DICER1 Syndrome: DICER1 Mutations in Rare Cancers

DICER1 syndrome is a rare genetic disorder that predisposes individuals to multiple cancer types. Through mutations of the gene encoding the endoribonuclease, Dicer, DICER1 syndrome disrupts the biogenesis and processing of miRNAs with subsequent disruption in control of gene expression. Since the first description of DICER1 syndrome, case reports have documented novel germline mutations of the DICER1 gene in patients with cancers as well as second site mutations that alter the function of the Dicer protein expressed. Here, we present a review of mutations in the DICER1 gene, the respective protein sequence changes, and clinical manifestations of DICER1 syndrome. Directions for future research are discussed.


Introduction
DICER1 syndrome, or pleuropulmonary blastoma familial tumor susceptibility syndrome (ORPHA: 284343; OMIM: 601200), is a rare genetic disorder predisposing individual to the development of tumors, both benign and malignant [1][2][3]. Recently, mutations documented in endocrine tumors (thyroid, parathyroid, pituitary, pineal gland, endocrine pancreas, paragangliomas, medullary, adrenocortical, ovarian, and testicular tumors) have been reviewed [4]. One copy of the altered gene is sufficient to cause an increased risk of developing tumors; however, many individuals who carry a mutation in the DICER1 gene do not develop abnormal growths. Patients may acquire a second mutation during tumorigenesis that has the potential to affect the catalytic activity of the enzyme. The prevalence of DICER1 syndrome is currently unknown and the full spectrum of clinical manifestation may not yet be fully defined [2, 5,6]. However, documented cases of DICER1 syndrome have been linked to pleuropulmonary blastomas, cystic nephroma, rhabdomyosarcoma, multinodular goiter, thyroid cancer, ovarian Sertoli-Leydig cell tumors, and other neoplasias [7][8][9]. DICER1 syndrome may also include neuroblastoma [10].
The DICER1 gene, located on chromosome 14, position q32.13, encodes the endoribonuclease Dicer protein of the ribonuclease III family. Discovered in 2001 by Bernstein, the Dicer endoribonuclease plays a role in protein translational control [11]. MicroRNAs (miRNAs) are created by the Dicer endoribonuclease protein [12][13][14]. Dicer is a component of the RNA-induced silencing complex (RISC) loading complex (RLC), which is composed of Dicer, Argonaute-2 (AGO2), and trans-activation-responsive RNA binding protein 2 (TARBP2). Dicer carries out its function downstream of DROSHA, a nuclear homolog of Dicer. miRNAs are produced from hairpin-folded pre-miRNAs that are approximately 60 nucleotides long. The pre-miRNAs are exported to the cytoplasm by exportin 5, where they are then processed by Dicer into the~22 nt double stranded RNAs (dsRNAs). dsRNAs are loaded into the Argonaut family member protein by the RLC. The resulting Figure 1. miRNA Production Pathway. DNA sequences are transcribed into RNA sequences that form a 'hairpin' structure of precursor miRNA. Drosha, a nucleic endoribonuclease, cleaves the hairpin from the primary RNA strand. Transported out of the nucleus by exportin 5, they are further processed by Dicer. After processing by Dicer and its accessory proteins, the hairpin structure of the precursor miRNA is degraded, leaving a single, linear piece of miRNA (the opposing piece is degraded by intracellular processes). This single piece is then bound by the RNA-induced silencing complex RISC. The RISC-miRNA complex binds to target mRNA strands, inhibiting translation by the ribosome.
Dicer plays additional diverse roles other than miRNA regulation [19]. Dicer plays a role in the processing of rRNA and, indirectly, ribosome biogenesis prior to export from the nucleus. In addition, Dicer plays a role in DNA processing and apoptosis [20]. Upon initiation of apoptosis, Dicer initiates the breakdown of chromosomal DNA, a key step in controlled cell death.

DICER1 Germline Mutations
Research into the causal factors and mechanisms of DICER1 syndrome has been represented in the scientific literature since 2009. Generation and dissemination of new knowledge about DICER1 syndrome is increasing as demonstrated by the increasing number of publications each year over the past decade. Mutations that lead to DICER1 syndrome are reviewed here.
Pathogenic germline mutations of the DICER1 gene linked to DICER1 syndrome are included in this review. Case studies provided detailed information regarding the germline mutation, patient(s), and physical manifestation. Mosaic DICER1 mutations have also been associated with DICER1 syndrome [21], in addition to the inherited germline mutations. Cases represented in the PubMed database [22] were identified using keywords "DICER1 syndrome", "DICER1 mutations", and "Pleuropulmonary Blastoma Familial Tumor Susceptibility Syndrome". A total of 244 articles were collected since the discovery of DICER1 syndrome, and 36 were selected that represented DICER1 pathogenic germline mutations. Articles were excluded from this review if they did not address germline mutations in the DICER1 gene, if they did not identify novel mutations, or if detailed documentation was absent from the report.
Eighty-eight DICER1 mutations are included in this review. While the majority of identified mutations are located within regions that encode one of Dicer's seven defined domains (helicase domains, the Dicer dimerization domain (DDD), the Piwi/Argonaute, Zwille (PAZ) domain, the RNase III domains, and the double-stranded RNA-binding domain), some mutations were found to lie outside these domains (see Figure 2). Mutations, resulting protein changes, patient information, and background, clinical manifestation, and references are presented in Tables 1-3.  Tables 1-3).

Manifestations of DICER1 Gene Mutations
While the range of clinical symptoms associated with DICER1 syndrome is varied, some symptoms are prevalent among patients. These include multinodular goiter, pleuropulmonary blastoma, cystic nephroma, and ovarian Sertoli-Leydig Cell Tumor.

Multinodular Goiter (MNG)
Multinodular goiters, abnormal, cancerous growths of the thyroid gland, are associated with DICER1 syndrome and are a subsection of thyroid growths, which have been reported as a prevalent manifestation of DICER1 syndrome. Seventy-five percent of women and 17% of men with DICER1 syndrome were shown to harbor abnormal thyroid growths, multinodular goiters included, compared to the control population: 8% and 0% for women and men, respectively [23]. Specifically, a recent study indicated a correlation between truncating germline DICER1 mutations and familial multinodular goiter, among other cancers [24]. Women are more likely to develop thyroid cancer than men, regardless of DICER1 variant status [25]. Frequent malignant neoplasms of the endocrine system [26] such as thyroid cancers are linked to both environmental and genetic factors, with studies indicating similar links between multinodular goiters and both environmental and genetic factors [27]. DICER1 mutations and subsequent global downregulation of miRNAs were found in multinodular goiter [28].

Pleuropulmonary Blastoma
Pleuropulmonary blastoma is a manifestation of DICER1 syndrome [29]. Primarily observed in children, it is a rare cancer that originates in the pleural cavity or the lungs [30]. Although rare, many cases of pleuropulmonary blastoma have been identified to be associated with DICER1 syndrome. First described in 1988 [31], cases are now documented in an international registry.
Four types of pleuropulmonary blastomas have been characterized. Type I is defined by cystic growths, has malignant potential, and may undergo malignant transformation in childhood. Type Ir is very similar to Type I, being defined by cystic growths, but contains no cancerous cells and is therefore not malignant. Type II consists of a hybrid of cystic and cancerous tumors, and type III solely consists of solid cancerous tumors. Types II and III have been associated with increased metastasis, primarily to the brain [32], and were found to be more aggressive than type I pleuropulmonary blastomas [33] (See Table 1).

Cystic Nephroma
Cystic nephromas have been reported in approximately 12% of children with pleuropulmonary blastomas or those with a family member with cystic nephroma [44,45]. The frequency of DICER1 germline mutations in cystic nephroma patients was found to be 73.2% [46,47].
A recent study of a patient with a cystic nephroma diagnosed as a multicystic left renal tumor was diagnosed with germline and somatic mutations in the DICER1 gene [52]. Additionally, a nascent anaplastic sarcoma of the kidney (ASK) was reported within a cystic nephroma, associated with the presence of a germline DICER1 mutation, or alternatively due to a somatic mutation [44,53].
Ovarian Sertoli-Leydig cell tumors are composed of several cell types, including Sertoli cells and Leydig cells [59]. They are responsible for an increase in testosterone production [60], and can lead to masculinization, voice deepening, and acne [61]. Interestingly, a recent study noted that the simultaneous occurrence of Sertoli-Leydig cell tumor and thyroid carcinoma is a reliable indicator of DICER1 syndrome [62] (See Table 2).  [7] del-deletion, dup-duplication, tran-transversion/transition.

Hodgkin Lymphoma
DICER1 syndrome includes novel symptoms which may facilitate early diagnosis. A rare form of Hodgkin lymphoma was diagnosed in an 11-year old boy with DICER1 syndrome in 2016 [41]. The patient had two DICER1 mutations (c.5299delC and c.4616C>T), and several of his family members shared these mutations. Prior to this diagnosis, Hodgkin lymphoma had not been linked to DICER1 syndrome; additionally, this form of Hodgkin lymphoma is considered rare. Most Hodgkin and Reed-Sternberg cells arise from mature B cells, but a rare subset of cells arise from T cells. In this patient, the cells were found to be of the T-cell lineage, indicating a unique symptom. All affected family members developed at least one type of tumor with differing origins [5].

Pineoblastoma
Pineoblastoma may be associated with a DICER1 mutation. Individuals with pineoblastomas were tested by de Kock and colleagues for the presence of DICER1 mutations. They suggested that germline DICER1 mutations make a clinically significant contribution to pineoblastoma; however, additional studies may confirm a causal relationship [69]. Additionally, the study of a single patient implicated a DICER1 germline mutation in a pineoblastoma. The mutation was found to be heterozygous for germline but hemizygous in the tumor itself [70]. Further studies may determine the relationship between DICER1 mutations and pineoblastomas (See Table 3).  del-deletion, dup-duplication, tran-transversion/transition.

Global Developmental Delay, Lung Cysts, Overgrowth, and Wilms Tumor (GLOW)
Documented by Klein and colleagues, symptoms include Global developmental delay, Lung cysts, Overgrowth, and Wilms tumor (GLOW). These symptoms were identified in patients with DICER1 mutations in the RNase IIIb domain of Dicer [84]. Mutations were associated with Lung cysts and Wilms tumors, but also developmental delays and overgrowth, marked by large body size and mass. While both mutations reported were de novo missense mutations, the symptoms were similar to that of other patients diagnosed with germline mutations in similar loci within the DICER1 gene. The current understanding of GLOW syndrome is limited, and more inquiry is required to determine how widespread GLOW syndrome is in relation to DICER1 syndrome.

Macrocephaly
A recently conducted study indicated that macrocephaly is associated with DICER1 syndrome [85]. Further studies are needed to confirm the link between macrocephaly and DICER1 syndrome, as this may help to identify individuals with DICER1 syndrome at an early stage.

Molecular Mechanisms of DICER1 Mutations-The Two-Hit Hypothesis
The two-hit hypothesis, also known as the Knudson hypothesis, was originally suggested in 1953, and formally postulated by Knudson in 1971 [86,87]. The hypothesis suggests that, for abnormalities to arise, an individual requires two separate mutations in each allele, and that only one mutation in a single allele is not sufficient to induce the formation of tumors. Generally, a single mutation is inherited, which by itself is relatively harmless. However, a second mutation may act in tandem with the first to give rise to cancer. In a number of cases, the two-hit hypothesis describes the mechanism by which tumor suppressor gene deactivation occurs [88,89]. DICER1 syndrome has been recognized as an autosomal-dominant disease, inherited and expressed in a haploinsufficient manner [1,5]. This proposed mechanism has been substantiated by a number of cases of individuals with DICER1 syndrome. Specifically, these cases involved individuals with only one apparent germline mutation, and symptoms characteristic of DICER1 syndrome. However, recent studies have indicated that patients with DICER1 syndrome have not only inherited mutations in one allele of the DICER1 gene but also acquired a somatic mutation in the second allele of the DICER1 gene. The two-hit hypothesis applies to DICER1 mutations and the role of Dicer as a tumor suppressor gene (see Figure 3).
Second-hit, somatic mutations have been found in the RNase IIIb domain of the DICER1 gene. A study involving three children with Wilms' tumor suggested that the two-hit hypothesis applied to DICER1 syndrome, in the formation of Wilms' tumor [90]. The patients were found to harbor germline DICER1 mutations, and upon screening for somatic DICER1 mutations, somatic mutations in the RNase IIIb domain on the second allele were found. Additionally, this finding highlights DICER1 somatic gene mutations occurring in Wilms Tumor patients in the RNase IIIb domain. The regions of the gene encoding the RNase III domains are genetic hotspots for somatic mutations within the DICER1 gene [91,92].
Biallelic DICER1 mutations are common in pleuropulmonary blastomas, with the second mutation occurring within the RNase IIIb domain [93]. A study on 11 pleuropulmonary blastoma patients revealed that, out of 11 patients with DICER1 gene mutations with sporadic pleuropulmonary blastomas, eight harbored biallelic DICER1 gene mutations in which one of the mutations was within the RNase IIIb domain.
A recent report of biallelic DICER1 mutations in an ovarian fibrosarcoma from a 9-year-old patient demonstrated a germline single base insertion in the DICER1 gene, causing a frameshift and premature stop codon as well as a second point mutation within the tumor that resulted in a substitution at amino acid position 1813 within the RNase IIIb domain (p.E1813G) [94]. Another case study involved a 14-month-old female patient diagnosed with pleuropulmonary blastoma and a previously removed cystic nephroma at 11 months [43]. Mutation analysis was conducted on available tissue and peripheral blood, revealing both alleles of the DICER1 gene to be compromised. A missense heterozygous somatic mutation (c.5425G>A; p.G1809R) was detected in the DNA obtained from the cystic nephroma in addition to the germline truncating mutation detected from peripheral blood (c.5347C>T; p.Q1783*) within exon 24, which encodes the RNase IIIb domain. This germline mutation was confirmed in the patient's mother and grandmother. In addition, the patient's 21-year-old female cousin was found to also harbor the germline mutation and had previously been treated for embryonal rhabdomyosarcoma at age 14 and multimodal goiter at age 20. A unique missense heterozygous somatic mutation was detected in the embryonal rhabdomyosarcoma (c.5428G>C; p.D1810H) from a cousin of the patient. This case demonstrates that biallelic mutations in DICER1 alleles, rather than haploinsufficiency, contribute to the mechanism of DICER1 syndrome.
Brenneman and colleagues [95] discussed the concept of biallelic mutations of the DICER1 gene, with a focus on second, somatic, hot-spot mutations in the RNase IIIb domain. A cohort of individuals diagnosed with pleuropulmonary blastoma underwent analysis to determine the mutation status of DICER1 alleles. Mutations within the RNase IIIb domain may represent hotspot mutations [96], and may be the rate-limiting step in the pathogenesis of DICER1 syndrome. Loss of function germline mutations in one allele of DICER1 was found to be common among patients, and the RNase IIIb hotspot mutations were less common, but more frequently found within the tumor. RNase IIIb mutations may also predispose patients to additional mutations due to the role of Dicer in DNA replication and repair.

Future Directions
The breadth of knowledge regarding DICER1 syndrome continues to grow since its recent discovery, but promising treatments and management options need further investigation. As Dicer deficiency stems from DICER1 mutations, experiments on Dicer in model animals have attempted to determine viable methods of upregulation of Dicer and related proteins to combat the effects of DICER1 syndrome. One such study conducted by Blandino and colleagues treated diabetic mice with metformin, reducing the incidence of cancer [97]. Mice treated with metformin showed reduced tumor growth and an upregulation of miRNAs and an increase in DICER1 gene expression. A similar, recent experiment confirmed these results, as metformin was found to induce higher Dicer levels in mouse models and human patients by altering the localization of AUF1, a DICER1 mRNA binding protein that down-regulates DICER1, leading to increased DICER1 mRNA stability [98,99]. While the treatments described in these experiments may not be beneficial to patients with biallelic DICER1 mutations, those with a single, functional DICER1 allele might benefit from metformin and compounds similar to it, as this single allele could hypothetically be targeted and upregulated to achieve increased Dicer protein production.
DICER1 germline mutations have been identified as nonsense mutations, leading to stop codons within the coding sequence and truncated proteins or nonsense-mediated RNA degradation. While these mutations lead to cancerous and non-cancerous tumors, certain antibiotic treatments have been shown to promote the read-through of similar premature stop-codons, leading to restored transcription and translation of otherwise unreadable sequences [100]. One study demonstrated this novel approach by treating human cells containing known nonsense mutations with Ataluren, a pharmaceutical drug utilized in treating genetic disorders. Ataluren successfully promoted the read-through of all three nonsense codons within the mutated alleles [101]. A more recent study built upon these findings by modifying the fluorine number and position of Ataluren, which led to the increased read-through ability of stop codons in nonsense mutations [102]. While no research has been conducted on DICER1 nonsense mutations utilizing drugs such as Ataluren, this drug and synonymous compounds may prove beneficial in treating patients with DICER1 syndrome stemming from nonsense mutations similar to those already tested. While treatments for pleuropulmonary blastoma and cystic nephroma have received wide attention from the scientific community, the root of these abnormal formations, DICER1 germline mutations and acquired somatic mutations, require more study.

Conclusions
The mechanisms of DICER1 syndrome and its presentation in patients have become more fully understood over the past several years as more cases are diagnosed with novel mutations and presentations discovered and documented, extending the phenotypic range of this disorder. Successful treatments for DICER1 syndrome will require a combination of basic, translational, and clinical research. The outcome of the inaugural International DICER1 Symposium was the development of consensus testing and surveillance as well as treatment recommendations. Recommendations for genetic testing-including intronic sequencing [103]; whole exome sequencing [104]; or screening products such as the ThyroSeq, which is a DNA-and RNA-based next-generation sequencing assay that analyzes 112 genes for genetic alterations, including point mutations, insertions/deletions, gene fusions, and copy number alterations [105], prenatal management-and surveillance for DICER1-associated symptoms have been developed and described in recent publications [106][107][108].
Funding: This research was funded by Institutional Development Awards (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health grant numbers P20GM103408 and P20GM109095.

Conflicts of Interest:
The authors declare no conflict of interest.