The mutations (including nucleotide substitutions) and clinical features of the mutation positive cases are summarised in table 1. Mutations were found in 37 of 43 (86%) patients with a clinical diagnosis of CS. No mutations were found in four patients with a possible diagnosis of either CS or CFC or five NS cases without a PTPN11 mutation.
Table 1Genotype and major phenotypic features in mutation positive cases of CS
Analysis of parental DNA samples was possible in 16 cases for both parents and in three cases for one parent, and confirmed the mutations as de novo in all of these cases.
As in the Japanese and North American series, the commonest mutation in our series was the G12S missense substitution, which we found in 30 of 37 mutation positive cases. Two of these 30 patients had a rhabdomyosarcoma (patients 1 and 8). Patient 1 was included in the report of Gripp et al
and is included here for completeness of phenotype analysis but was only counted once in the analyses of malignancy frequency (table 2). Three patients, two of whom had a rhabdomyosarcoma, had a G12A mutation. Three novel mutations were observed: G12C in two patients (one with a rhabdomyosarcoma), G12E in one patient, and K117R in one patient.
Table 2Mutation spectrum and malignancy frequency in CS
In our series, all the mutation positive cases had failure to thrive, and the facial appearance and hands characteristic of CS. Macrocephaly, relative or absolute, was found in 32 of 37 mutation positive cases; current head size is unknown in two cases, and another, patient 11, had cerebral atrophy secondary to ischaemia.
Warts in unusual sites are one of the defining features of CS. They were present in 12 patients with a G12S mutation, but absent in 18 analysed or seen at 9 years of age or younger. They were also absent in the 9 year old patient with the K117R mutation, and one 5 year old patient with a G12A mutation. The youngest patient with warts was aged 6 years, with a G12C mutation.
Cardiomyopathy was common, occurring in 19 patients, and was found in patients with all five mutations. Atrial arrhythmia occurred in 11 patients, all of whom had the common G12S mutation, as did the patients with arrhythmia in the series of Gripp et al
Congenital heart disease occurred in only eight patients, and in the presence of mutations G12S, G12A, and K117R.
Hypoglycaemia occurred in three patients, growth hormone deficiency in four, and precocious puberty in one. Chiari malformation occurred in two patients, and ventricular dilatation in three. Four patients had epilepsy. None of these findings was sufficiently common for correlation with mutation type to be possible.
Several clinical features are of interest. Patient 7 (G12A) died aged 25 years of a short and severe respiratory illness, just prior to testing being undertaken. At postmortem, extensive lung fibrosis was described. This has not previously been reported in CS.
In two cases, the phenotype was so severe that the clinical diagnosis has been doubted. Patient 10 (G12S) remains hospitalised in the second year of life because of severe airway obstruction secondary to subglottic stenosis and persisting respiratory compromise despite surgery and tracheostomy. Patient 11 (G12E) presented as a newborn with severe hypoglycaemia secondary to hyperinsulinism, requiring treatment throughout her life. She had hepatomegaly and cardiomegaly noted on the first day of life, as well as persistent and severe feeding difficulty. At 5 weeks, she developed aponea during a respiratory infection, associated with a cardiac arrest. Despite initial improvement, she required re‐ventilation. She had severe biventricular cardiomyopathy, and on bronchography and bronchoscopy, bronchomalacia of the left main bronchus with narrowing of the right main bronchus was found. Her head growth and development ceased, with evidence on magnetic resonance imaging scan of cerebral atrophy, assumed due to hypoxia. She developed a chylous ascites, and remained ventilator dependent despite maximum therapy, including tracheostomy. She died of respiratory failure. At postmortem, the clinical findings diagnosed in life were confirmed. She had a nesidioblastosis‐like lesion, with hypertrophy and hyperplasia of the Langherhans' islets of the pancreas, evidence of cortical ischaemia, and hypertrophic cardiomyopathy. Unexpected findings were a fibromuscular dysplasia of the coronary arteries, an inflammatory subglottic tracheal polyp, multiple cysts of both ovaries, and a severe canicular cholestasis.
The K117R mutation described in patient 16 lies in exon 3 and was not described in the original report. This patient's physical phenotype was unusual in that there was microretrognatism and both plantar and palmar creases were slightly less pronounced than usually seen in CS patients. The behavioural phenotype included autistic traits, with verbal stereotypies and hand biting. Otherwise she had classical CS features, with cardiac involvement (cardiomyopathy and ventricular septal defect), but no neurological malformation.
Residue 117 lies in a region of HRAS where no other mutation associated with CS has been found found. The mutation changes a lysine into an arginine. The de novo status of the mutation was demonstrated by its absence in both parents of this child. The lysine at position 117 is highly conserved through evolution. Interestingly, mutations at codons 116 and 119 were found in cancer.20
This variant was not found in 100 independent control chromosomes tested. Altogether, these elements indicate that K117R is highly likely to be a mutation and not a polymorphism, although it does not change the amino acid class (both lysine and arginine are positively charged).