Lung Cancer

Diagnosis

Histopathology and characteristics of NTRK+ tumours

Even though the first NTRK gene fusion in CRC was identified more than 35 years ago, only limited clinicopathologic data of NTRK fusion-positive tumours are available [3]. A study including 7,008 CRC cases identified only 16 cases with NTRK fusions by using pan-TRK immunohistochemistry (IHC), followed by NGS. The positive cases were also screened for various other markers. Histologically, all NTRK-positive tumours showed moderate-to-poor differentiation (n=11) with a partly or entirely solid pattern (n=5) and mucinous component (n=10), including 1 case with sheets of signet ring cells. Of note, DNA mismatch repair deficiency (MMRd) was seen in 13/16 (81.25%) of cases. Tumour-infiltrating CD4/CD8 lymphocytes were detected in 9 cases. PD-L1(+) tumour-infiltrating immune cells and focal tumour cell positivity were observed in most cases. CDX2 expression and loss of CK20 and MUC2 expression were also noted. CK7 was expressed in 5 cases. In terms of the molecular profile of NTRK fusion cases, no mutations were identified in BRAF, RAS, and PIK3CA. However, other genes of PI3K-AKT/MTOR pathway were mutated. In several cases, components of Wnt/β-catenin (APC, AMER1, CTNNB1), p53, and TGFβ (ACVR2A, TGFBR2) pathways were mutated. However, no SMAD4 mutations were found. Two tumours also harboured FBXW7 tumour suppressor gene mutations [9].

Another study of 4,569 unselected CRC cases used pan-TRK IHC and identified 9 positive cases. In line with the previous report, eight (89%) of these were Mismatch Repair Deficient (MMRd) and lacked the BRAF^V600E mutation and one was MMR proficient and was also confirmed to be microsatellite stable on formal molecular testing. Trk IHC positive CRCs were mostly right sided, larger in size, and mainly showed infiltrative growth. Mucinous differentiation (n = 3 cases), a tendency to a solid-cribriform growth pattern (n = 3), and prominent tumour infiltrating lymphocytes were among the observed features of NTRK-positive CRCs with MMRd. Apart from these well reported features, already known to be associated with MMRd status, the study did not report any histological features specific to TRK IHC positive CRCs [10]. Another study in Chinese CRC patients has reported a high incidence (7%) of NTRK gene fusions in those patients with Microsatellite Instability high (MSI‐H) CRC; a 20-fold higher incidence than the Microsatellite Instability Stable (MSI-S) patients [20]. Altogether these findings indicate that MMRd CRCs are enriched with NTRK fusions. Importantly, studies on the genomic context of NTRK1/2/3 fusion-positive tumours have elucidated that this enrichment takes place in the subset of spontaneous MSI-H CRC (in which MMRd is caused by methylation of the MLH1 promoter) lacking BRAF mutation [21].

Current testing algorithms

The low prevalence of NTRK fusions in GI tumours makes universal screening of uncertain cost-effectiveness. However, prevalence is higher among certain subgroups of patients, supporting the rationale for universal screening for gene fusions. Pan-TRK IHC using different anti- pan-TRK antibodies has been implemented in various studies to screen for NTRK gene rearrangements in CRC, followed by RNA-sequencing to confirm the results. This two-step approach of IHC screening prior to molecular sequencing is also in line with international guidelines, such as those published by ESMO [22 - 25].

As a part of Lynch syndrome screening programs, many institutions test for microsatellite instability (MSI) or MMRd on all CRCs, and subsequently perform molecular or IHC testing for BRAF^V600E mutation in cases with dual PMS2 and MLH1 loss of expression. Accordingly, testing for NTRK gene fusions should be considered for all patients with  MMRd tumors lacking BRAF mutations [10, 21].

As in other tumours with low NTRK fusion prevalence, NGS is recommended upfront to screen CRCs together with the assessment of MSI status. Alternatively, standard testing for key oncogenic drivers (KRAS, NRAS, and BRAF) can be performed first. At the time of disease progression after first- or second-line therapies, NGS testing may be considered to assess additional molecular alterations, including NTRK gene fusions, especially in MSI-high patients who are negative for key oncogenic drivers [25].

Challenges

In well-resourced institutions, most patients with advanced CRC already undergo NGS or advanced molecular testing, which may include NTRK fusion testing as part of a broader panel. However, in many centres, it is considered difficult to justify the cost of routine molecular testing for NTRK fusions in all patients with CRC. In this regard, pan-TRK IHC offers an inexpensive way to screen for such fusions and follow up with molecular testing only in IHC-positive samples [24]

Treatment

TRK inhibitor efficacy and safety data

Clinical data that supported the registration of larotrectinib and entrectinib in all cancers including CRC, is described in Module 1.

Ongoing clinical trials with TRK inhibitors in all cancers, including CRC are described in Module 1.

References

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