Loss of heterozygocity in early and Invasive breast cancers
“Loss of heterozygocity” (LOH) is basically a decrease in the genetic diversity of a particular cell in a given area of the body. It occurs when a given cell has lost the normal function of one allele of a gene in which the other allele was already inactivated, which is a common occurrence in cancers, including breast cancer. Generally, loss of heterozygocity within a given region indicates that a functional tumor suppression gene is absent in the area.
I just want to let you know that I have created a newer version of this page with more up-to-date information on Breast Cancer Recurrence and Heterozygocity. However, this page is very long, and still has very great information, and so because of that, I would still use it as well.
For established invasive breast cancers, there will more than likely be a corresponding loss of heterozygocity in areas of cancer spread. Loss of heterozygocity is usually determined by a tissue-staining process, but might also be revealed in blood serum samples. There are a couple of different investigative avenues within the context of breast cancer development and LOH. On the one hand, LOH in some chromosomes may be associated with the very earliest stages of breast tumor development.
At the other end of the spectrum, loss of heterozygocity in blood serum may prove to be a useful tool in the evaluation of breast cancer treatments and in the early detection of metastasis.
Loss of heterozygocity results from the loss of one allele, or ‘allelic imbalance‘
Humans have two copies of each gene, and normally these two copies are expressed at the same level. In some cases individuals may be born with a certain allelic imbalance, but certain genetic diseases can also create scenarios which may increase or decrease expression of one particular allele slightly, resulting in imbalance.
Sometimes this is called “Allelic imbalance“, and it is indicative of a situation where one member (allele) of a gene pair is lost or amplified. Many people will still remain healthy with such a loss, however, because one functional gene is still left on the other chromosome of the chromosome pair. But, this remaining copy of the tumor suppressor gene may later be inactivated by a point mutation, leaving no tumor suppressor gene to protect the body, making one more susceptible to breast cancer development.
Loss of heterozygocity found in a high percentage of breast cancers
Many recent molecular-genetic breast cancer studies have reported significant levels of loss of heterozygosity within tumors and ductal carcinoma in situ. What seems to be of the most interest is the apparent loss of heterozygocity in very early stage breast lesions, even those that are not yet considered breast cancer or even a pre-cancerous situation. In an attempt to find breast cancer in the earliest possible appearance, loss of heterozygosity has been researched and discovered not just in DCIS, but also in very early hyperplastic breast lesions.
However, it does also appear that there are often relatively high levels of loss of heterozygosity in benign hyperplastic lesions as well, so the significance of these findings remains unclear. Around 40% of atypical ductal hyperplasias may show significant loss of heterozygocity, while about 37% of breast lesions with ‘usual‘ hyperplasia may also show loss of heterozygocity. However, the observation that loss of heterozygocity is ‘increasing‘, slightly, with atypical hyperplasias may still prove to be a potentially useful as a breast cancer risk indicator.
Other studies have shown that while both typical and atypical ductal hyperplasia may show loss of heterozygocity rates of around 50%, the LOH rate in DCIS lesions may be as high as 80% of cases. There is no doubt a connection there, but the mechanics of this relationship, should one exist, remains a mystery.
Loss of heterozygocity as a diagnostic tool and for estimating breast cancer risk
Increasingly, genetic loss of heterozygocity is being investigated as potential diagnostic tool, and as another component for estimating breast cancer risk. Analysis for the loss of heterozygocity in various chromosomes due to breast cancer can be done through blood serum tests and also bone marrow tests.
The overall incidences of loss of heterozygocity in breast tumor tissues has been demonstrated in about 27% of cases, while blood serum and bone marrow samples show loss of heterozygosity in about 9% and 5% of cases respectively. The presence of loss of heterozygocity does not seem have any specific association with tumor size, which in turn suggests that factors other than the ‘tumor load‘ influence the loss of heterozygocity levels in blood serum.
Breast cancer treatments often monitor the amount of free tumor DNA in the blood
‘Free tumor DNA‘ circulating in the blood of metastasized breast cancer tumors suggests that tumor DNA is a signature of apoptotic or necrotic cells from both the primary breast tumor and other metastatic sources. So, it then becomes possible to monitor free DNA in patients undergoing systemic breast cancer treatment therapy, suggesting that LOH analysis will eventually become a useful diagnostic tool for breast cancer.
Blood samples from metastasized breast cancers seem to show lower LOH rates than non metastasized breast cancers
Curiously, recent studies in blood serum levels for loss of heterozygocity in non-metastasized (M0) vs. metastasized (M1) breast cancers actually shows a lower LOH level in metastasized breast cancer patients. This could indicate one of two things. First, it suggests that DNA loss might be a contributing factor in early breast cancer tumorigenesis (loss of heterozygocity is most associated with development of the initial tumor, probably through the loss of tumor suppression genes).
Secondly, the finding of a lower LOH rate in blood samples with metastasized breast tumors might reflect the positive influence of breast cancer treatments already underway.
Loss of heterozygocity will likely be widely accepted predictor of breast cancer aggressiveness in the years to come
The factors that give rise to the development of a breast cancer tumor remain largely unknown and unpredictable. That is to say, we can now observe changes which are occurring through the use of immunochemical markers and such, but we really cannot predict how, why, or when these changes will occur.
Some of the known morphological and molecular characteristics and markers of breast cancer tumors include S-phase-fraction, Ki-67, epidermal growth factor receptor (EGFR), the HER2 oncogene, along with invasion and metastasis markers like tumor-associated pro teases and associated inhibitors. Molecular-genetic research showing changes and ‘loss of heterozygocity‘ may soon also prove to be an accepted factor in tumor development.
Down-regulation of tumor suppressor genes is associated with early breast tumor development
Breast cancer carcinogenesis is a complex, multi-step process associated with all sorts of genetic alterations, along with an early hematogenous dissemination of the tumor cells. (The presence of breast tumor cells changes the characteristics of blood serum). Alterations in DNA (loss of heterozygocity), along with the down-regulation of tumor-suppressor genes as another characteristic change in the early development of a breast tumor.
These changes will likely occur to the genes which encode molecules involved in cell adhesion, and cell-cycle arrest and apoptosis.
Certain chromosomes have been specifically identified as important for the proliferation of breast tumor cells.
A number of chromosomal regions that seem to be associated with the proliferation of breast tumor cells have now been identified, and the list is likely to grow, or at least to show more subtle areas of overlap and clustering. Three chromosomes that seem clearly to be associated with the occurrence of loss of heterogeneity in breast tumors are D3S1255, D9S171 and D17S855. Moreover, D3S1255, D9S171 and D17S855 also seem to be correlated with a higher grade of breast tumor.
Evidence also suggests that loss of heterozygocity at the locus D3S1255 might be correlated with positive axillary lymph node status. Allele losses at the marker D9S171 (whose product is a negative regulator of the cell cycle) might be associated with the pathogenesis and spread of sporadic breast cancer (metastasis). There also seems to be a significant correlation between the marker D17S855 and positive progesterone receptor status.
Other genetic breast cancer studies have shown a loss of heterozygocity around chromosome 16q at a rate of about 55%. All of these indicators suggest that at the very least, specific markers for loss of molecular-genetic heterozygocity has strong potential as a diagnostic and evaluative tool in breast cancer staging and treatment.
Loss of 16q chromosome is common in higher grade invasive ductal and lobular breast cancers
The chromosome 16q seems to be implicated in a significant number of invasive breast cancers, and the specific characteristics of this particular chromosome seems to be of great interest among genetic breast cancer researchers at the moment. Loss of heterozygosity at the long arm of chromosome 16 occurs over half of all breast tumors, and it is believed that this loss targets one or more tumor suppressor genes. In recent studies, about 80% of ductal cancers and all lobular breast cancers have demonstrated loss of heterozygocity in at least part of 16q.
What is most intriguing about loss of heterozygocity and specifically loss at the 16q chromosome, is that it is probably one of the earliest genetic events in the development of breast cancer, occurring already in the pre-invasive stage, (DCIS) and probably even before that. LOH at 16q has been known to occur in the FNA diploid sub-population of tumor cells, before aneuploidization (before the loss of heterozygocity in other chromosomes), which suggests it plays a role in early stages of breast cancer tumor development (tumorigenesis).
Various breast tumor grades may correlated with changes in the 16q chromosome
Furthermore, different grades of breast tumors seem to be associated with specific changes in the 16q chromosome arm. Grade I tumors will often lose the entire 16q arm, while grade II and III breast cancer tumors tend to show less frequent whole-arm loss, but have other more complex changes. With grade II and III breast tumors, it is more typical to see small regions of gain, together with large regions of loss.
Invasive lobular carcinomas seem to show the simplest changes on 16q, and it is speculated that lobular carcinomas probably diverge from the invasive ductal breast carcinoma lineage close to the stage of 16q loss. Another angle on the 16q issue is the notion that while most higher grade invasive breast (ductal) carcinomas develop from low-grade lesions, some grade II and III breast tumors may simply ‘arise‘, without a grade I precursor.
Estrogen receptor positive breast cancers often show 16q LOH
Positive estrogen content also seems to be prevalent with many breast tumors showing 16q LOH. Curiously, mucinous breast cancer tumors do not seem to show any 16q LOH at all, at least given current data.
Evidence of loss of heterogeneity exists for columnar cell breast lesions
A columnar cell breast lesion is the result of a change in the cellular formation of the breast terminal duct lobular units, characterized by the proliferation of columnar cells lining the lobules. These columnar cell changes may or may not show atypical features, with ‘atypical‘ presentation generally considered to be a little more worrisome in terms of eventual breast cancer development.
Most of these columnar cell changes are considered completely benign, and there is some diversity of opinion as to whether or not the development of a columnar cell breast lesion is considered a pre-cancerous risk factor. Molecular-genetic research directed towards the earliest possible stages of breast cancer development and with particular concern for loss of heterozygocity has placed new focus upon columnar cell changes, with the hypothesis that loss in and around particular chromosomes are possible precursors to invasive breast carcinoma.
Columnar cell breast lesions consistently exhibit low numbers of genetic alterations and recurrent 16q loss
It has to be said that in some cases, genetic breast cancer studies are perhaps a little guilty of ‘smoothing the data‘ when it comes to making a case for a given molecular-genetic claim. There is a growing body of breast cancer research which aims to prove a hypothetical continuum (roughly) from benign columnar cell change, to columnar cell hyperplasia, to atypical ductal hyperplasia, and finally to ductal carcinoma in situ and eventual invasive breast cancer.
These studies attempt to show that there are overlapping chromosomal alterations between columnar cell breast lesions and more advanced lesions within individual terminal duct lobular units, and therefore also must be a common molecular-genetic evolution between them. There is some evidence to support this, but not overwhelming, conclusive evidence by any means.
However, it is an intriguing idea that no doubt has elements of truth to it, and with years of continued investigation down this path there are sure to be new insights surrounding early chromosomal alterations in columnar cell and other breast lesions to the tumor genesis of breast cancer.
Histologically normal breast epithelium might still contain aberrant clones
Research into the molecular-genetic profile of hyperplastic epithelial breast tumors including columnar cell lesions suggest that even within histologically normal breast epithelium, a high percentage of women may posses occult aberrant clones within these tissues.
Individual clones are rarely a precursor to a co-existing breast cancer, but, they may constitute a reservoir from which proliferative lesions or second cancers develop once additional genetic abnormalities come into play. These occult aberrant-clone cells might then contribute to intratumoral genetic heterogeneity, which would be consistent with the theoretical role of genetic instability early in breast cancer tumorigenesis.
Chromosomes 16q, 9q, 10q, 17p,and 17q losses are found in some atypical breast columnar cell lesions.
Columnar cell breast lesions have been shown to exhibit low numbers of genetic alteration including recurrent 16q loss, which is a similar morphological finding to low grade in situ and invasive breast carcinoma. There is some evidence to support a parallel development between the degree of loss of heterozygocity in certain chromosomes and the degree of proliferation and atypia in columnar cell breast lesions.
It does appear that allelic loss damage in some atypical columnar cell breast lesions may preferentially target loci at the 9q, 10q, 17p, and 17q chromosomes, suggesting that atypical columnar breast hyperplasia might be a morphologic precursor to invasive breast carcinoma.
Differences in loss of heterozygocity may help identify breast cancer cell types
At present, there is no internationally agreed upon standard of definition for the different types of columnar cell breast lesions, so perhaps by looking at various losses in heterozygocity and gene mutation a more precise and ultimately predictive definition of columnar cell hyperplasia might be developed. In fact, there may ultimately be enough evidence along these molecular-genetic lines to change the way of thinking on columnar cell lesions not to be considered ‘hyperplasia‘, but rather as ‘neoplasia‘.
Given sufficient molecular profiling, the development of certain columnar cell ‘neoplasms‘ may ultimately prove to be a kind of non-obligate, intermediary step in the development some forms of low grade in situ and invasive breast carcinomas.
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