Developmental and epileptic encephalopathy in two siblings with a novel,homozygous missense variant in SCN1B

Developmental and epileptic encephalopathies are genetic disorders in which both the developmental disability and the frequent epileptic activity are the effect of a specific gene variant. While heterozygous variants in SCN1B have been described in families with generalized epilepsy with febrile seizures plus, Type 1, only three cases of homozygous, missense variants in SCN1B have been reported in association with autosomal recessive inheritance of a severe developmental and epileptic encephalopathy. We present two siblings who are homozygous for a novel, missense variant in SCN1B, c.265C>T, predicting p.Arg89Cys. The proband is an 11‐year‐old female with infantile‐onset, fever‐induced, intractable generalized tonic–clonic seizures, myoclonic seizures, and developmental slowing and autism spectrum disorder occurring later in the course of the disease. Her 4‐year‐old brother had a similar epilepsy phenotype, but still displays normal development. This variant has not been previously reported in the homozygous state in control databases. The variant was predicted to be damaging and occurred in the vicinity of other epileptic encephalopathy‐associated missense variants that are biallelic and located in the extracellular immunoglobulin loop domain of the protein, which mediates interaction of the beta‐1 subunit with cellular adhesion molecules. Our report is the first set of siblings with homozygosity for the p.Arg89Cys variant in SCN1B and further implicates biallelic mutations in this gene as a cause of epileptic encephalopathy mimicking Dravet syndrome. Interestingly, the phenotype we observed was milder compared to that previously described in patients with recessive SCN1B mutations.     

TOPORS as a novel causal gene for Joubert syndrome

Joubert syndrome (JBTS) is a Mendelian disorder of the primary cilium defined by the clinical triad of hypotonia, developmental delay, and a distinct cerebellar malformation called the molar tooth sign. JBTS is inherited in an autosomal recessive, autosomal dominant, or X-linked recessive manner. Though over 40 genes have been identified as causal for JBTS, molecular diagnosis is not made in 30%–40% of individuals who meet clinical criteria. TOPORS encodes topoisomerase I-binding arginine/serine-rich protein, and homozygosity for a TOPORS missense variant (c.29C > A; p.(Pro10Gln)) was identified in individuals with the ciliopathy oral-facial-digital syndrome in two families of Dominican descent. Here, we report an additional proband of Dominican ancestry with JBTS found by exome sequencing to be homozygous for the identical p.(Pro10Gln) TOPORS missense variant. Query of the Mount Sinai BioMe biobank, which includes 1880 individuals of Dominican ancestry, supports a high carrier frequency of the TOPORS p.(Pro10Gln) variant in individuals of Dominican descent. Our data nominates TOPORS as a novel causal gene for JBTS and suggests that TOPORS variants should be considered in the differential of ciliopathy-spectrum disease in individuals of Dominican ancestry.     

Genome‐wide linkage and sequence analysis challenge CCDC66 as a human retinal dystrophy candidate gene and support a distinct NMNAT1‐related fundus phenotype

To uncover the genotype underlying early‐onset cone‐rod dystrophy and central nummular macular atrophic lesion in 2 siblings from an endogamous Arab family, we performed targeted next‐generation sequencing (NGS) of 44 retinal dystrophy genes, whole‐exome sequencing (WES) and genome‐wide linkage analysis. Targeted NGS and WES in the index patient highlighted 2 homozygous variants, a CCDC66 frameshift deletion and a novel missense NMNAT1 variant, c.500G>A (p.Asn167Ser). Linkage and segregation analysis excluded the CCDC66 variant and confirmed the NMNAT1 mutation. Biallelic NMNAT1 mutations cause Leber congenital amaurosis with a central nummular macular atrophic lesion (LCA9). The NMNAT1 mutation reported here underlied cone‐rod dystrophy rather than LCA but the fundus lesion was compatible with that of LCA9 patients, highlighting that such a fundus appearance should raise suspicion for biallelic mutations in NMNAT1 when in the context of any retinal dystrophy. Although Ccdc66 mutations have been proposed to cause retinal disease in dogs, our results and public databases challenge CCDC66 as a candidate gene for human retinal dystrophy.     

Pathogenic mutations cause rapid degradation of lysosomal storage disease‐related membrane protein CLN6

One variant form of late infantile neuronal ceroid lipofuscinosis is an autosomal recessive inherited neurodegenerative lysosomal storage disorder caused by mutations in the CLN6gene. The function of the polytopic CLN6 membrane protein localized in the endoplasmic reticulum is unknown. Here we report on expression studies of three mutations (c.368G>A, c.460‐462delATC, c.316insC) found in CLN6 patients predicted to affect transmembrane domain 3 (p.Gly123Asp), cytoplasmic loop 2 (p.Ile154del) or result in a truncated membrane protein (p.Arg106ProfsX26), respectively. The rate of synthesis and the stability of the mutant CLN6 proteins are reduced in a mutation‐dependent manner. None of the mutations prevented the dimerization of the CLN6 polypeptides. The particularly rapid degradation of the p.Arg106ProfsX26 mutant which is identical with the mutation in the murine orthologue Cln6 gene in the nclf mouse model of the disease, can be strongly inhibited by proteasomal and partially by lysosomal protease inhibitors. Both degradative pathways seem to be sufficient to prevent the accumulation/aggregation of the mutant CLN6 polypeptides in the endoplasmic reticulum. © 2009 Wiley‐Liss, Inc.     

Screening of a Large Cohort of Leber Congenital Amaurosis and Retinitis Pigmentosa Patients Identifies Novel LCA5 Mutations and New Genotype–Phenotype Correlations

This study was undertaken to investigate the prevalence of sequence variants in LCA5 in patients with Leber congenital amaurosis (LCA), early‐onset retinal dystrophy (EORD), and autosomal recessive retinitis pigmentosa (arRP); to delineate the ocular phenotypes; and to provide an overview of all published LCA5 variants in an online database. Patients underwent standard ophthalmic evaluations after providing informed consent. In selected patients, optical coherence tomography (OCT) and fundus autofluorescence imaging were possible. DNA samples from 797 unrelated patients with LCA and 211 with the various types of retinitis pigmentosa (RP) were screened by Sanger sequence analysis of all LCA5 exons and intron/exon junctions. Some LCA patients were prescreened by APEX technology or selected based on homozygosity mapping. In silico analyses were performed to assess the pathogenicity of the variants. Segregation analysis was performed where possible. Published and novel LCA5 variants were collected, amended for their correct nomenclature, and listed in a Leiden Open Variation Database (LOVD). Sequence analysis identified 18 new probands with 19 different LCA5 variants. Seventeen of the 19 LCA5 variants were novel. Except for two missense variants and one splice site variant, all variants were protein‐truncating mutations. Most patients expressed a severe phenotype, typical of LCA. However, some LCA subjects had better vision and intact inner segment/outer segment (IS/OS) junctions on OCT imaging. In two families with LCA5 variants, the phenotype was more compatible with EORD with affected individuals displaying preserved islands of retinal pigment epithelium. One of the families with a milder phenotype harbored a homozygous splice site mutation; a second family was found to have a combination of a stop mutation and a missense mutation. This is the largest LCA5 study to date. We sequenced 1,008 patients (797 with LCA, 211 with arRP) and identified 18 probands with LCA5 mutations. Mutations in LCA5 are a rare cause of childhood retinal dystrophy accounting for ～2% of disease in this cohort, and the majority of LCA5 mutations are likely null. The LCA5 protein truncating mutations are predominantly associated with LCA. However, in two families with the milder EORD, the LCA5 gene analysis revealed a homozygous splice site mutation in one and a stop mutation in combination with a missense mutation in a second family, suggesting that this milder phenotype is due to residual function of lebercilin and expanding the currently known phenotypic spectrum to include the milder early onset RP. Some patients have remaining foveal cone structures (intact IS/OS junctions on OCT imaging) and remaining visual acuities, which may bode well for upcoming treatment trials.     

Hidden Genetic Variation in LCA9‐Associated Congenital Blindness Explained by 5′UTR Mutations and Copy‐Number Variations of NMNAT1

Leber congenital amaurosis (LCA) is a severe autosomal‐recessive retinal dystrophy leading to congenital blindness. A recently identified LCA gene is NMNAT1, located in the LCA9 locus. Although most mutations in blindness genes are coding variations, there is accumulating evidence for hidden noncoding defects or structural variations (SVs). The starting point of this study was an LCA9‐associated consanguineous family in which no coding mutations were found in the LCA9 region. Exploring the untranslated regions of NMNAT1 revealed a novel homozygous 5′UTR variant, c.‐70A>T. Moreover, an adjacent 5′UTR variant, c.‐69C>T, was identified in a second consanguineous family displaying a similar phenotype. Both 5′UTR variants resulted in decreased NMNAT1 mRNA abundance in patients’ lymphocytes, and caused decreased luciferase activity in human retinal pigment epithelial RPE‐1 cells. Second, we unraveled pseudohomozygosity of a coding NMNAT1 mutation in two unrelated LCA patients by the identification of two distinct heterozygous partial NMNAT1 deletions. Molecular characterization of the breakpoint junctions revealed a complex Alu‐rich genomic architecture. Our study uncovered hidden genetic variation in NMNAT1‐associated LCA and emphasized a shift from coding to noncoding regulatory mutations and repeat‐mediated SVs in the molecular pathogenesis of heterogeneous recessive disorders such as hereditary blindness.     

The Sac1 Domain of SYNJ1 Identified Mutated in a Family with Early‐Onset Progressive Parkinsonism with Generalized Seizures

This study aimed to elucidate the genetic causes underlying early‐onset Parkinsonism (EOP) in a consanguineous Iranian family. To attain this, homozygosity mapping and whole‐exome sequencing were performed. As a result, a homozygous mutation (c.773G>A; p.Arg258Gln) lying within the NH₂‐terminal Sac1‐like inositol phosphatase domain of polyphosphoinositide phosphatase synaptojanin 1 (SYNJ1), which has been implicated in the regulation of endocytic traffic at synapses, was identified as the disease‐segregating mutation. This mutation impaired the phosphatase activity of SYNJ1 against its Sac1 domain substrates in vitro. We concluded that the SYNJ1 mutation identified here is responsible for the EOP phenotype seen in our patients probably due to deficiencies in its phosphatase activity and consequent impairment of its synaptic functions. Our finding not only opens new avenues of investigation in the synaptic dysfunction mechanisms associated with Parkinsonism, but also suggests phosphoinositide metabolism as a novel therapeutic target for Parkinsonism.     

RP1L1 Variants are Associated with a Spectrum of Inherited Retinal Diseases Including Retinitis Pigmentosa and Occult Macular Dystrophy

In one consanguineous family with retinitis pigmentosa (RP), a condition characterized by progressive visual loss due to retinal degeneration, homozygosity mapping, and candidate gene sequencing suggested a novel locus. Exome sequencing identified a homozygous frameshifting mutation, c.601delG, p.Lys203Argfs*28, in RP1L1 encoding RP 1‐like1, a photoreceptor‐specific protein. A screen of a further 285 unrelated individuals with autosomal recessive RP identified an additional proband, homozygous for a missense variant, c.1637G>C, p.Ser546Thr, in RP1L1. A distinct retinal disorder, occult macular dystrophy (OCMD) solely affects the central retinal cone photoreceptors and has previously been reported to be associated with variants in the same gene. The association between mutations in RP1L1 and the disorder OCMD was explored by screening a cohort of 28 unrelated individuals with the condition; 10 were found to harbor rare (minor allele frequency ≤0.5% in the 1,000 genomes dataset) heterozygous RP1L1 missense variants. Analysis of family members revealed many unaffected relatives harboring the same variant. Linkage analysis excluded the possibility of a recessive mode of inheritance, and sequencing of RP1, a photoreceptor protein that interacts with RP1L1, excluded a digenic mechanism involving this gene. These findings imply an important and diverse role for RP1L1 in human retinal physiology and disease.     

Genetic screening of LCA in Belgium: predominance of CEP290 and identification of potential modifier alleles in AHI1 of CEP290‐related phenotypes

Leber Congenital Amaurosis (LCA), the most severe inherited retinal dystrophy, is genetically heterogeneous, with 14 genes accounting for 70% of patients. Here, 91 LCA probands underwent LCA chip analysis and subsequent sequencing of 6 genes (CEP290, CRB1, RPE65, GUCY2D, AIPL1and CRX), revealing mutations in 69% of the cohort, with major involvement of CEP290 (30%). In addition, 11 patients with early‐onset retinal dystrophy (EORD) and 13 patients with Senior‐Loken syndrome (SLS), LCA‐Joubert syndrome (LCA‐JS) or cerebello‐oculo‐renal syndrome (CORS) were included. Exhaustive re‐inspection of the overall phenotypes in our LCA cohort revealed novel insights mainly regarding the CEP290‐related phenotype. The AHI1 gene was screened as a candidate modifier gene in three patients with the same CEP290 genotype but different neurological involvement. Interestingly, a heterozygous novel AHI1 mutation, p.Asn811Lys, was found in the most severely affected patient. Moreover, AHI1 screening in five other patients with CEP290‐related disease and neurological involvement revealed a second novel missense variant, p.His758Pro, in one LCA patient with mild mental retardation and autism. These two AHI1 mutations might thus represent neurological modifiers of CEP290‐related disease. © 2010 Wiley‐Liss, Inc.     

Mutation in the SYNJ1 Gene Associated with Autosomal Recessive,Early‐Onset Parkinsonism

Autosomal recessive, early‐onset Parkinsonism is clinically and genetically heterogeneous. Here, we report the identification, by homozygosity mapping and exome sequencing, of a SYNJ1 homozygous mutation (p.Arg258Gln) segregating with disease in an Italian consanguineous family with Parkinsonism, dystonia, and cognitive deterioration. Response to levodopa was poor, and limited by side effects. Neuroimaging revealed brain atrophy, nigrostriatal dopaminergic defects, and cerebral hypometabolism. SYNJ1 encodes synaptojanin 1, a phosphoinositide phosphatase protein with essential roles in the postendocytic recycling of synaptic vesicles. The mutation is absent in variation databases and in ethnically matched controls, is damaging according to all prediction programs, and replaces an amino acid that is extremely conserved in the synaptojanin 1 homologues and in SAC1‐like domains of other proteins. Sequencing the SYNJ1 ORF in unrelated patients revealed another heterozygous mutation (p.Ser1422Arg), predicted as damaging, in a patient who also carries a heterozygous PINK1 truncating mutation. The SYNJ1 gene is a compelling candidate for Parkinsonism; mutations in the functionally linked protein auxilin cause a similar early‐onset phenotype, and other findings implicate endosomal dysfunctions in the pathogenesis. Our data delineate a novel form of human Mendelian Parkinsonism, and provide further evidence for abnormal synaptic vesicle recycling as a central theme in the pathogenesis.     

SOFT syndrome in a patient from Chile

SOFT syndrome (MIM614813) is an extremely rare primordial dwarfism caused by biallelic mutations in the POC1A gene. It is characterized by prenatal short stature, onychodysplasia, facial dysmorphism, hypotrichosis, and variable skeletal abnormalities including hypoplastic pelvis and sacrum, small hands, and cone‐shaped epiphyses, as well as delayed bone age. To the best of our knowledge, only eight POC1A mutations have been reported in humans to date. We report a 7‐year‐old Chilean girl with SOFT syndrome arising from a novel POC1A mutation c. 649C>T, p.Arg217Trp. Although her clinical features were largely compatible with SOFT syndrome, hand X‐ray examinations at 3.5 and 6 years unexpectedly showed normal bone age. Automated bone age determination was performed using image analysis software, BoneXpert. This case highlights the importance of the accumulation of patients with POC1A mutations to further elucidate the detailed clinical features of SOFT syndrome.     

Mutations in the COPII Vesicle Component Gene SEC24B are Associated with Human Neural Tube Defects

Neural tube defects (NTDs) are severe birth malformations that affect one in 1,000 live births. Recently, mutations in the planar cell polarity (PCP) pathway genes had been implicated in the pathogenesis of NTDs in both the mouse model and in human cohorts. Mouse models indicate that the homozygous disruption of Sec24b, which mediates the ER‐to‐Golgi transportation of the core PCP gene Vangl2 as a component of the COPII vesicle, will result in craniorachischisis. In this study, we found four rare missense heterozygous SEC24B mutations (p.Phe227Ser, p.Phe682Leu, p.Arg1248Gln, and p.Ala1251Gly) in NTDs cases that were absent in all controls. Among them, p.Phe227Ser and p.Phe682Leu affected its protein stability and physical interaction with VANGL2. Three variants (p.Phe227Ser, p.Arg1248Gln, and p.Ala1251Gly) were demonstrated to affect VANGL2 subcellular localization in cultured cells. Further functional analysis in the zebrafish including overexpression and dosage‐dependent rescue study suggested that these four mutations all displayed loss‐of‐function effects compared with wild‐type SEC24B. Our study demonstrated that functional mutations in SEC24B might contribute to the etiology of a subset of human NTDs and further expanded our knowledge of the role of PCP pathway‐related genes in the pathogenesis of human NTDs.     

Clinical and molecular findings in a cohort of 152 Brazilian severe early onset inherited retinal dystrophy patients

Leber congenital amaurosis (LCA) and early‐onset retinal dystrophy (EORD) are severe inherited retinal dystrophy that can cause deep blindness childhood. They represent 5% of all retinal dystrophies in the world population and about 10% in Brazil. Clinical findings and molecular basis of syndromic and nonsyndromic LCA/EORD in a Brazilian sample (152 patients/137 families) were studied. In this population, 15 genes were found to be related to the phenotype, 38 new variants were detected and four new complex alleles were discovered. Among 123 variants found, the most common were CEP290: c.2991+1655A>G, CRB1: p.Cys948Tyr, and RPGRIP1: exon10‐18 deletion.     

PEX6 is Expressed in Photoreceptor Cilia and Mutated in Deafblindness with Enamel Dysplasia and Microcephaly

Deafblindness is part of several genetic disorders. We investigated a consanguineous Egyptian family with two siblings affected by congenital hearing loss and retinal degeneration, initially diagnosed as Usher syndrome type 1. At teenage, severe enamel dysplasia, developmental delay, and microcephaly became apparent. Genome‐wide homozygosity mapping and whole‐exome sequencing detected a homozygous missense mutation, c.1238G>T (p.Gly413Val), affecting a highly conserved residue of peroxisomal biogenesis factor 6, PEX6. Biochemical profiling of the siblings revealed abnormal and borderline plasma phytanic acid concentration, and cerebral imaging revealed white matter disease in both. We show that Pex6 localizes to the apical extensions of secretory ameloblasts and differentiated odontoblasts at early stages of dentin synthesis in mice, and to cilia of retinal photoreceptor cells. We propose PEX6, and possibly other peroxisomal genes, as candidate for the rare cooccurrence of deafblindness and enamel dysplasia. Our study for the first time links peroxisome biogenesis disorders to retinal ciliopathies.     

ALMS1 null mutations: a common cause of Leber congenital amaurosis and early‐onset severe cone–rod dystrophy

In our previous studies, mutations in known candidate genes were detected in approximately 50% of Chinese patients with various forms of retinal degeneration. The next stage, identifying additional causative mutations in patients with various forms of genetic eye diseases based on whole exome sequencing of 1220 samples, revealed frequent homozygous or compound heterozygous null mutations in ALMS1, which are known to associate with Alström syndrome as well as individuals diagnosed with Leber congenital amaurosis (LCA) or early‐onset severe cone–rod dystrophy (CORD) without signs of systemic phenotypes except that one had a congenital heart abnormity. Sanger sequencing, co‐segregation analysis and analysis of normal individuals identified a total of 13 null mutations in ALMS1 in 11 probands, including 4 probands with homozygous mutations and 7 with compound heterozygous mutations. Follow‐up examinations revealed absent or mild systemic manifestations of Alström syndrome in those available: 9 of 15 patients in 11 families. These findings not only expand the spectrum of phenotypes associated with ALMS1 mutations but also suggest that ALMS1 should be regarded as a candidate causative gene in patients diagnosed with isolated LCA and early‐onset severe CORD.     

A Homozygous Mutation in the TUB Gene Associated with Retinal Dystrophy and Obesity

Inherited retinal dystrophies are a major cause of childhood blindness. Here, we describe the identification of a homozygous frameshift mutation (c.1194_1195delAG, p.Arg398Serfs*9) in TUB in a child from a consanguineous UK Caucasian family investigated using autozygosity mapping and whole‐exome sequencing. The proband presented with obesity, night blindness, decreased visual acuity, and electrophysiological features of a rod cone dystrophy. The mutation was also found in two of the proband's siblings with retinal dystrophy and resulted in mislocalization of the truncated protein. In contrast to known forms of retinal dystrophy, including those caused by mutations in the tubby‐like protein TULP‐1, loss of function of TUB in the proband and two affected family members was associated with early‐onset obesity, consistent with an additional role for TUB in energy homeostasis.     

Insight into vitamin B6‐dependent epilepsy due to PLPBP (previously PROSC) missense mutations

Vitamin B₆‐dependent genetic epilepsy was recently associated to mutations in PLPBP (previously PROSC), the human version of the widespread COG0325 gene that encodes TIM‐barrel‐like pyridoxal phosphate (PLP)‐containing proteins of unclear function. We produced recombinantly, purified and characterized human PROSC (called now PLPHP) and its six missense mutants reported in epileptic patients. Normal PLPHP is largely a monomer with PLP bound through a Schiff‐base linkage. The PLP‐targeting antibiotic d ‐cycloserine decreased the PLP‐bound peak as expected for pseudo‐first‐order reaction. The p.Leu175Pro mutation grossly misfolded PLPHP. Mutations p.Arg241Gln and p.Pro87Leu decreased protein solubility and yield of pure PLPHP, but their pure forms were well folded, similarly to pure p.Pro40Leu, p.Tyr69Cys, and p.Arg205Gln mutants (judged from CD spectra). PLPHP stability was decreased in p.Arg241Gln, p.Pro40Leu, and p.Arg205Gln mutants (thermofluor assays). The p.Arg241Gln and p.Tyr69Cys mutants respectively lacked PLP or had a decreased amount of this cofactor. With p.Tyr69Cys there was extensive protein dimerization due to disulfide bridge formation, and PLP accessibility was decreased (judged from d ‐cycloserine reaction). A 3‐D model of human PLPHP allowed rationalizing the effects of most mutations. Overall, the six missense mutations caused ill effects and five of them impaired folding or decreased stability, suggesting the potential of pharmacochaperone‐based therapeutic approaches.     

Genetic studies of multiple consanguineous Pakistani families segregating oculocutaneous albinism identified novel and reported mutations

Oculocutaneous albinism (OCA) is an autosomal‐recessive disorder of a defective melanin pathway. The condition is characterized by hypopigmentation of hair, dermis, and ocular tissue. Genetic studies have reported seven nonsyndromic OCA genes, among which Pakistani OCA families mostly segregate TYR and OCA2 gene mutations. Here in the present study, we investigate the genetic factors of eight consanguineous OCA families from Pakistan. Genetic analysis was performed through single‐nucleotide polymorphism (SNP) genotyping (for homozygosity mapping), whole exome sequencing (for mutation identification), Sanger sequencing (for validation and segregation analysis), and quantitative PCR (qPCR) (for copy number variant [CNV] validation). Genetic mapping in one family identified a novel homozygous deletion mutation of the entire TYRP1 gene, and a novel deletion of exon 19 in the OCA2 gene in two apparently unrelated families. In three further families, we identified homozygous mutations in TYR (NM_000372.4:c.1424G > A; p.Trp475*), NM_000372.4:c.895C > T; p.Arg299Cys), and SLC45A2 (NM_016180:c.1532C > T; p.Ala511Val). For the remaining two families, G and H, compound heterozygous TYR variants NM_000372.4:c.1037‐7T > A, NM_000372.4:c.1255G > A (p.Gly419Arg), and NM_000372.4:c.1255G > A (p.Gly419Arg) and novel variant NM_000372.4:c.248T > G; (p.Val83Gly), respectively, were found. Our study further extends the evidence of TYR and OCA2 as genetic mutation hot spots in Pakistani families. Genetic screening of additional OCA cases may also contribute toward the development of Pakistani specific molecular diagnostic tests, genetic counseling, and personalized healthcare.     

PPP1R21 homozygous null variants associated with developmental delay,muscle weakness,distinctive facial features,and brain abnormalities

We present 3 children with homozygous null variants in the PPP1R21 gene. A 3‐year‐old girl had profound developmental delay, hypotonia and weakness, poor feeding, recurrent chest infections and respiratory failure, rotatory nystagmus, absent reflexes, and a homozygous nonsense variant c.2089C>T (p.Arg697*). A 2‐year‐old boy had profound developmental delay, weakness and hypotonia, recurrent chest infections and respiratory distress, undescended testes, rotatory nystagmus, hyporeflexia, and a homozygous nonsense variant c.427C>T (p.Arg143*). An 11‐year‐old girl with profound developmental delay, weakness and hypotonia, stereotypic movements, growth failure, hyporeflexia, and a homozygous frameshift variant c.87_88delAG (p.Gly30Cysfs*4). In addition, these children shared common facial features (thick eyebrows, hypertelorism, broad nasal bridge, short nose with upturned nasal tip and broad low‐hanging columella, thick lips, low‐set ears, and coarse facies with excessive facial hair), and brain abnormalities (cerebellar vermis hypoplasia, ventricular dilatation, and reduced white matter volume). Although PPP1R21 has not yet been linked to human disease, the consistency in the phenotype of individuals from unrelated families, the nature of the variants which result in truncated proteins, and the expected vital role for PPP1R21 in cellular function, all support that PPP1R21 is a novel disease‐associated gene responsible for the phenotype observed in these individuals.