Gain-of-function mutations in RIT1 cause Noonan syndrome, a RAS/MAPK pathway syndrome

Abstract

RAS GTPases mediate a wide variety of cellular functions, including cell proliferation, survival, and differentiation. Recent studies have revealed that germline mutations and mosaicism for classical RAS mutations, including those in HRAS, KRAS, and NRAS, cause a wide spectrum of genetic disorders. These include Noonan syndrome and related disorders (RAS/mitogen-activated protein kinase [RAS/MAPK] pathway syndromes, or RASopathies), nevus sebaceous, and Schimmelpenning syndrome. In the present study, we identified a total of nine missense, nonsynonymous mutations in RIT1, encoding a member of the RAS subfamily, in 17 of 180 individuals (9%) with Noonan syndrome or a related condition but with no detectable mutations in known Noonan-related genes. Clinical manifestations in the RIT1-mutation-positive individuals are consistent with those of Noonan syndrome, which is characterized by distinctive facial features, short stature, and congenital heart defects. Seventy percent of mutation-positive individuals presented with hypertrophic cardiomyopathy; this frequency is high relative to the overall 20% incidence in individuals with Noonan syndrome. Luciferase assays in NIH 3T3 cells showed that five RIT1 alterations identified in children with Noonan syndrome enhanced ELK1 transactivation. The introduction of mRNAs of mutant RIT1 into 1-cell-stage zebrafish embryos was found to result in a significant increase of embryos with craniofacial abnormalities, incomplete looping, a hypoplastic chamber in the heart, and an elongated yolk sac. These results demonstrate that gain-of-function mutations in RIT1 cause Noonan syndrome and show a similar biological effect to mutations in other RASopathy-related genes.

Figure 1

Photographs of Six Individuals in whom RIT1 Mutations Were Identified (A–D) KCC38 at 3 years of age. Broad forehead, sparse eyebrows, ptosis, hypertelorism, and hyperpigmentation were observed (A and B). Prominent finger pads were observed (C and D). (E–H) NS358 at 4 years of age. Hypertelorism, epicanthus, sparse eyebrows, and low-set ears were observed. (I) NS414 at 3 years of age. (J) NS465 at 1 year of age. (K) NS276 at 5 months. (L) NS265 at 5 years of age. (M) Structure and identified germline alterations in RIT1 and HRAS. HRAS alterations identified in individuals with Costello syndrome were described before or shown in The RAS/MAPK Syndromes Homepage (see Web Resources). HRAS alterations identified in individuals with congenital myopathy with excess of muscle spindles are indicated in purple. We obtained specific consent for photographs from six individuals.

Figure 2

Stimulation of ELK Transcription in NIH 3T3 Cells Expressing RIT1 Germline Mutations (A) The ELK1-GAL4 vector and the GAL4 luciferase trans-reporter vector were transiently transfected with various RIT1 germline mutations and activating mutations in BRAF and MAP2K1 in NIH 3T3 cells. c.1910T>A (p.Val637Glu) in mouse Braf corresponds to oncogenic c.1799T>A (p.Val600Glu) in human BRAF. Relative luciferase activity was calculated by normalization to the activity of a cotransfected control vector, phRLnull-luc, containing distinguishable R. reniformis luciferase. (B) ELK1 transactivation in cells expressing p.Ser35Thr, identified in individuals with Noonan syndrome, and p.Ser35Asn, were examined. p.Ser35Asn corresponds to dominant-negative alteration p.Ser17Asn in RAS. Results are expressed as the means of quadruplicate (A) and triplicate (B) samples. Error bars represent the SDs of mean values. Red bars indicate germline RIT1 mutations identified in Noonan syndrome. The following abbreviation is used: WT, wild-type. ^∗p < 0.01 by t test.

Figure 3

Morphology of Embryos Injected with the WT or Mutant RIT1 mRNA In vitro transcription of each mRNA was performed with the mMESSAGE mMACHINE kit (Applied Biosystems) according to the manufacturer’s instructions. Synthesized mRNAs were purified with G-50 Micro Columns (GE Healthcare) and subsequently adjusted to a 300 ng/μl concentration for microinjection. Approximately 1 nl (300 pg) of RNA in water with 0.2% phenol red was injected into the cytoplasm of 1-cell-stage zebrafish embryos. Injected embryos were incubated at 28°C until observation. (A) At 11 hpf, the shapes of the embryos injected with the WT sense or antisense mRNA were round, a normal morphology as observed in the uninjected embryos. In contrast, embryos expressing mutations (c.236A>T [p.Gln79Leu], c.242A>G [p.Glu81Gly], and c.284G>C [p.Gly95Ala]) are oval and compressed along the dorsal-ventral axis, indicative of a gastrulation defect. Note that cells have a hump in the head region at the anterior end of the body axis, the earliest manifestation of a craniofacial defect. (B) Lateral views at 48 hpf are shown. Embryos expressing mutations (c.236A>T [p.Gln79Leu], c.242A>G [p.Glu81Gly], and c.284G>C [p.Gly95Ala]) formed swollen yolk sacs equally along the anterior posterior axis but did not show narrowing in the caudal half, which was clearly visible in the uninjected embryos and in those injected with the WT sense or antisense mRNA. In the craniofacial area, misshapen head and jaw structures and small eyes with hypoplasia on the ventral side were observed (middle panel); these phenotypes are consistent with the gastrulation defect. Shapes of the hearts (highlighted by red dotted lines) are shown in the right panel at a higher magnification. Normal looping of the heart tube and correct formation of two distinct chambers are observed in embryos injected with the WT sense or antisense mRNA. When mutations (c.236A>T [p.Gln79Leu], c.242A>G [p.Glu81Gly], and c.284G>C [p.Gly95Ala]) were expressed, looping was incomplete, resulting in stretched straight heart tubes. Constrictions at the atrial-ventricular canal are obscure, and the heart chambers are hypoplastic. Abbreviations are as follows: A, atrium; and V, ventricle.