Peripheral neuroblastic tumours; are derived from developing neuronal cells of the sympathetic nervous system and are found mostly (but not exclusively) in infants and young children.In the text below, we will stick to the well-known term neuroblastoma instead of using the more precise but less common term peripheral neuroblastic tumours.

Neuroblastoma risk stratification utilizes age, stage, histology, MYCN gene amplification status, tumor cell ploidy, and segmental chromosomal abnormalities (Cohn et al., 2009). The International Neuroblastoma Risk Group (INRG) classification system stratifies patients into low-, intermediate-, and high-risk groups. Low- and intermediate-risk patients have excellent outcomes and current clinical trials aim to reduce therapy-related toxicities. High-risk neuroblastoma survival remains poor, necessitating new and improved treatment (Pinto et al., 2015).

Neuroblastoma cells arise from embryonal neural crest cells of the sympathetic lineage (sympathetic neuronal precursor cells).

The pathogenesis is not fully understood, but it is clear that many cytogenetic and molecular factors (discussed below) drive the development of neuroblastoma.
Most neuroblastomas occur sporadically, with only 1-2% occurring in families in an autosomal dominant pattern (Matthay et al., 2016).

Neuroblastoma is the most frequently diagnosed tumor in infancy and the most common extracranial solid tumor and most common cancer diagnosed within the first year of life. These tumors account for 7-10% of all childhood cancers. The incidence, which is almost uniform in industrialized countries, is 5-10 per million children per year. The median age at diagnosis is approximately 17 months. 50% of patients are diagnosed by the age of 2, and 90% of patients are diagnosed before 6 years of age.

The International Neuroblastoma Pathology Classification (INPC - Shimada system) groups neuroblastic tumors into four histopathologic categories according to the degree of cellular differentiation into ganglionic cells, organoid maturation with the development of a Schwann cell stroma, and co-existence of clones of different maturity or of distinct aggressiveness: 

- Neuroblastoma (Schwannian stroma-poor)
- Ganglioneuroblastoma intermixed (Schwannian stroma-rich)
- Ganglioneuroma (Schwannian stroma-dominant), maturing subtype OR Ganglioneuroblastoma, well differentiated (Schwannian stroma-rich)
- Ganglioneuroblastoma nodular (composite).

Two of these categories are further divided into subgroups according to cellular differentiation signs, i.e. Neuroblastoma into: undifferentiated, poorly differentiated and differentiating; and Ganglioneuroblastoma into maturing and mature.
The degree of differentiation and stromal component of tumors can be used for prognostic assessment. Additionally, the age of the patient at diagnosis (below 1.5, between 1.5 and 5 years or over 5 years) is included as well as the number of mitotic and karyorrhectic (apoptotic) cells in the category of Neuroblastoma and the nodular part of Ganglioneuroma nodular.
Favorable features include:

Poorly differentiated or differentiating neuroblastoma, low or intermediate mitosis-karyorrhexis index (MKI), and age ≤ 1.5 years

Differentiating neuroblastoma, low MKI, and age 1.5 to 5 years

Ganglioneuroblastoma, intermixed

Ganglioneuroma
Unfavorable features include:

Undifferentiated tumor or high MKI tumor

Poorly differentiated tumor or intermediate MKI tumor in patients age 1.5 to 5 years

Patients ≥ 5 years with any grade of differentiation or MKI class

Nodular ganglioneuroblastoma
Prognostic Factors: As neuroblastoma is a heterogeneous disease, risk stratification according to clinical features of the patient and biologic features of the tumor has allowed for an accurate assessment of prognosis and risk-adapted therapy. To facilitate clinical research and improve the outcome of children with neuroblastoma, investigators from the major cooperative groups developed a consensus pre-treatment International Neuroblastoma Risk Group (INRG) classification system. The task force collected data on 36 prognostic variables on 8,800 children enrolled on cooperative group studies and identified the seven factors that were highly statistically significant and also considered clinically relevant (Cohn et al., 2009).

• Tumor Stage:
• -Stage L1: Locoregional tumor not involving vital structures as defined by the image-defined risk factors (IDRF) and confined to one body part
• -Stage L2: Locoregional tumor with presence of one or more IDRF
• -Stage M: Distant metastatic disease (except Stage MS)
• -Stage MS: Metastatic disease confined to skin, liver, and/or bone marrow
• -Age at diagnosis - Age

Somatic Genomic Variations:
Multiple somatically acquired genomic alterations that contribute to tumorigenesis and progression of disease have been described. Somatic changes in the tumor genome such as gene mutations, loss or gain of alleles, and variations in ploidy have been identified as important factors in in the development of neuroblastoma and contribute to its diverse phenotype.

Patients with low-risk neuroblastoma are often managed with surgical resection or, as is being evaluated in an ongoing clinical trial (NCT01728155), observation alone. Intermediate-risk patients are treated with chemotherapy (number of cycles is dependent on prognostic markers) and surgical resection of the primary tumor (Twist et al., 2019). Low- and intermediate-risk neuroblastomas have excellent outcomes and clinical trials aim to reduce therapy-related toxicities (Pinto et al., 2015). High-risk standard-of-care treatment consists of three treatment blocks. The first block, induction, includes high-dose chemotherapy and resection of the primary tumor. The second block is consolidation, which consists of tandem courses of myeloablative dose chemotherapy with autologous stem-cell rescue and external-beam radiotherapy (Park et al., 2019). Lastly, post-consolidation therapy includes anti-ganglioside 2 (GD2) immunotherapy with cytokines and cis-retinoic acid (Yu et al., 2010). Current clinical trials are evaluating the addition of MIBG therapy to induction (NCT01175356), anti-GD2 to induction chemotherapy (NCT01857934), or removing IL2 from immunotherapy and altering chemotherapy regimens (NCT01704716). Refractory and relapsed neuroblastoma is common in high-risk patients, indicating the need for further research on acquired drug and/or clonal resistance. While many patients can be salvaged with combination chemotherapy and anti-GD2 antibody (Mody et al., 2017), the long-term outcomes of relapsed neuroblastoma remain dismal.

Neuroblastoma mass screening: Neuroblastoma mass screening was attempted in order to identify patients in a preclinical stage with the intention to decrease mortality. The results obtained by these efforts indicated that while the incidence of low-risk neuroblastoma increased dramatically, there was no decrease in the incidence or mortality of high-risk disease, suggesting that screening was ineffective and that low-risk and high-risk neuroblastoma are two biologically independent entities.

Special features: As demonstrated by mass screening efforts, neuroblastoma tumors range from indolent masses that may never be detected to rapidly growing and aggressive tumors. Spontaneous regression (without cytotoxic treatment) can be observed in neuroblastoma diagnosis within the first 18 months of life, even with metastatic disease. These tumor cells are characterized by distinct genomics. Spontaneous maturation of neuroblastoma into ganglioneuroblastoma and ganglioneuroma can be observed. These neuroblastic/ganglionic cells share genetic characteristics with spontaneously regressing neuroblastomas. Moreover, such tumor cells are capable of recruiting non-neoplastic Schwann cells from the tumor adjacent tissue.

Segmental chromosomal aberrations: Tumors with segmental and subsegmental chromosomal aberrations have been found to be associated with high-risk disease. Additionally, new segmental aberrations have often been found at time of relapse. The most common alterations are gain of chromosomes 2p, which is the location of the MYCN oncogene; 17q gain; 1p loss of heterozygosity (LOH); and 11q loss (Schleiermacher et al., 2012; Bown, 2001). 1p LOH is found primarily in MYCN amplified tumors whereas loss of 11q is correlated with non-MYCN amplified high-risk disease (Maris et al., 2000; Depuydt et al., 2018).
2p24 Amplification-(MYCN): The MYCN oncogene on chromosome 2p24 encodes a transcription factor that is known to cause malignant transformation. Amplification of the MYCN gene is noted in 20% of all primary neuroblastoma tumors. It is present in 50% of all high-risk tumors, and it is associated with aggressive disease and poor prognosis (Brodeur et al., 1984). MYCN amplification is used as a biomarker for risk stratification in the INRG classification system (Cohn et al., 2009).
17q gain: The gain of the distal portion of chromosome 17q is the most common copy number aberration in neuroblastoma. It is present in 50% of primary neuroblastomas, primarily those that are otherwise classified as high-risk (Bown et al., 1999).
1p LOH: LOH at chromosome 1p36 correlates with MYCN-amplification, metastatic disease, and older age. It is present in 23-35% of primary neuroblastoma tumors (Maris et al., 2000).
11q loss occurs in about 33% of neuroblastomas, primarily non-MYCN-amplified, high-risk tumors. Despite significant research efforts, no confirmed driver tumor suppressors have been identified on 1p or 11q (Attiyeh et al., 2005).

Heredity: Most cases of neuroblastoma develop sporadically, but familial neuroblastoma accounts for 1-2% of all cases (Matthay et al, 2016). Familial neuroblastoma presents at a younger age, and there is often a family history of the same tumor. Its inheritance pattern is most often autosomal dominant with incomplete penetrance.

• PHOX2B: Loss-of-function mutations in paired-like homeobox 2B (PHOX2B) were the first discovered neuroblastoma predisposition genes. PHOX2B mutations are responsible for of 6-10% of familial neuroblastoma cases. These mutations also occur in approximately 2% of sporadic neuroblastoma cases (Trochet et al., 2004).
• ALK: In 2008, the anaplastic lymphoma kinase gene (ALK) was identified as the major familial neuroblastoma predisposition gene (Mossé et al., 2008). Three missense mutations in in ALK (R1192P, R1275Q, G1128A) were found to be present in most familial neuroblastoma cases identified. ALK mutations are also present in 10-12% of cases of sporadic neuroblastoma.
• KIF1B: KIF1B is a tumor suppressor gene that is likely involved in the development of neural crest tumors. Germline mutations in KIF1B have been found in patients who have developed neuronal tumors, including neuroblastoma and pheochromocytoma (Schlisio et al., 2008; Barr and Applebaum, 2018).
• Other cancer predisposition syndromes: While the mutations in PHOX2B, ALK, and KIF1B are specific for predisposition to developing neuroblastoma, there are several syndromes that can lead to the development of neuroblastoma including RAS pathway mutations ( Costello syndrome, Noonan syndrome, neurofibromatosis type I, and congenital central hypoventilation syndrome), Li-Fraumeni syndrome, ROHHAD syndrome (rapid-onset obesity with hypothalamic dysfunction, hypoventilation, autonomic dysregulation), Wiedemann-Beckwith syndrome, Weaver syndrome, and Familial Paraganglioma/Pheochromocytoma.

Sporadic (Nonfamilial) Neuroblastoma
Sporadic neuroblastoma is recognized to be a complex genetic disease. Large genome-wide association studies (GWAS) have identified common single nucleotide polymorphisms that influence neuroblastoma susceptibility and phenotype.

• BARD1 contains one of the most replicated signals. It has generally been thought of as a tumor suppressor, but in neuroblastoma, risk alleles are associated with increased expression of BARD1, which promotes growth (Capasso et al., 2009).
• LMO1 is a neuroblastoma oncogene and polymorphisms in the gene are associated with the development of high risk, clinically aggressive disease (Wang et al., 2011).
• LIN28B polymorphisms are strongly correlated to with predisposition to high-risk neuroblastoma. Variants in HACE1, TP53 have also been identified (Diskin et al., 2016).
• Variants in DUSP12 (1q23), HSD17B12 (11p11), DDX4 / IL31RA (5q11.2) are associated with low-risk neuroblastoma (Nguyen et al., 2011).
• Variants in CASC15, NBAT1 (6p22) influence susceptibility to high-risk neuroblastoma (Russell et al., 2015).
• Polymorphisms in KIF15 are associated with the development of MYCN amplified neuroblastoma (Hungate et al., 2017) and those in MMP20 are associated with somatic 11q deletion (Chang et al., 2017).

ALK (Anaplastic Lymphoma Kinase)

2p23.1

The major cause of familial neuroblastoma is heritable gain-of-function mutations of ALK. ALK mutations are also present in 10-12% of cases of sporadic neuroblastoma. It is the gene that is most commonly mutated in neuroblastoma and inhibition of ALK signaling with crizotinib or newer generation ALK inhibitors are being tested in late phase trials.

Cell surface receptor tyrosine kinase

Three missense mutations in in ALK ( R1192P, R1275Q, G1128A) were found to be present in most familial neuroblastoma cases

Genetic missense alterations (most commonly at R1275, F1174, and F1245) or gene amplification events

ALK: Genetic missense alterations (most commonly at R1275, F1174, and F1245) or gene amplification events within anaplastic lymphoma kinase (ALK) can occur. It is the gene that is most commonly mutated in neuroblastoma and inhibition of ALK signaling with crizotinib or newer generation ALK inhibitors are being tested in late phase trials (Bresler et al., 2014).
ATRX: Loss-of-function mutations or deletions in the RNA helicase alpha-thalassemia/mental retardation syndrome X-linked (ATRX) have been observed in neuroblastoma (10% of cases) and are often found in older patients (age at diagnosis >5). ATRX plays a role in chromatin remodeling, nucleosome assembly, and telomere maintenance. ATRX mutations are mutually exclusive with MYCN-amplification and TERT mutations (Pugh et al., 2013).
TERT: Genomic rearrangements in telomerase reverse transcriptase (TERT), a target of MYCN, can lead to neuroblastoma. It has been proposed that all neuroblastomas need a pathway to activate TERT or bypass the pathway via ATRX mutation (Valentijn et al., 2015).

MYCN (v-myc myelocytomatosis viral related oncogene, neuroblastoma derived (avian))

2p24.3

Amplification of the MYCN gene is noted in 20% of all primary neuroblastoma tumors. It is present in 50% of all high-risk tumors, and it is associated with aggressive disease and poor prognosis (Brodeur et al., 1984). MYCN amplification is used as a biomarker for risk stratification in the INRG classification system. The oncogene is either amplified in form of acentric double minute chromosomes (dmin) or chromosomally integrated as homogeneously staining regions (hsr).

Nuclear protein; helix-loop-helix and a leucine zipper domain; transcription factor

Copy_Number_Variation Amplification

ATRX (alpha-thalassemia/mental retardation syndrome X-linked)

Xq21.1

Loss-of-function mutations or deletions in ATRX have been observed in neuroblastoma (10% of cases) and are often found in older patients (age at diagnosis >5 years). ATRX plays a role in chromatin remodeling, nucleosome assembly, and telomere maintenance. ATRX mutations are mutually exclusive with MYCN-amplification and TERT mutations.

RNA helicase.

Loss of function alterations (mutations, multi-exon deletions).

TERT (telomerase reverse transcriptase)

5p15.33

Genomic rearrangements in TERT, a target of MYCN, can lead to neuroblastoma. It has been proposed that all neuroblastomas need a pathway to activate TERT or bypass the pathway via ATRX mutation.

Subunit of the enzyme telomerase.

Rearrangements and copy number mutations.

This research was supported in part by the NIH Grant K08CA226237 (MAA). The contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIH.