Genetic landscape of RASopathies in Chinese: Three decades' experience in Hong Kong.

RASopathies are a group of genetic disorders due to dysregulation of the RAS-MAPK signaling pathway, which is important in regulating cell growth, proliferation, and differentiation. These include Noonan syndrome (NS), Noonan syndrome with multiple lentigines (NSML), cardiofaciocutaneous (CFC) syndrome, and Costello syndrome (CS), clinical manifestations include growth retardation, developmental delay, cardiac defects, and specific dysmorphic features. There were abundant publications describing the genotype and phenotype from the Western populations. However, detailed study of RASopathies in Chinese population is lacking. We present here the largest cohort of RASopathies ever reported in Chinese populations, detailing the mutation spectrum and clinical phenotypes of these patients. The Clinical Genetic Service, Department of Health, and Queen Mary Hospital are tertiary referral centers for genetic disorders in Hong Kong. We retrospectively reviewed all the genetically confirmed cases of RASopathies, including NS, NSML, CFC syndrome, and CS, over the past 29 years (from 1989 to 2017). Analyses of the mutation spectrum and clinical phenotypes were performed. One hundred and ninety-one ethnic Chinese patients with genetically confirmed RASopathies were identified, including 148 patients with NS, 23 NSML, 12 CFC syndrome, and eight CS. We found a lower incidence of hypertrophic cardiomyopathy in individuals with NSML (27.3%), and NS caused by RAF1 mutations (62.5%). Another significant finding was for those NS patients with myeloproliferative disorder, the mutations fall within Exon 3 of PTPN11 but not only restricted to the well-known hotspots, that is, p.Asp61 and p.Thr731, which suggested that re-evaluation of the current tumor surveillance recommendation maybe warranted.

A novel missense mutation in CCDC88C activates the JNK pathway and causes a dominant form of spinocerebellar ataxia.

BACKGROUND: Spinocerebellar ataxias (SCAs) are a group of clinically and genetically diverse and autosomal-dominant disorders characterised by neurological deficits in the cerebellum. At present, there is no cure for SCAs. Of the different distinct subtypes of autosomal-dominant SCAs identified to date, causative genes for only a fraction of them are currently known. In this study, we investigated the cause of an autosomal-dominant SCA phenotype in a family that exhibits cerebellar ataxia and pontocerebellar atrophy along with a global reduction in brain volume. METHODS AND RESULTS: Whole-exome analysis revealed a missense mutation c.G1391A (p.R464H) in the coding region of the coiled-coil domain containing 88C (CCDC88C) gene in all affected individuals. Functional studies showed that the mutant form of CCDC88C activates the c-Jun N-terminal kinase (JNK) pathway, induces caspase 3 cleavage and triggers apoptosis. CONCLUSIONS: This study expands our understanding of the cause of autosomal-dominant SCAs, a group of heterogeneous congenital neurological conditions in humans, and unveils a link between the JNK stress pathway and cerebellar atrophy.

Oculopharyngeal muscular dystrophy: underdiagnosed disease in Hong Kong.

Despite the advances in the understanding of the molecular basis for oculopharyngeal muscular dystrophy in the last decade, it remains an underdiagnosed disease, especially among the Chinese. In the presence of a positive family history and late-onset ptosis, dysphagia, and proximal muscle weakness (its cardinal features), we suggest that PABPN1 gene analysis should be the first-line investigation to rule out this condition. Muscle biopsy can be reserved for atypical cases. Non-specific mitochondrial changes in the muscle specimens of these patients should be appreciated, so as to avoid diagnostic confusion. It is hoped that greater awareness among medical professionals and judicious use of PABPN1 gene analysis will lead to earlier diagnosis, better management, and avoidance of unnecessary invasive investigations of affected patients.

Congenital fibrosis of extraocular muscle type 1A due to KIF21A mutation: first case report from Hong Kong.

With the advancement of ophthalmological genetics, the molecular basis for more and more eye diseases can be elucidated. Congenital fibrosis of extraocular muscle (CFEOM) is an example. It is characterised by a congenital non-progressive restrictive ophthalmoplegia and ptosis. It is an autosomal dominant disease, caused by mutations of the KIF21A gene. With positive family history and typical ophthalmological findings, mutational analysis of KIF21A gene should be performed, not only to confirming the diagnosis, but also to offer a prognosis, for genetic counselling, and the possibility of prenatal diagnosis. Here we report the first KIF21A mutation associated with CFEOM1A in Hong Kong.

Metaphyseal chondrodysplasia McKusick type in a Chinese fetus, caused by novel compound heterozygosity 64T> A and 79G >T in RMRPgene.

We present the first confirmed case by molecular analysis of a metaphyseal chondrodysplasia, McKusick type, in a 22-week fetus. Two novel compound heterozygous mutations, 64T> A and 79G > T, were found in the highly conserved regions of the RMRP gene. Twenty-two heterozygous g.1018 T> C mutations, two homozygous g.1018 T> C mutations, two heterozygous insertion mutations g.799_g.800insC and one heterozygous insertion mutation g.849_g.850insT were found among 100 normal controls. Careful radiological examination of the fetus for skeletal dysplasia allowed definitive diagnosis, proper genetic counselling and future prenatal diagnosis.

Triple-A syndrome--the first Chinese patient with novel mutations in the AAAS gene.

We report on the first Chinese patient with triple-A syndrome, who presented at 22 months with status epilepticus secondary to hyponatraemia and hypoglycaemia. Subsequent endocrine investigations confirmed primary adrenal insufficiency and aldosterone deficiency. In the presence of achalasia and alacrima, this patient satisfies the diagnostic criteria of triple-A syndrome. Further molecular testing detected compound heterozygous mutations in the AAAS gene: a c.580C --> T transition in exon 7 and a c.771delG single nucleotide deletion in exon 8. Testing of parents and brother confirmed their heterozygous carrier status.

High rate of detection of subtelomeric aberration by using combined MLPA and subtelomeric FISH approach in patients with moderate to severe mental retardation.

OBJECTIVES: (1) To evaluate the prevalence of subtelomeric deletion in moderate to severe mental retardation population, (2) to assess the feasibility and cost-effectiveness of combined methodology in routine workup of this sub-population. METHOD: Twenty unrelated patients using strict selection criteria were recruited for the study from the Clinical Genetic Service. Patients were initially screened by Multiplex Ligation-dependent Probe Amplification (MLPA) for subtelomeric imbalance followed by FISH analysis for anatomical integrity. This is then followed by parental subtelomeric FISH analysis. RESULTS: Three subtelomeric deletions were identified. They were Deletion 1p36, Deletion 1q44 and Deletion 10q26; these were previously unidentified by conventional technique. CONCLUSIONS: The prevalence of subtelomeric deletion in our cohort of moderate to severe mental retardation patients is consistent with published findings of around 10%. The figure is on the higher side if more stringent criteria is used. The combination of strict clinical criteria, MLPA and selective subtelomeric FISH was shown to be feasible and cost-effective.

Detecting exon deletions and duplications of the DMD gene using Multiplex Ligation-dependent Probe Amplification (MLPA).

OBJECTIVES: To evaluate the efficacy of Multiplex Ligation-dependent Probe Amplification (MLPA) technique in comparison with the traditional multiplex PCR assay in detection of exon deletions and duplications of the DMD gene. DESIGN AND METHODS: The sensitivity and accuracy of MLPA were assessed and compared with the multiplex PCR in a total of 63 subjects including 43 subjects with Duchenne muscular dystrophy (DMD) or Becker muscular dystrophy (BMD) and 20 female carriers. RESULTS: MLPA was able to detect all the known deletions and duplications; it detected four additional mutations that had been missed by multiplex PCR. In addition, the extent of the deletions and duplications could be more accurately defined which in turn facilitated a genotype-phenotype correlation. CONCLUSIONS: MLPA is superior to multiplex PCR. It should be the method of choice for the detection of exon deletions and duplications of the DMD gene in patients with DMD or BMD, as well as in female carriers.

Spectrum of NSD1 gene mutations in southern Chinese patients with Sotos syndrome.

BACKGROUND: Sotos syndrome is an overgrowth syndrome with characteristic facial gestalt and mental retardation of variable severity. Haploinsufficiency of the NSD1 gene has been implicated as the major cause of Sotos syndrome, with a predominance of microdeletions reported in Japanese patients. This study was conducted to investigate into the spectrum of NSD1 gene mutations in southern Chinese patients with Sotos syndrome. METHODS: Thirty-six Chinese patients with Sotos syndrome and two patients with Weaver syndrome were subject to molecular testing. RESULTS: NSD1 gene mutations were detected in 26 (72%) Sotos patients. Microdeletion was found in only 3 patients, while the other 23 had point mutations (6 frameshift, 8 nonsense, 2 spice site, and 7 missense). Of these, 19 mutations were never reported. NSD1 gene mutations were not found in the two patients with Weaver syndrome. CONCLUSIONS: Most cases of Sotos syndrome are caused by NSD1 gene defects, but the spectrum of mutations is different from that of Japanese patients. Genotype-phenotype correlation showed that patients with microdeletions might be more prone to congenital heart disease but less likely to have somatic overgrowth. The two patients with Weaver syndrome were not found to have NSD1 gene mutations, but the number was too small for any conclusion to be drawn.