Immunohistochemical detection of ALK protein identifies APC mutated medulloblastoma and differentiates the WNT-activated medulloblastoma from other types of posterior fossa childhood tumors

Abstract

Expression of the ALK gene strongly correlates with the WNT-activated medulloblastomas, which are routinely identified by detection of CTNNB1 mutation. However, some tumors have mutations in other than CTNNB1 genes. Therefore, we investigated if ALK expression may identify WNT-activated tumors without CTNNB1 mutation. In addition, we examined if ALK expression may differentiate WNT-activated medulloblastoma from other malignant posterior fossa tumors. ALK expression was analyzed using immunohistochemistry (clone D5F3) in 70 patients with posterior fossa tumours. Among 55 medulloblastomas, 6 tumors showed ALK expression in > 50% of tumor cells. In one tumor, with ALK positive reaction, negative nuclear reaction against β-catenin and the lack of CTNNB1 mutation, next generation sequencing revealed a presence of pathogenic variant c.3366_3369del in the APC gene, with homozygous deletion leading to inactivation of both copies in tumor cells. MLPA analysis displayed the presence of chromosome 6 monosomy, therefore, confirming the WNT type of this tumor. All analyzed 19 anaplastic ependymomas, 4 choroid plexus carcinomas and 2 atypical teratoid rhabdoid tumors were immunonegative for ALK expression. Therefore, we propose, that immunohistochemical detection of ALK protein should be highly recommended in routine investigation, in parallel to already established methods for identification and differentiation of WNT-activated medulloblastoma.

Keywords: ALK; APC; Immunohistochemistry; WNT-activated medulloblastoma.

Fig. 1

Medulloblastoma tumor with APC mutation. Magnetic resonance image of the tumor with cerebellar midline location (a); ALK immunopositive reaction present in > 80% of tumor cells (b); result of NGS analysis displaying the presence of the c.3366_3369del APC variant in heterozygous state in DNA extracted from patient’s leukocytes (c) and in homozygous state in tumor sample (d). Preparation was scanned at original magnification × 40 and digital magnification presented on image is × 20

Fig. 2

Representative ALK immunostaining in medulloblastoma and anaplastic ependymomas, all located in CPA region. Location of tumors is shown on magnetic resonance images (MRI). Immunohistochemical preparations were scanned at original magnification × 40. Digital magnification is indicated on each image