Sanger sequencing in exonic regions of STK11 gene uncovers a novel de-novo germline mutation (c.962_963delCC) associated with Peutz-Jeghers syndrome and elevated cancer risk: case report of a Chinese patient

Peutz-Jeghers Syndrome (PJS, OMIM 175200) is an autosomal dominant disorder characterized by gastrointestinal (GI) hamartomatous polyps, mucocutaneous pigmentation, and an increased risk for the development of GI and various extra-GI malignancies [1]. It is a rare condition with a low incidence of 1/50,0009 to 1/200,000 [2]. The elevated cancer risk in PJS patients has been observed in several cohort studies, and it is estimated to be 9–18 times higher than the general population [3, 4].

Since cloned by Jenne et al. and Hemminki et al. respectively in 1997, serine-threonine kinase 11 (STK11/LKB1, OMIM 602216) on chromosome 19p13.3 have been considered to be the pathogenic gene of PJS [5, 6]. The gene contains 10 exons, within which exon 1–9 encode a 433-amino-acid-residue protein, a tumor suppressor. Mutation detection rates of 10%–94% have been achieved at different centers, depending on the screening method, with considerable uncharacterized genetic heterogeneity remaining in this syndrome [7, 8]. Most mutations are frameshift or nonsense changes, which result in an abnormal truncated protein and the consequent loss of kinase activity, and they can be detected in PJS patients with or without family histories [9]. Different cancer risks due to different STK11 mutations have been recognized through hundreds of cases reported previously, however the STK11 mutation spectrum and genotype-phenotype correlation are still poorly understood.

PJS is mainly caused by heterozygous germline mutations in STK11, and in this study we identified a novel de-novo mutation of STK11, c.962_963delCC (p.P321HfsX38), in a Chinese male without a family history. In general, identification of a STK11 gene mutation in an index patient offers the possibility of a predictive diagnosis in PJS pedigrees, but sometimes an isolated patient not a pedigree of a positive family history is found clinically. In these cases, genetic testing often discovers a germline mutation carried only by the proband but not the parents, which means a de-novo mutation. Though lacking the reminder of a family history, PJS is relatively easy to diagnose according to the obvious mucocutaneous pigmentation and GI hamartomas. In these cases, clinicians should take de-novo mutation into consideration and be aware of the possible cancer risk, so that an appropriate surveillance recommendation can be made.

The STK11 protein is mainly comprised of three major domains: an N-terminal non-catalytic domain, a catalytic kinase domain, and a C-terminal non-catalytic regulatory domain (CRD) [12]. Among the over 400 reported STK11 mutations detected in patients with PJS or other disorders (HGMD Professional 2016.2) [13], most variants are located in the region of catalytic kinase domain (amino acids 49–309) and result in the absence of kinase activity and disrupting the formation of a complex to maintain kinase activation [14]. Several autophosphorylation sites within this area have been described [15]. As to the CRD of STK11 protein, it is important for binding STRADα/β [16, 17], and mutation within this area could also lessen the AMPKα activity, affect cell polarity,and impair downstream signaling [16]. So this novel mutation broaden the pathological mutation spectrum of the STK11 gene. It much likely results in the premature termination, which leads to dramatic change and partial loss of CRD, and may contribute to polyp formation and tumorigenesis through various mechanisms, such as loss of growth arrest, apoptosis, and loss of cell polarity.

Although the exact function of STK11 remains largely unknown, studies suggest that it plays a role in cell signaling and apoptosis [18], and several proteins interacting with it (P53, Cdc37, Hsp90, and PTEN) are responsible for PJS phenotype. It is believed that the development of PJS phenotype is due to the elimination of STK11 kinase activity, associating with a loss of growth suppression function [19]. STK11 induces cell cycle arrest and apoptosis on the dependence of P53, and transgenic mice experiments has proven a greater life span reduction and tumor incidence increasing in STK11 and P53 both knockout than single gene knockout [20]. Chromatin immunoprecipitation analyses has proven that STK11 is recruited directly to the P21 gene promoter by P53 and activated P21 expression [21]. Overexpression of STK11 could elevate the transcriptional activity of P53, while its depletion inhibited P53 functions, which has been observed in patients of PJS-associated GI carcinomas [22, 23]. In our study, computational predictions by software suggested that these mutant proteins lost partial CRD, which possibly impairs the interaction between STK11 and its partners (STRADα/β for example), and eventually cause PJS phenotype, especially the cancerogenesis.

Most of the time, PJS is diagnosed when the patient suffers from intestinal obstruction and comes to the surgical emergency, just like the case reported here [24]. With the widely use of endoscopy, PJS patients can be managed intensively to avoid emergency condition and potential cancer risk. People with a positive family history should receive first upper GI endoscopy and colonoscopy at age 8 years, and regular follow-up is necessary [25]. More important, double-balloon enteroscopy (DBE) is the key method to detect and remove small bowel polyps nonoperatively [26‐29]. 131 PJS patients who had abdominal surgery for intestinal obstruction before have admitted to our hospital, and with the help of DBE, 113 of them (86.3%) avoid the second open surgery, indicating proper follow-up ensures the patients free from intestinal obstruction and malignancies [30].