Genome sequencing identifies biallelic variants in SCLT1 in a patient with syndromic nephronophthisis: Reflections on the SCLT1-related ciliopathy spectrum.

Ciliopathies represent a major category of rare multisystem disease. Arriving at a specific diagnosis for a given patient is challenged by the significant genetic and clinical heterogeneity of these conditions. We report the outcome of the diagnostic odyssey of a child with obesity, renal, and retinal disease. Genome sequencing identified biallelic splice site variants in sodium channel and clathrin linker 1 (SCLT1), an emerging ciliopathy gene. We review the literature on all patients reported with biallelic SCLT1 variants highlighting a frequent clinical presentation that overlaps Bardet-Biedl and Senior-Loken syndromes. We also discuss current concepts in syndrome designation in light of these data.

MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement.

MSTO1 encodes a cytosolic mitochondrial fusion protein, misato homolog 1 or MSTO1. While the full genotype-phenotype spectrum remains to be explored, pathogenic variants in MSTO1 have recently been reported in a small number of patients presenting with a phenotype of cerebellar ataxia, congenital muscle involvement with histologic findings ranging from myopathic to dystrophic and pigmentary retinopathy. The proposed underlying pathogenic mechanism of MSTO1-related disease is suggestive of impaired mitochondrial fusion secondary to a loss of function of MSTO1. Disorders of mitochondrial fusion and fission have been shown to also lead to mitochondrial DNA (mtDNA) depletion, linking them to the mtDNA depletion syndromes, a clinically and genetically diverse class of mitochondrial diseases characterized by a reduction of cellular mtDNA content. However, the consequences of pathogenic variants in MSTO1 on mtDNA maintenance remain poorly understood. We present extensive phenotypic and genetic data from 12 independent families, including 15 new patients harbouring a broad array of bi-allelic MSTO1 pathogenic variants, and we provide functional characterization from seven MSTO1-related disease patient fibroblasts. Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy. MSTO1 protein was not detectable in the cultured fibroblasts of all seven patients evaluated, suggesting that pathogenic variants result in a loss of protein expression and/or affect protein stability. Consistent with impaired mitochondrial fusion, mitochondrial networks in fibroblasts were found to be fragmented. Furthermore, all fibroblasts were found to have depletion of mtDNA ranging from 30 to 70% along with alterations to mtDNA nucleoids. Our data corroborate the role of MSTO1 as a mitochondrial fusion protein and highlight a previously unrecognized link to mtDNA regulation. As impaired mitochondrial fusion is a recognized cause of mtDNA depletion syndromes, this novel link to mtDNA depletion in patient fibroblasts suggests that MSTO1-deficiency should also be considered a mtDNA depletion syndrome. Thus, we provide mechanistic insight into the disease pathogenesis associated with MSTO1 mutations and further define the clinical spectrum and the natural history of MSTO1-related disease.

Expansion of the GLE1-associated arthrogryposis multiplex congenita clinical spectrum.

Mutations in GLE1 cause two recessive subtypes of arthrogryposis multiplex congenita (AMC), a condition characterized by joint contractures at birth, and all previously reported patients died in the perinatal period. GLE1 related AMC has been almost exclusively reported in the Finnish population and is caused by a relatively common pathogenic splicing mutation in that population. Here, we report two non-Finnish brothers with novel compound heterozygous splicing mutations in GLE1, one of whom has survived to 12 years of age. We also demonstrate low levels of residual wild type transcript in fibroblasts from the surviving brother, suggesting that this residual wild-type transcript may contribute to the relatively longer-term survival in this family. We provide a detailed clinical report on the surviving patient, providing the first insight into the natural history of this rare neuromuscular disease. We also suggest that lethal congenital contracture syndrome 1 (LCCS1) and lethal arthrogryposis with anterior horn disease (LAAHD), the two AMC subtypes related to GLE1, do not have sufficient clinical or molecular differentiation to be considered allelic disorders. Rather, GLE1 mutations cause a variable spectrum of AMC severity including a non-lethal variant described herein.

Debunking Occam's razor: Diagnosing multiple genetic diseases in families by whole-exome sequencing.

BACKGROUND: Recent clinical whole exome sequencing (WES) cohorts have identified unanticipated multiple genetic diagnoses in single patients. However, the frequency of multiple genetic diagnoses in families is largely unknown. AIMS: We set out to identify the rate of multiple genetic diagnoses in probands and their families referred for analysis in two national research programs in Canada. MATERIALS & METHODS: We retrospectively analyzed WES results for 802 undiagnosed probands referred over the past 5 years in either the FORGE or Care4Rare Canada WES initiatives. RESULTS: Of the 802 probands, 226 (28.2%) were diagnosed based on mutations in known disease genes. Eight (3.5%) had two or more genetic diagnoses explaining their clinical phenotype, a rate in keeping with the large published studies (average 4.3%; 1.4 - 7.2%). Seven of the 8 probands had family members with one or more of the molecularly diagnosed diseases. Consanguinity and multisystem disease appeared to increase the likelihood of multiple genetic diagnoses in a family. CONCLUSION: Our findings highlight the importance of comprehensive clinical phenotyping of family members to ultimately provide accurate genetic counseling.

Utility of whole-exome sequencing for those near the end of the diagnostic odyssey: time to address gaps in care.

An accurate diagnosis is an integral component of patient care for children with rare genetic disease. Recent advances in sequencing, in particular whole-exome sequencing (WES), are identifying the genetic basis of disease for 25-40% of patients. The diagnostic rate is probably influenced by when in the diagnostic process WES is used. The Finding Of Rare Disease GEnes (FORGE) Canada project was a nation-wide effort to identify mutations for childhood-onset disorders using WES. Most children enrolled in the FORGE project were toward the end of the diagnostic odyssey. The two primary outcomes of FORGE were novel gene discovery and the identification of mutations in genes known to cause disease. In the latter instance, WES identified mutations in known disease genes for 105 of 362 families studied (29%), thereby informing the impact of WES in the setting of the diagnostic odyssey. Our analysis of this dataset showed that these known disease genes were not identified prior to WES enrollment for two key reasons: genetic heterogeneity associated with a clinical diagnosis and atypical presentation of known, clinically recognized diseases. What is becoming increasingly clear is that WES will be paradigm altering for patients and families with rare genetic diseases.

HostSeq: a Canadian whole genome sequencing and clinical data resource.

HostSeq was launched in April 2020 as a national initiative to integrate whole genome sequencing data from 10,000 Canadians infected with SARS-CoV-2 with clinical information related to their disease experience. The mandate of HostSeq is to support the Canadian and international research communities in their efforts to understand the risk factors for disease and associated health outcomes and support the development of interventions such as vaccines and therapeutics. HostSeq is a collaboration among 13 independent epidemiological studies of SARS-CoV-2 across five provinces in Canada. Aggregated data collected by HostSeq are made available to the public through two data portals: a phenotype portal showing summaries of major variables and their distributions, and a variant search portal enabling queries in a genomic region. Individual-level data is available to the global research community for health research through a Data Access Agreement and Data Access Compliance Office approval. Here we provide an overview of the collective project design along with summary level information for HostSeq. We highlight several statistical considerations for researchers using the HostSeq platform regarding data aggregation, sampling mechanism, covariate adjustment, and X chromosome analysis. In addition to serving as a rich data source, the diversity of study designs, sample sizes, and research objectives among the participating studies provides unique opportunities for the research community.

Mutation of DNAJC19, a human homologue of yeast inner mitochondrial membrane co-chaperones, causes DCMA syndrome, a novel autosomal recessive Barth syndrome-like condition.

BACKGROUND: A novel autosomal recessive condition, dilated cardiomyopathy with ataxia (DCMA) syndrome, has been identified in the Canadian Dariusleut Hutterite population, characterised by early onset dilated cardiomyopathy with conduction defects, non-progressive cerebellar ataxia, testicular dysgenesis, growth failure, and 3-methylglutaconic aciduria. OBJECTIVE: To map DCMA syndrome and identify the mutation underlying this condition. METHODS: A genome wide scan was undertaken on consanguineous Hutterite families using a homozygosity mapping approach in order to identify the DCMA associated chromosomal region. Mutation analysis was carried out on positional candidate genes in this region by sequencing. Reverse transcriptase polymerase chain reaction and bioinformatics analyses were then used to characterise the mutation and determine its effect on the protein product. RESULTS: The association of DCMA syndrome with a 2.2 Mb region of chromosome 3q26.33 was found. A disease associated mutation was identified: IVS3-1 G-->C in the DNAJC19 gene, encoding a DNAJ domain containing protein of previously unknown function (Entrez Gene ID 131118). CONCLUSIONS: The DNAJC19 protein was previously localised to the mitochondria in cardiac myocytes, and shares sequence and organisational similarity with proteins from several species including two yeast mitochondrial inner membrane proteins, Mdj2p and Tim14. Tim14 is a component of the yeast inner mitochondrial membrane presequence translocase, suggesting that the unique phenotype of DCMA may be the result of defective mitochondrial protein import. It is only the second human disorder caused by defects in this pathway that has been identified.

Diagnostic criteria for respiratory chain disorders in adults and children.

BACKGROUND: Respiratory chain (RC) disorders are clinically, biochemically, and molecularly heterogeneous. The lack of standardized diagnostic criteria poses difficulties in evaluating diagnostic methodologies. OBJECTIVE: To assess proposed adult RC diagnostic criteria that classify patients into "definite," "probable," or "possible" categories. METHODS: The authors applied the adult RC diagnostic criteria retrospectively to 146 consecutive children referred for investigation of a suspected RC disorder. Data were collected from hospital, genetics, and laboratory records, and the diagnoses predicted by the adult criteria were compared with the previously assigned assessments. RESULTS: The authors identified three major difficulties in applying the adult criteria:lack of pediatric-specific criteria; difficulty in segregating continuous data into circumscribed major and minor criteria; and lack of additivity of clinical features or enzyme tests. They therefore modified the adult criteria to allow for pediatric clinical and histologic features and for more sensitive coding of RC enzyme and functional studies. Reanalysis of the patients' data resulted in congruence between the diagnostic certainty previously assigned by the authors' center and that defined by the new general RC diagnostic criteria in 99% of patients. CONCLUSIONS: These general diagnostic criteria appear to improve the sensitivity of the adult criteria. They need further assessment in prospective clinical and epidemiologic studies.

Two pairs of male monozygotic twins discordant for Wiedemann-Beckwith syndrome.

Wiedemann-Beckwith syndrome (WBS) is a congenital anomaly syndrome which classically consists of exomphalos, macroglossia, and gigantism. The syndrome is also associated with a variety of minor anomalies and affected individuals have an increased risk of developing rare embryonal cell tumors. To date, 15 monozygotic (MZ) twin pairs have been reported of which 13 are discordant for WBS. All except one pair of the discordant WBS twin pairs have been female. We report two pairs of male MZ twins, each discordant for WBS.

Single-blind study of dystrophin staining in carriers of Duchenne muscular dystrophy.

A single-blind study of dystrophin staining in skeletal muscle was performed in 13 biopsies from carriers of Duchenne Muscular Dystrophy (DMD) and controls. The results indicate that immunohistochemical analysis of dystrophin staining is a valuable diagnostic test for DMD carriers when DNA for testing is unavailable from critical family members or is uninformative, when creatine kinase (CK) values are conflicting or when CK values must be used in isolation.

Correlates of age at diagnosis of autism spectrum disorders in six Canadian regions.

INTRODUCTION: Early identification of autism spectrum disorders (ASD) is important, since earlier exposure to behavioural intervention programs may result in better outcomes for the child. Moreover, it allows families timely access to other treatments and supports. METHODS: Using generalized linear modeling, we examined the association between child and family characteristics and the age at which 2180 children were diagnosed with ASD between 1997 and 2005 in six Canadian regions. RESULTS: A diagnosis of pervasive developmental disorder-not otherwise specified (PDD-NOS) or Asperger syndrome, rural residence, diagnosis in more recent years, and foreign birthplace were associated with a later age at diagnosis. Children who are visible minorities or who have siblings with ASD were more likely to be diagnosed earlier. Collectively, these factors explained little of the variation in age at diagnosis, however. CONCLUSION: While it is encouraging that ethnocultural identity, neighbourhood income, urban or rural residence, and sex of the child were not major contributors to disparities in the age when children were identified with ASD, more work is needed to determine what does account for the differences observed. Regional variations in the impact of several factors suggest that aggregating data may not be an optimal strategy if the findings are meant to inform policy and clinical practice at the local level.

Association between congenital foot anomalies and gestational age at amniocentesis.

OBJECTIVES: Our objectives were to confirm the reported association between early amniocentesis and congenital foot anomalies as well as to report, for the first time, on the outcome of amniocenteses performed during the 13th and 14th weeks of gestation. METHODS: We conducted a triple cohort retrospective study of 4457 amniocenteses. Cohort definitions: early amniocentesis (EA), 11 weeks and 0/7 days to 12 weeks to 6/7 days; early midtrimester amniocentesis (EMA), 13 weeks and 0/7 days to 14 weeks and 6/7 days; and midtrimester amniocentesis (MA), 15 weeks and 0/7 days to 19 weeks and 6/7 days. Outcome measures were obtained by searching the Alberta Congenital Anomalies Surveillance System (ACASS) database for children born with foot anomalies represented by International Classification of Diseases version 9 (ICD-9) codes 754.5, 754.6 and 754.7. RESULTS: Incidences of congenital foot anomalies were: EA 11/980 (1.1%), EMA 11/2515 (0.4%), and MA 1/962 (0.1%). There is a significant difference between the EA and EMA cohorts (p=0.019) and between the EA and MA cohorts (p=0.003); however, these data suggest there is no difference between EMA and MA cohorts (p=0.11). CONCLUSIONS: Our incidence of congenital foot anomalies of 1.1% for women who underwent EA is similar to previously reported data, which further validates this association; however, our data also suggest that the foot anomaly risk may be limited to amniocenteses performed before the 13th week of gestation.

Bowen-Conradi syndrome: a clinical and genetic study.

The purpose of the study was to delineate the anomalies and the natural life history of persons with the Bowen-Conradi syndrome [Bowen and Conradi 1976: Birth Defects: Orig Artic Ser XII(6):101-108]. We ascertained 39 cases and personally examined almost all. For those who were not seen, their clinical record were scrutinized. Pedigree analysis of all 39 was done and kinship coefficients computed. The birth prevalence was estimated to be 1/355 live births.

Mutations in the novel protocadherin PCDH15 cause Usher syndrome type 1F.

We have determined the molecular basis for Usher syndrome type 1F (USH1F) in two families segregating for this type of syndromic deafness. By fluorescence in situ hybridization, we placed the human homolog of the mouse protocadherin Pcdh15 in the linkage interval defined by the USH1F locus. We determined the genomic structure of this novel protocadherin, and found a single-base deletion in exon 10 in one USH1F family and a nonsense mutation in exon 2 in the second. Consistent with the phenotypes observed in these families, we demonstrated expression of PCDH15 in the retina and cochlea by RT-PCR and immunohistochemistry. This report shows that protocadherins are essential for maintenance of normal retinal and cochlear function.