apparent loss-of-function mutations in PRR12 in three patients with intellectual disability and iris abnormalities.

PRR12 encodes a proline-rich protein nuclear factor suspected to be involved in neural development. Its nuclear expression in fetal brains and in the vision system supports its role in brain and eye development more specifically. However, its function and potential role in human disease has not been determined. Recently, a de novo t(10;19) (q22.3;q13.33) translocation disrupting the PRR12 gene was detected in a girl with intellectual disability and neuropsychiatric alterations. Here we report on three unrelated patients with heterozygous de novo apparent loss-of-function mutations in PRR12 detected by clinical whole exome sequencing: c.1918G>T (p.Glu640*), c.4502_4505delTGCC (p.Leu1501Argfs*146) and c.903_909dup (p.Pro304Thrfs*46). All three patients had global developmental delay, intellectual disability, eye and vision abnormalities, dysmorphic features, and neuropsychiatric problems. Eye abnormalities were consistent among the three patients and consisted of stellate iris pattern and iris coloboma. Additional variable clinical features included hypotonia, skeletal abnormalities, sleeping problems, and behavioral issues such as autism and anxiety. In summary, we propose that haploinsufficiency of PRR12 is associated with this novel multisystem neurodevelopmental disorder.

Kaufman oculocerebrofacial syndrome: Novel UBE3B mutations and clinical features in four unrelated patients.

The "blepharophimosis-mental retardation" syndromes (BMRS) consist of a group of clinically and genetically heterogeneous congenital malformation syndromes, where short palpebral fissures and intellectual disability associate with a distinct set of other morphological features. Kaufman oculocerebrofacial syndrome represents a rare and recently reevaluated entity within the BMR syndromes and is caused by biallelic mutations of UBE3B. Affected individuals typically show microcephaly, impaired somatic growth, gastrointestinal and genitourinary problems, ectodermal anomalies and a characteristic face with short, upslanted palpebral fissures, depressed nasal bridge. and anteverted nares. Here we present four patients with five novel UBE3B mutations and propose the inclusion of clinical features to the characteristics of Kaufman oculocerebrofacial syndrome, including prominence of the cheeks and limb anomalies.

Latent transforming growth factor binding protein 4 regulates transforming growth factor beta receptor stability.

Mutations in the gene for the latent transforming growth factor beta binding protein 4 (LTBP4) cause autosomal recessive cutis laxa type 1C. To understand the molecular disease mechanisms of this disease, we investigated the impact of LTBP4 loss on transforming growth factor beta (TGFß) signaling. Despite elevated extracellular TGFß activity, downstream signaling molecules of the TGFß pathway, including pSMAD2 and pERK, were down-regulated in LTBP4 mutant human dermal fibroblasts. In addition, TGFß receptors 1 and 2 (TGFBR1 and TGFBR2) were reduced at the protein but not at the ribonucleic acid level. Treatment with exogenous TGFß1 led to an initially rapid increase in SMAD2 phosphorylation followed by a sustained depression of phosphorylation and receptor abundance. In mutant cells TGFBR1 was co-localized with lysosomes. Treatment with a TGFBR1 kinase inhibitor, endocytosis inhibitors or a lysosome inhibitor, normalized the levels of TGFBR1 and TGFBR2. Co-immunoprecipitation demonstrated a molecular interaction between LTBP4 and TGFBR2. Knockdown of LTBP4 reduced TGFß receptor abundance and signaling in normal cells and supplementation of recombinant LTBP4 enhanced these measures in mutant cells. In a mouse model of Ltbp4 deficiency, reduced TGFß signaling and receptor levels were normalized upon TGFBR1 kinase inhibitor treatment. Our results show that LTBP4 interacts with TGFBR2 and stabilizes TGFß receptors by preventing their endocytosis and lysosomal degradation in a ligand-dependent and receptor kinase activity-dependent manner. These findings identify LTBP4 as a key molecule required for the stability of the TGFß receptor complex, and a new mechanism by which the extracellular matrix regulates cytokine receptor signaling.