Noonan syndrome (NS) and the clinically overlapping disorders cardio-facio-cutaneous syndrome, LEOPARD syndrome, Costello syndrome and Neurofibromatosis-Noonan syndrome share the clinical features of short stature, the same spectrum of congenital heart defects, and a similar pattern of craniofacial anomalies. It is now known that all these disorders are caused by mutations in components of the RAS-MAPK signaling pathway. This pathway was previously known for its involvement in tumorigenesis. This article reviews the current knowledge on underlying genetic alterations and possible pathogenetic mechanisms responsible for NS and related disorders. It discusses the relationship between a group of developmental disorders and oncogenes. Potential future treatment prospects are based on the possibility of inhibiting RAS-MAPK signaling by pharmaceuticals.

Short stature is a major characteristic of Noonan syndrome (NS), the biological basis of which is not yet clear. In around half of all individuals with NS, the cytoplasmic tyrosine phosphatase SHP2 encoded by PTPN11 is mutated and predicted to be overactive. While SHP2 enhances Ras-MAPK signaling, it downregulates Jak2/STAT5b signaling of the growth hormone (GH) receptor, according to in vitro data. Decreased IGF-I levels have been measured in those children with NS who carried PTPN11 mutations suggesting a mode of mild GH insensitivity. The short-term responsiveness to GH therapy in NS with respect to PTPN11 mutations has been addressed in 3 studies in the past. The number of treated children was small and gene analysis was restricted to PTPN11, excluding the recent discovered candidate genes KRAS, RAF1 and SOS1. All 3 studies showed that GH responsiveness was mildly reduced in the presence of PTPN11 mutations; relevant long-term data, however, are missing. In a small subgroup of patients with NS, tumor risk is increased and related to specific mutations of Ras-MAPK pathway genes, including PTPN11. Therefore, when long-term GH therapy is intended to promote growth in children with NS, it has to be considered in relation to the genotype, the effective promotion of growth and the potentially increased tumor risk. Progress in the understanding of cell regulation by Ras-MAPK signaling will hopefully provide more evidence on which therapy might be helpful in the care of children with NS.

Growth hormone treatment in Noonan syndrome increases growth velocity significantly during the first 2 years of treatment and, to some extent, until puberty. This increase is more pronounced if treatment is started at an early age. Treatment before the age of 5 years is not recommended due to an increased risk of malignancies. In contrast to other growth hormone-treated patients, a slight but significant further increase in height gain can be expected during pubertal growth (at least in boys). Final height improvement varies between 1 and 2 SDS in different studies. Cardiac function does not seem to be impaired during treatment. No significant adverse events have been reported.

Short stature is one of the key features of Noonan syndrome (NS). Attempts have therefore been made to improve height by means of recombinant human growth hormone (rhGH) treatment. Most of these endeavors were carried out either as case studies or observational studies. The overall experience in treating NS is still rather limited, and, in general, it can be said that the NS patients who received GH treatment represent a very narrow segment. The dosages applied in both the case studies and observational studies tended to be higher than those used in the replacement therapy of GH-deficient patients, but lower than in Turner syndrome patients. The NS studies have shown that the overall height gain of patients is small (5-10 cm), and that treatment usually begins at the age of about 10 years, at a height of approximately -3.0 SDS. This small response to treatment reflects the external treatment conditions (i.e. late age at GH start, low GH dose), but may also be associated with the fact that impaired sensitivity to GH is common in NS. Both case studies and observational studies are necessary in order to obtain further evidence about the efficacy and safety of GH treatment in NS.

Growth failure in Noonan syndrome is mainly postnatal of character and is dominated by slow maturation and late puberty. The postnatal early decline seems to be an intrinsic part of the syndrome. Reported adult heights are about -2 SD and are indicative of a secular trend.

The hypothalamo-pituitary-adrenal axis has not been studied systematically in Noonan syndrome (NS), despite potential concerns about other aspects of hypothalamo-pituitary function. While adrenarche may be delayed in children with constitutional growth of puberty and in isolated GH deficiency, this does not generally seem to be the case in hypergonadotrophic hypogonadism due to Turner syndrome (TS) and this is (anecdotally) the usual hormonal profile in NS children and adults. Precocious or 'exaggerated' adrenarche can be associated with intrauterine growth retardation and is a forerunner of syndrome X. Although NS neonates often have 'normal' birth weights, in some it can be artificially inflated by subcutaneous edema (as in TS, where intrauterine growth retardation is characteristic). Overall, however, a controlling role for adrenarche (whether precocious or delayed) in gonadarche in NS seems unlikely. Neither normally descended testes nor normal (even if delayed) pubertal development implies normal fertility in NS men. Interactions between fetal, neonatal, childhood and pubertal testis development and gonadal axis maturation are complex. There is probably a spectrum of abnormalities in NS, but most commonly primary gonadal failure and hypergonadotrophic hypogonadism - characteristic NS molecular genetic abnormalities - may be important for normal germ cell proliferation, development and migration. The identification of different gene defects facilitates understanding of NS phenotypic diversity and provides opportunities for prospective studies on gonadal and adrenal axes in better defined populations less subject to ascertainment bias. At a clinical level, more longitudinal data are still needed with regard to the natural history of pubertal timing, its tempo of progression and the pattern of pubertal growth.

The current paper introduces concise neuropsychological assessment as an essential tool for studying the contribution of cognition and behavior in the expression of genetic syndromes, like Noonan syndrome (NS). Cognitive and behavioral findings in NS show intelligence scores across a wide range, with a mildly lowered average level. Language and motor development are often delayed, but no longer dysfunctional in adulthood. Continuing mild problems in selective and sustained attention are noted, as well as suboptimal organization skills and compromised abilities to structure complex information. These problems seem to culminate in learning difficulties, requiring attention for special needs in education. It seems that a complex of psychosocial immaturity, alexithymia and amenable traits is typical of NS patients. Consequently, psychopathology or psychological problems in leading a self-serving life may often remain underreported. This is why the authors advocate the integration of the domain of social cognition and personality in NS assessment.

The overall risk of cancer in children with Noonan (NS), cardio-facial-cutaneous, Costello or LEOPARD syndrome is high, although no precise estimates are available. There are few data on cancer in adults with NS, but the reported numbers of malignancies in adults do not seem excessive. Juvenile myelomonocytic leukemia (JMML) is a rare aggressive leukemia in young children. A JMML-like myeloproliferative disorder has been described in about 30 neonates with NS and the PTPN11 mutation. The disorder often regresses spontaneously, but fatal complications may occur. A review of the literature indicates an increased risk of acute lymphoblastic leukemia and acute myeloid leukemia in NS. Young children with Costello syndrome have an extremely high risk of rhabdomyosarcoma, and also an increased risk of neuroblastoma and bladder carcinoma. Registry-based studies of patients with NS and related disorders diagnosed with molecular genetics and a high-quality long-term follow-up are necessary to further estimate the incidence of malignancy.

Noonan syndrome (NS) is a fairly common (1 per 1,000-2,500 live births) autosomal dominantly inherited disorder and the most common syndromal cause of congenital heart disease after Down's syndrome. The clinical features vary with age, but typical signs of NS include characteristic facial features with hypertelorism, down-slanting palpebral fissures, low-set posteriorly rotated ears, chest and spinal deformities, short stature, specific heart defects, learning disabilities and mild mental retardation. This article gives a brief introduction to NS and its basic clinical features using the established and generally accepted NS scoring system based on family history and facial, cardiac, growth, chest wall and other criteria. Aspects discussed include the definition, epidemiology, etiology, diagnosis and genetics of NS, as well as growth, skeletal and gonadal anomalies, pubertal development, ophthalmic and cutaneous abnormalities and the incidence of cancer in patients with NS.